FOUS11 KWBC 311839
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
239 PM EDT Thu Oct 31 2024

Valid 00Z Fri Nov 01 2024 - 00Z Mon Nov 04 2024

...U.P. of Michigan...
Day 1...

A compact and strengthening surface low will move rapidly
northeast from northern WI through the U.P. and into southeast
Canada early on D1. Although this low will be pulling away the
first 12 hours of the forecast period, a strong comma head driven
by an elongated axis of mid-level deformation will help dynamically
cool precipitation on the NW side of the departing to low to result
in a band of heavy snow pivoting across the U.P. Although the
strongest ascent is progged to occur before the 00Z Friday (start
of the forecast period) some residual lift into the DGZ combined
with some enhanced moisture on northerly flow off of Lake Superior
will result in moderate snow accumulations, especially in the
higher terrain of the Huron Mountains where WPC probabilities for
4+ inches are as high as 10-30%.


...Much of the terrain of the West...
Days 1-3...

Slow amplification of a mid-level trough will occur through the
weekend across much of the western CONUS, with the trough both
deepening and broadening through the forecast period. This
evolution begins with split flow encroaching onto the Pacific coast characterized by dual jet streaks pushing onshore the Pacific
Northwest and southern California, respectively. As the trough
begins to amplify along the Pacific coast, especially by Saturday
/D2/, the northern stream jet will dig into the Great Basin while
mid-level energy begins to phase into one larger trough. This
evolution will then continue into D3, with the upper trough
beginning to take on a more neutral tilt towards the Four Corners
by the end of the forecast period.

The amplification and evolution of the synoptic pattern will help
to push a surface low southward along the coast, with this feature
digging into the southern Great Basin by Sunday. A cold front
accompanying the low will act as a funnel for increasing PW
reaching as high as +1 sigma, but moisture will otherwise remain
modest across the West as reflected by near normal IVT/PW according
to NAEFS ensemble tables. Still, sufficient moisture within the
region of impressive synoptic lift will result in expanding
precipitation, with two waves likely through the period.

The first wave will be D1 across primarily the Pacific NW as a
weakening surface low pivots towards NW Washington, spreading
precipitation as far south as northern CA and as far east as the
Northern Rockies. This will continue in a weakening form into D2 as
the forcing weakens (dive further south) and moisture remains
modest. Still, the amplifying upper trough will help cool snow
levels to 3500-4500 ft on Friday before slowly warming late D1 in
advance of the next round of WAA/precip. WPC probabilities D1 for
6+ inches of snow are generally 50-90% in the Cascades of OR and WA
above 4500 ft, with additional high probabilities continuing into
the Blue Mountains of OR.

Then during Saturday and Sunday, snow levels vary within periods
of WAA and then cooling as the trough amplifies, but will generally
waver within the 4000-6000 ft range in areas where precipitation
will occur. Rounds of precipitation will expand eastward through
the weekend as well, resulting in axes of moderate to heavy snow,
with the heaviest snow accumulations expected in the higher terrain
or where favorable upslope occurs. WPC probabilities for more than
6 inches are high both D2 and D3, focused along the OR portion of
the Cascades, as well as across the northern WA Cascades, where
3-day snowfall in excess of 2-3 feet is likely in the higher
terrain. Additional heavy snow of 1 foot or more is also possible
across the terrain around Yellowstone NP including the Tetons and
Wind Rivers, with several inches of snow also likely across the
Sierra, Uintas, and San Juans.

With snow levels generally remaining moderately high,
impacts to the passes appear to be modest, which is reflected by
low WSSI-P for moderate impacts except at Santiam Pass.

The probability of significant ice across the CONUS is less than
10 percent.

Weiss



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FOUS11 KWBC 010623
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
223 AM EDT Fri Nov 1 2024

Valid 12Z Fri Nov 01 2024 - 12Z Mon Nov 04 2024

...Mountainous West...
Days 1-3...

Active period in the Western CONUS over the next few days
downstream of a very strong upper ridge (~99th percentile) between
Alaska and Hawaii. This will help guide several northern stream
systems into the West Coast and into the Intermountain West this
weekend into early next week. With the source region in the mid-
latitudes, moisture anomalies with each system will be modest and
thus the brunt of the snowfall will be driven by orographic upslope
into the Cascades initially then into the Rockies thereafter. Snow
levels will be on the higher side but still low enough at times to
bring some snow to the higher mountain passes.

D1 system will have the largest QPF amounts, primarily into the
WA/OR Cascades, as a cold front comes ashore. D1 probabilities of
at least 8 inches of snow are >50% above ~5000-6000ft. Into the
weekend, the lead system will move through the Great Basin with
light to moderate snow across central Idaho/western Montana and
into the Tetons southward through the Uintas and Wasatch. WPC
probabilities for at least 8 inches of snow D2 are low (<40%) in
these areas but a bit higher into the OR Cascades with the next
system quickly approaching the coast. By D3, that system will slip
through the Great Basin and into the central Rockies, with more
moderate totals possible into northern CO (Medicine Bow). Light
snow will work its way into the Front Range by the end of the
period as low pressure deepens over southeastern CO. WPC
probabilities of at least 8 inches of snow are >50% above 10,000ft
or so and for at least 4 inches are >50% above 8000ft.

The probability of significant ice across the CONUS is less than
10 percent.

Fracasso






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FOUS11 KWBC 011922
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 PM EDT Fri Nov 1 2024

Valid 00Z Sat Nov 02 2024 - 00Z Tue Nov 05 2024

...Mountainous West...
Days 1-3...

A series of upper level disturbances will carve out a longwave
trough across the western third of the U.S. that will result in
periods of moderate-to-heavy mountain snow this weekend from the
Pacific Northwest to the Central Rockies. This afternoon and
through Saturday, mountain ranges such as the Cascades, northern
Sierra Nevada, Blue, Sawtooth, Bitterroots, Tetons, Wind River,
Lewis Range, and as far south as the Wasatch are all likely to see
measurable snowfall. Given the more supportive orthogonal flow
pattern into the Oregon Cascades, the Blue Mountains, and the
Boise/Sawtooth of Idaho, these mountain ranges have the higher odds
(high chances, or >70%) for snowfall >8" through Saturday
night. While the initial leading disturbance amplifies the
longwave trough over the Southwest U.S. Saturday evening, the next
trailing shortwave trough will arrive in the Pacific Northwest
bringing additional mountain snow to the Cascades and northern
Rockies through Sunday morning.

As the trailing disturbance dives southeast, it will further
deepen the longwave trough and foster an impressive area of upper
level divergence over the south-central High Plains. A surface low
in lee of the Rockies will strengthen over eastern Colorado while
500mb heights continue to fall across the Four Corners region
Sunday evening. By Sunday evening, 500mb and 700mb heights along
the Mexico/Arizona border dip to the 1st climatological percentile
according to the ECMWF SATs. As mid level moisture begins to wrap
around the 500mb low over central AZ and central NM Sunday night,
this should result in periods of heavy snow in parts of the
Southern Rockies. A little farther north, as high pressure builds
in over the northern High Plains, easterly upslope flow will
influence heavier snowfall along the Front Range of the Colorado
Rockies as well Sunday night and into Monday morning. By Monday
afternoon, the upper low exits to the east, and snowfall will
gradually taper off in the Central and Southern Rockies.

WPC PWPF from late Sunday and into Monday sports moderate-to-high
chances (50-70%) for snowfall >6" for elevations above 9,000ft in
the Colorado Rockies. This is also the case along the Sangre de
Cristo Mountains of northern New Mexico. Most of these mountains
ranges are showing moderate-to-high chances (50-70%) for Minor
Impacts according to the WSSI-P late Sunday into Monday. Residents
along the Palmer Divide of central Colorado and south towards Raton
Pass should also keep a close eye on this system for Sunday night
into Monday. This would be the first measurable snowfall of the
season for parts of these areas, and with low-to-moderate
probabilities (30-50%) present for snowfall totals >4", even
lesser snowfall totals could result in travel headaches for
commuters in these areas Monday morning.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax



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FOUS11 KWBC 020637
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 AM EDT Sat Nov 2 2024

Valid 12Z Sat Nov 02 2024 - 12Z Tue Nov 05 2024

...Mountainous West...
Days 1-3...

An active pattern remains for much of the West this period,
downstream of a strong upper ridge between Alaska and Hawaii. This
will take a series of systems on a NW to SE path from the PacNW
through the Great Basin and into the Rockies this weekend into
early next week. For day 1, ongoing system in the NW will continue
to bring snow to the Cascades eastward to the northern Rockies and
southward through the Bitterroots, Tetons, Wind River, into the
Wasatch. WPC probabilities for at least 8 inches of snow are >50%
above 5000ft or so in the NW to about 9000ft over the Uintas.

On day 2, the trough will dig into the Four Corners region and
support light to modest snow for the CO Rockies, especially above
10,000ft where WPC probabilities for at least 8 inches of snow are

50%. Light snow is probable into the I-25 corridor but amounts are

quite uncertain. Accumulating snow is favored farther south into
the Palmer Divide. By day 3, that upper trough will continue to
move slowly through NM with a focus for snowfall over the Sangre de
Cristos into the Raton Mesa. There, WPC probabilities for at least
8 inches of snow are moderate (40-70%). Back to the PacNW, the
next system will move ashore with another surge in moisture to the
area and significant snow for the WA Cascades. Snow levels will
rise to 4500ft or so with a decent moisture plume (300-400 kg-m/s
IVT) then fall behind the front. Higher mountains passes will
likely be affected with significant accumulations, but lowering
snow levels will likely affect some lower passes as well. WPC
probabilities for at least 8 inches of snow are >50% above
3000-3500ft or so.


The probability of significant ice across the CONUS is less than
10 percent.

Fracasso


$$

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FOUS11 KWBC 021920
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 PM EDT Sat Nov 2 2024

Valid 00Z Sun Nov 03 2024 - 00Z Wed Nov 06 2024

...Mountainous West...
Days 1-3...

A series of upper level disturbances will carve out a longwave
trough across Intermountain West this weekend, with another
imposing shortwave trough hot on its heels that arrives in the
Pacific Northwest to kickoff the first full week of November.
Numerous mountain ranges across the West, including but are not
limited to: Cascades, Blue, Northern Rockies, Tetons, Wind River,
Uinta, and Wasatch, can expect periods of snow this evening that
linger into Sunday morning. As a trailing shortwave trough over the
northern Great Basin dives southeast Sunday morning, 500mb and
700mb height falls will ensue across the Four Corners states. Snow
will pick up in intensity along the Wasatch Sunday morning, the
over the southern Wyoming and central Colorado Rockies Sunday
afternoon. Snowfall rates will increase along the Front Range of
the Colorado Rockies Sunday afternoon and evening as low pressure
strengthens over the eastern Colorado High Plains and a favorable
combination of low level upsloping northeasterly flow combined with
strong upper level divergence ahead of the trough sets up over
central Colorado. This will likely result in not only heavy snow
as far south as the Sangre de Cristo and Raton Pass, but minor
snow accumulations along the Palmer Divide and portions of the
Denver/Boulder metro area Monday morning. Snow will linger in parts
of the southern Rockies through early afternoon, then come to an
end by Monday evening. WPC PWPF shows moderate-to-high chances
(50-70%) for >8" of snowfall in parts of the Wasatch and central
Rockies above 9,000ft. There are similar probabilities for parts of
the Sangre De Cristo and Raton Pass through Monday morning.

In the Pacific Northwest, a brief break in the active pattern
arrives Sunday night and into early Monday morning, but the next
northeast Pacific storm system arrives late Monday morning and will
bring another round of heavy snow to the Washington Cascades. NAEFS
shows IVT values of 300-400 kg/m/s that are above the 90th
climatological percentile over much of Washington State. Heavy
snow will spill over into the Northern Rockies Monday afternoon and
persist into Tuesday as a storm system in the Canadian Prairies
keeps moist and upsloping NWlry 700-300mb flow over the Northern
Rockies. Over the course of the next 72 hours, WPC PWPF above
5,000ft in the Washington Cascades sports high chances (>70%) for
snowfall totals >18". Similar high chance probabilities for >12"
of snowfall over the next 72 hours are present over the Northern
Rockies, especially in the Lewis Range, the Bitterroots, and Blue
Mountains at elevations >6,000ft. Treacherous travel conditions
around some passes in the northern Rockies and the Cascade Range
are possible. Farther south for elevations >7,000ft the Tetons,
Wind River Range, and ridge lines in the northern Great Basin sport
moderate- to-high chances (50-70%) for >8" of snowfall through
Tuesday afternoon.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax



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FOUS11 KWBC 030705
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
205 AM EST Sun Nov 3 2024

Valid 12Z Sun Nov 03 2024 - 12Z Wed Nov 06 2024

...Mountainous West...
Days 1-3...

Upper troughing over the Great Basin this morning will continue to
dig into the Four Corners region into tomorrow, promoting broad
lift over the Wasatch and especially the CO Rockies. WPC D1
probabilities of at least 8 inches of snow are >50% above 10,000ft
or so.

Into D2, upper low will be moving out of NM with lingering snow
for the San Juans, Raton Mesa, and Jemez Mountains where WPC
probabilities for an additional 8 inches of snow are moderate
(40-70%). Concurrently, a shortwave out of the northeastern Pacific
will push into Washington with a brief surge in moisture aimed at
the WA Cascades before weakening as the cold front moves inland.
Snow levels will rise from ~4000 to 6000ft as the core of the WAA
surges in ahead of the front, then will fall back below 4000ft
post-FROPA. Moisture should reach into the northern ID mountains
and into NW MT as well, where WPC probabilities for at least 8
inches of snow are >50% above 4000-5000ft or so.

By D3, the system in the NW will continue toward the Rockies with
upstream vorticity at the nose of a 140kt jet streaming in across
the PacNW, helping to amplify the pattern over the eastern Pacific
into western North America. This, in turn, will carve out a broad
upper trough that will dig into the Four Corners region through the
end of the period (and then beyond). Broad lift combined with more
localized upslope will maximize snow over the higher terrain as
snow levels fall to the valley floors in most areas of the Northern
Tier. WPC probabilities for at least 8 inches of snow are highest

70%) over the Little Belt Mountains in MT as well as the Big Snow

Mountains due to the favorable northerly flow. Lighter snow is
forecast for the central Idaho ranges, NW MT around Glacier NP,
southward through western WY and into the CO Rockies.

The probability of significant ice across the CONUS is less than
10 percent.

Fracasso


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FOUS11 KWBC 031938
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
238 PM EST Sun Nov 3 2024

Valid 00Z Mon Nov 04 2024 - 00Z Thu Nov 07 2024

...Mountainous West...
Days 1-3...

An anomalous upper level low (NAEFS shows 500mb heights near the
1st climatological percentile over southwest New Mexico tonight)
will be the primary cause for periods of heavy snow in the higher
elevations of the Colorado and northern New Mexico Rockies. Periods
of snow ill pick up in intensity Monday morning along the Sangre De
Cristo, including around Raton Pass where treacherous travel
conditions are possible at pass level. Snow will taper off in these
areas by Monday afternoon. WPC PWPF shows moderate-to-high chances
(50-70%) for snowfall >8" along Raton Pass and at elevations

9,000ft in the Colorado Rockies. In fact, there are moderate

probabilities (40-60%) for >12" above 10,000ft in central Colorado
and just west of Raton Pass. The Probabilistic WSSI shows moderate
chances (40-60%) for Moderate Impacts (hazardous disruptions to
daily life) in the Wet Mountains of Colorado and along Raton Pass.

Farther north, a potent shortwave trough over the northeastern
Pacific will race through Washington State on Monday with a plume
of Pacific moisture and modest surge in cold air advection
resulting in snow levels dropping as low as ~4,000ft. Heavy snow
Monday morning in the Cascades will linger into the afternoon hours
while the same moisture plume reaches the Northern Rockies Monday
afternoon, prompting periods of heavy mountain snow in the
Bitterroots, Lewis Range (including Glacier NP) and as far south as
the Sawtooth and Tetons. As low pressure strengthens over the
Canadian Prairies Monday night and into Tuesday morning, NWrly
flow will support multiple hours of upslope flow into portions of
the Northern Rockies.

By Tuesday afternoon, the upper low will begin to plunge south as
an anti-cyclonic wave break (ridging over the northeast Pacific
races over southwest Canada and forces the upper low south) leads
to a southward track in the cold front and another area of low
pressure forming in southeast Colorado by Tuesday night. This setup
will lead to periods of snow once again in the Central Rockies,
including the Palmer Divide and along both the Front Range of
Colorado and the Sangre De Cristo on Wednesday. WPC PWPF shows
high chances (>70%) for >8" of snowfall above 9,000ft. Along the
Palmer Divide and including the Denver/Boulder metro area, there
are moderate chance (40-60%) for snowfall total >4". The Palmer
Divide, given the higher elevation compared to Denver's metro area,
sport low chances (10-30%) for >8" of snow through Wednesday
evening.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax



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FOUS11 KWBC 040704
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
204 AM EST Mon Nov 4 2024

Valid 12Z Mon Nov 04 2024 - 12Z Thu Nov 07 2024

...CO/NM border...
Day 1...

Upper low over NM this morning will lift northeastward across the
TX Panhandle this evening. Snow on the NW side of the low within a
region of modest PVA and enhanced upslope into the Raton Mesa will
continue for the first part of D1 before tapering off later this
evening and overnight. Lighter snow will extend eastward into the
far western OK Panhandle as rain changes to snow. WPC probabilities
for at least 8 inches of snow are >50% above 9000-10,000ft over
portions of central/northern NM and across the CO line.


...Northwest and Northern/Central Rockies...
Days 1-2...

Next system coming into the PacNW will be the last for at least a
little bit in what has been quite a busy stretch. Warm front will
move ashore today bringing in a surge of moisture and rising snow
levels up to around 4000ft to the north and 6000ft to the south
over the WA Cascades, but only rising to around 5000ft farther east
toward the Divide. Strong and favorable upslope flow will maximize
over the WA Cascades where a few feet of snow will be possible at
the higher peaks. By late in the evening and overnight into
Tuesday, snow levels will fall to around 3000ft bringing
accumulating snow to many of the lower passes across the Cascades.
WPC probabilities for at least 8 inches of snow are >50% above
3500ft or so. Upper jet will continue inland and promote moderate
snow across the northern ID ranges into northwestern Montana. Into
D2, the mid-level vort will deepen and split, with the northern
portion closing off along the MT/Canadian border before pivoting
back southward Tuesday evening over central MT. This will maintain
modest snow over the central MT terrain (i.e., Little Belt and Big
Snowy Mountains) on upslope northerly flow. There, WPC
probabilities for at least 8 inches of snow D2 are >50% above
4000ft or so. The southern portion of the mid-level trough will dig
into the Great Basin through early Wednesday, spreading snow
across UT/CO where several inches are likely, especially into the
CO Rockies. By the end of D2, an upper low will start to close off
over the Four Corners region, setting up the next phase of the
system.


...Four Corners...
Day 3...

Aforementioned upper low is forecast to be situated over eastern
UT early Wednesday morning and will likely sink southward through
the day to a position over eastern AZ by the end of D3. This will
keep a relatively consistent SW to S flow over northern NM into
southern CO, which is quite favorable for upslope enhancement over
the San Juans and into the Sangre de Cristos among other ranges.
Multiple waves of PVA will rotate around the upper low and across
the region, favoring moderate to heavy snow at times with a high
likelihood (>60% chance) of moderate impacts per the
probabilistic WSSI. WPC probabilities for at least 8 inches of
snow are >50% above 6000-8000ft and probabilities for at least 4
inches >50% down to around 5000-7000ft. Colder air filtering into
northeastern CO will also promote light to perhaps modest snow over
the I-25 corridor with heavier totals likely into the Palmer
Divide. Heavy snow is forecast to continue past the end of D3 as
the system slowly moves through the region. Please see the Winter
Weather Outlook and the Probabilistic Winter Storm Severity Index
(WSSI-P) for more information.

The probability of significant ice across the CONUS is less than
10 percent.

Fracasso


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FOUS11 KWBC 041937
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 PM EST Mon Nov 4 2024

Valid 00Z Tue Nov 05 2024 - 00Z Fri Nov 08 2024

...CO/NM border...
Day 1...

Upper low over NM this morning is lifting northeastward across the
TX Panhandle this afternoon and will become a more progressive open
wave as it enters the High Plains tonight. Snow on the NW side of
the low within a region of modest PVA and enhanced upslope into the
Raton Mesa will persist for a little while longer this afternoon
before tapering off overnight. Light snow will extend eastward
into the far western OK Panhandle as rain changes to snow but minor accumulations are expected in these areas.

...Northwest and Northern/Central Rockies...
Days 1-2...

A vigorous shortwave trough racing through the Pacific Northwest
today is responsible for blanketing the Cascades and tallest peaks
of the Olympics in heavy snow today. Snow levels will fall to as
low as 3,000ft this evening and into tonight, allowing to snowfall accumulations to occur across many of the lower passes across the
Cascades. As the 250-500mb trough moves inland today, the
diffluent left-exit region of the approaching jet streak will be
favorably placed of the Northern Rockies and, in turn, helps spawn
low pressure over the Canadian Prairies this evening. The upper
level divergence associate with the upper level jet will march east
into the Northern Rockies tonight, which combined with a surge in
700mb moisture will support periods of heavy snow along the
Bitterroots, Lewis Range, and Tetons. Lingering Pacific moisture on
the backside of the 700mb low near the Montana/Saskatchewan border
will stick around much of the day Tuesday with periods of heavy
snow unfolding along the Tetons, Little Belt, Big Snowy, and Big
Horn Mountains. Snowfall should gradually taper off in these areas
by Wednesday morning.

WPC PWPF shows high chances (>70%) for snowfall totals >8" over
parts of the Bitterroots, Lewis Range, Little Belt, and Big Snowy
mountains. These ranges, especially >6,000ft, have moderate-to-high
chances (50-70%) for >12" through Wednesday afternoon. Some of the
tallest peaks of these ranges could surpass 24" locally. The WSSI
highlights Moderate Impacts for many of these ranges (above 4,000ft
in the Cascades, above 6,000ft in the Northern Rockies ranges
listed) while the peaks of all ranges mentioned sport Major Impact
potential as a result of this early November snowstorm.

...Four Corners...
Day 3...

The same upper level feature responsible for heavy snow in the
northern Rockies will dive south on tuesday through the heart of
the Intermountain West and into the Four Corners region by early
Wednesday morning. By Wednesday, the upper level trough will cut
off into an anomalous upper level low thanks to a staunch anti-
cyclonic wave break event over the northwestern US and southwest
Canada. There does remain some uncertainty on whether this upper
low will feel the influence of a lingering disturbance over the
Central Plains (GEFS) or be fully cut off from the mean steering
flow pattern for multiple days (ECMWF EPS). While the exact
evolution is unclear beyond Day 3, this synoptic scale setup is
likely to produce the first significant winter storm of the season
in the Southern Rockies. The stage becomes set initially by a cold
front diving south to usher in a colder air-mass throughout the
region. Then, as the upper level low cuts off over the Four Corners
region Wednesday morning, strong upper level divergence aloft will
support excellent large scale ascent atop the atmosphere. Next, as
the upper low late Wednesday into Thursday deepens, the moisture
flow over the southern High Plains will accelerate, prompting
southerly IVT values above the 90th climatological percentile (on
both NAEFS and the ECMWF SATs) to be directed at the southern
Rockies.

Look for periods of snow to initially occur along and in wake of
the cold frontal passage from the Wasatch on east to not just the
Central Rockies but parts of the central High Plains as well. As
high pressure builds in to the north, sub-freezing air rushing
south along the Front Range of the Rockies will provide the
opportunity for periods of snow within the Denver/Boulder metro
area Tuesday night and into Wednesday morning. This upslope
component does provide a good setup for heavy snowfall in parts of
the San Juans, the Front Range, and the Sangre De Cristo through
the day Wednesday and into Thursday. The key on Thursday will be
where the upper low sets up and the extent of the moisture/upslope
flow into the Southern Rockies. Latest guidance trended a little
drier over parts of the south-central Colorado Rockies, but the
prolonged upslope into late Thursday-Friday may still result in
additional heavy snowfall by the end of the week.

WPC PWPF for the event through 00Z Fri shows high chances (>70%)
for snowfall >8" in the San Juans, the Sangre De Cristo, and along
the Front Range of the Rockies. For >12" probabilities, there are
high chances (>70%) along the Sangre De Cristo. The Denver metro
area sports low-to-moderate chances (30-50%) for snowfall >4" with
the southern suburbs on the higher end of that range. WSSI-P
moderate probabilities show moderate chances (40-60%) along the
Sangre De Cristo and along Raton Mesa Wednesday night and into
Thursday.


The probability of significant ice across the CONUS is less than
10 percent.


Mullinax




$$

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FOUS11 KWBC 050721
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
221 AM EST Tue Nov 5 2024

Valid 12Z Tue Nov 05 2024 - 12Z Fri Nov 08 2024


...Northern/Central Rockies...
Days 1-2...

Vigorous upper trough moving through the NW will start its
transition to separate systems... the northern entity becoming an
upper low that will wobble through eastern Montana later today and
overnight. Broad divergence will favor light snow over much of
Montana but the eastward then southward movement of the forming
upper low will turn the flow northerly over central portions of the
state, which will enhance upslope flow into the Little Belt, Big
Snowy, and Big Horn Mountains. Snow will fall at a modest to heavy
rates per the 00Z HREF which shows >1"/hr probs of 50-90%. Lighter
snow is expected farther south through WY into CO as the southern
portion of the trough begins to split off and sink toward the Four
Corners, with some higher totals in the high terrain. Snowfall
should gradually taper off in most areas by Wednesday morning
(north) or overnight (south). WPC PWPF shows high chances (>70%)
for snowfall totals >8" over many of the MT ranges, especially

6,000ft, through Wednesday afternoon. Some of the tallest peaks of

these ranges could surpass 24" locally.

...Four Corners...
Day 2-3...

The southern part of the upper level feature responsible for the
heavy snow in the northern Rockies will sink southward through
eastern UT on Wednesday into AZ by early Thursday. After that, it
should slowly turn eastward into NM, but the guidance remains
inconsistent in the evolution/track. A cold front will bring in
colder air to the southern Rockies onto the High Plains as strong
upper level divergence aloft will support broad ascent over the
region. Moisture levels will be modest, but IVT on southwest to
southerly flow is forecast to be near the 90th percentile which
should act in concert with upslope enhancement to promote moderate
to heavy snow over the southern Rockies. Trend has been a bit less
and farther south with the QPF, resulting in a bit less snow for
northern areas in CO and continued uncertainty in snow amounts over
NM. Nevertheless, significant snow is still expected in at least
portions of southern CO into northern NM with this dynamic system.

The upslope component should favor both the SW/S-facing slopes of
the San Juans and Sangre de Cristos as well as farther north into
the Front Range via the cold front. The key on Thursday will be
where the upper low sets up and the extent of the moisture/upslope
flow into the Southern Rockies. By Friday morning, the upper low is
forecast to drift into NM with the brunt of the snowfall over NM.

WPC PWPF for the event through 12Z Fri shows high chances (>70%)
for snowfall >8" in the San Juans, the Sangre De Cristo, and into
parts of the far western High Plains. Highest totals are likely
over the Sangre De Cristo where 2ft+ is possible (>50% chance).
Around the Denver metro area, WPC PWPF for >4" of snow are around
30% but increase to the south (i.e., Palmer Divide) and then
increase again near/south of Colorado Springs and Pueblo to reach

70%.

The probability of significant ice across the CONUS is less than
10 percent.


Fracasso


$$

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FOUS11 KWBC 052046
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 PM EST Tue Nov 5 2024

Valid 00Z Wed Nov 06 2024 - 00Z Sat Nov 09 2024


...Montana and Wyoming Mountain Ranges...
Day 1...

Vigorous upper trough moving through the northern Rockies today is
starting its transition to split into two separate systems, with
the northern entity becoming an upper low that will wobble eastward
into through the northern Plains overnight. Broad divergence will
favor light snow over much of Montana but the eastward then
southward movement of the forming upper low will turn the flow
northerly over central portions of the state, which will enhance
upslope flow into the Little Belt, Big Snowy, and Big Horn
Mountains. Snow will fall at a modest to heavy rates per the 12Z
HREF which shows >1"/hr probs of 50-90%. Lighter snow is expected
farther south through WY as the southern portion of the trough
begins to split off and sink toward the Four Corners, with some
higher totals in the high terrain. Snowfall should gradually taper
off in most areas of MT and WY by Wednesday morning. WPC PWPF shows
high chances (>70%) for snowfall totals >8" over many of the MT
ranges, especially >6,000ft, through Wednesday afternoon. Some of
the tallest peaks of these ranges could surpass an additional 15"
locally.

...Central/Southern Rockies and Adjacent High Plains...
Days 1-3...

The southern part of the upper level feature responsible for the
heavy snow in the northern Rockies will sink southward through
eastern UT on Wednesday into AZ by early Thursday. After that, it
should slowly turn eastward into NM as a closed upper low and set
the stage for a potentially significant and long-duration heavy
snowfall event for parts of the southern Rockies and nearby High
Plains. The guidance still remains somewhat uncertain on the
evolution of this system, but has narrowed in on a slower/amplified
solution with today's 12z runs. A cold front will bring in colder
air to the southern Rockies and adjacent High Plains as strong
upper level divergence aloft will support broad ascent over the
region. An initial area of mesoscale banding associated with this
cold front, a leeside cyclone, and left exit region of a
strengthening upper jet over the central High Plains on Wednesday
may produce a swath of heavy snow from the Palmer Divide to the
KS/CO border. Hires guidance, including the 12z HREF snowband tool,
highlight this region where 1-2" snowfall rates could occur.
Medium probabilities (30-50%) of at least 8" of snow exist across
parts of east-central Colorado through early Thursday according to
WPC's PWPF, which is an increase from previous runs.

More directly related to the upper low forecast to impact NM and
southern parts of CO between Wednesday night and Friday, moisture
levels will be modest and IVT on southwest to southerly flow is
forecast to near the 90th percentile. This should act in concert
with upslope enhancement to promote moderate to heavy snow over the
southern Rockies. Trend has been for greater QPF over the High
Plains of southeast CO and northeast NM, as well as into the
terrain of northern NM and south-central CO. Some uncertainty
remains in the ensemble guidance and cluster analysis regarding the
western extent of heaviest QPF, with a notable westward trend in
global guidance today in response to higher heights forecast over
the central United States. However, even more uncertainty exists
across the High Plains regarding low-level thermals associated
with this westward trend as the event carries on and warmer air
advects into the High Plains. Nevertheless, significant snow is
still expected in at least portions of southern CO into northern NM
with this dynamic system.

The upslope component should favor both the SW/S-facing slopes of
the San Juans and Sangre de Cristos as well as farther north into
the Front Range via the cold front. The key on Thursday will be
where the upper low sets up and the extent of the moisture/upslope
flow into the Southern Rockies. By Friday morning, the upper low is
forecast to drift into NM with the brunt of the snowfall over NM.

WPC PWPF for the event through 12Z Fri shows high chances (>70%)
for snowfall >8" in the San Juans, the Sangre De Cristo, and into
parts of the far western High Plains (including the Raton Mesa).
Highest totals are likely over the Sangre De Cristo where 2ft+ is
possible (>50-70% chance). Around the Denver metro area, WPC PWPF
for >4" of snow are around 30% but increase to the south (i.e.,
Palmer Divide) and then increase again near/south of Colorado
Springs and Pueblo to reach 50-70%.


The probability of significant ice across the CONUS is less than
10 percent.


Snell




$$

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FOUS11 KWBC 060807
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 AM EST Wed Nov 6 2024

Valid 12Z Wed Nov 06 2024 - 12Z Sat Nov 09 2024


...Central/Southern Rockies and Adjacent High Plains...
Days 1-3...

Upper low at the base of a larger western CONUS trough lies over
southeastern UT this morning, and will continue sinking southward
today into AZ. It will slowly detach from the northern stream and
cut off as it wobbles southward then eastward into southern NM by
early Friday. Then, feeling the push of the subtropical jet, it
will lift northeastward across the northern TX Panhandle and far
western OK Panhandle to the CO/KS border by early Saturday. The
slow movement of the upper low combined with a fairly steady
stretch of moisture as well as upslope enhancement will lead to
significant snow accumulations in parts of the NM mountains and
across the CO line along the Sangre de Cristo range. Impacts may be
major to extreme in some locations.

For D1, frontal boundary over southeastern CO on the edge of the
height falls will see modest to locally heavy snow continue this
morning in an area of surface convergence and beneath bouts of PVA.
Through the day, as the upper low tucks farther southwest of CO,
heights should rise and the snow should lessen. Nevertheless,
snowfall rates of >1"/hr should yield an additional several inches
near/south of DEN and eastward over the High Plains. There, WPC
probabilities for at least 8 inches of snow after 12Z are moderate
(40-70%). Farther south over NM, closer to the stream of moisture
and vort maxes rotating around the upper low center, snow will
increase in intensity over much of the NM terrain where snowfall
just on D1 will likely exceed 8 inches (probabilities >70%) in the
terrain (e.g., Sangre de Cristos/Raton Mesa/Jemez Mnts).

By D2, slow movement of the upper low will allow for continued
snowfall to its east via persistent IVT values near/above the 90th
percentile. This could result in 1-2ft of snow over the NM terrain
(WPC probabilities for at least 12 inches are >70% over much of
northeastern NM) along with areas of blowing/drifting snow eastward
into the western OK Panhandle. Snow will continue over northern NM
into southern CO along the Sangre de Cristos and onto the High
Plains as the upper low sloshes toward the TX/OK Panhandles.
Moisture source will add in then largely switch to the western
Gulf, wrapping around the upper low and capitalizing on upslope
from the northeast through the column. Snow will finally start to
taper off by the end of the period (early Saturday).

All told, 3-day WPC probabilities for at least 24 inches of snow
are >50% above 6000-7000ft over the Sangre de Cristos and Raton
Mesa, with a large footprint of at least 6 inches over much of
eastern CO southward into central NM.


The probability of significant ice across the CONUS is less than
10 percent.


Fracasso



$$

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FOUS11 KWBC 061922
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
222 PM EST Wed Nov 6 2024

Valid 00Z Thu Nov 07 2024 - 00Z Sun Nov 10 2024


...Central/Southern Rockies and Adjacent High Plains...=20
Days 1-3...

Upper low at the base of a larger western CONUS trough lies over=20
the Four Corners this afternoon and will continue sinking=20
southward tonight into eastern AZ. It will slowly detach from the=20
northern stream and cut off as it wobbles southward then eastward=20
into southern NM by early Friday. Then, feeling the push of the=20
subtropical jet, it will lift northeastward across the northern TX=20
Panhandle and far western OK Panhandle to the CO/KS border by early
Saturday. The slow movement of the upper low combined with a=20
fairly steady stretch of moisture as well as upslope enhancement=20
will lead to significant snow accumulations in parts of the NM=20
mountains and across the CO line along the Sangre de Cristo range.=20
Impacts may be major to extreme in some locations and exaggerated=20
by the fact that this is the first significant winter storm of the=20
season for this region.

For early D1, frontal boundary slowly sinking southward from
western KS to northeast NM on the edge of the height falls will=20
continue to promote light to modest snow through this evening over
northeast CO and border regions of KS and NE in an area of surface
convergence and beneath bouts of PVA. Through the evening, as the=20
upper low tucks farther southwest of CO, heights should rise and=20
the snow should lessen. Nevertheless, snowfall rates near 1"/hr=20
should yield an additional few inches near/south of DEN and=20
eastward over the High Plains.=20

Farther south over NM, closer to the stream of moisture and vort=20
maxes rotating around the upper low center, snow will increase in=20
intensity over much of the NM terrain on D1 and last through D2
with similar intensity before the upper low lifts north and weakens
by D3. The slow movement of the upper low will allow for continued
heavy snowfall to its east via persistent IVT values near/above=20
the 90th percentile. This could result in 1-3ft of snow over the NM
terrain (WPC probabilities for at least 12 inches are >90% over=20
much of northeastern NM and southeastern CO) along with areas of=20 blowing/drifting snow eastward into the far western OK Panhandle.=20
Snow will continue over northern NM into southern CO along the=20
Sangre de Cristos and onto the High Plains as the upper low sloshes
toward the TX/OK Panhandles. Moisture source will add in then=20
largely switch to the western Gulf, wrapping around the upper low=20
and capitalizing on upslope from the northeast through the column.=20
Snow will finally start to taper off by the end of the period=20
(early Saturday).

The greatest uncertainty remains along the eastern gradient of
snowfall where low-level warmth creeps into the High Plains and
leads to a transition toward rain from the KS/CO border to eastern
NM and the TX/OK Panhandles. A brief period of sleet is possible=20
as a warm nose aloft ebbs and flows across the High Plains during=20
the event. Exactly where this rain/snow/mix line occurs is still a
question, with today's guidance trending westward and a gradually
warming transition as the storm weakens by Saturday.


All told, 3-day WPC probabilities for at least 24 inches of snow=20
are >50% above 6000-7000ft over the Sangre de Cristos and Raton=20
Mesa, with a large footprint of at least 6 inches over much of=20
eastern CO southward into central NM. Latest WSSI shows these
snowfall values as producing widespread Major to Extreme impacts
across much of central and northern NM as well as south-central
Colorado.


The probability of significant ice across the CONUS is less than=20
10 percent.


Fracasso/Snell




...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!8Hc9ZkQo36coF6I3ZSGf-crdXe3E7XBE-MJnNGCMXtpMz= aIb7FEN_4y6X78ba-EfIn6P0aGYap-cKQKWI0jybsvbLsE$=20

$$

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FOUS11 KWBC 070825
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 AM EST Thu Nov 7 2024

Valid 12Z Thu Nov 07 2024 - 12Z Sun Nov 10 2024


...Central/Southern Rockies and Adjacent High Plains...=20
Days 1-2...

...Two phase winter storm to bring heavy snow and major impacts to
the Southern Rockies...

The forecast begins with an impressive closed and cutoff low
sinking slowly southward across the Four Corners region. This low
is progged to continue to amplify today, reaching as low as -4
sigma with respect to 500mb heights across AZ, and then sinking as
far south as Sonora, Mexico before finally rounding through the=20
base of the longwave trough across the west and beginning to eject=20 northeast. This is a classic position for heavy precipitation into=20
the Southern Rockies as downstream height falls and robust=20
divergence combine with the LFQ of an intensifying subtropical jet=20
streak to produce large scale impressive ascent. This will impinge=20
into a moistening column as well as both the subtropical jet streak
and southerly 850-700mb flow surge moisture into the region=20
leading to PWs that are +1 to +2 sigma today into Friday. This=20
synoptic lift into the moistening column will combine with=20
persistent and intensifying moist isentropic lift, especially near=20
the 300K level where wind speeds will approach 50 kts, pushing=20
mixing ratios above 6g/kg Thursday aftn.=20

The result of this evolution will be an expanding precipitation
shield across NM/CO, with heavy snow the primary precipitation type
except across far eastern NM and far southeast CO. Evaluation of
forecast soundings during this time suggests impressive omega
crossing the DGZ, with a near isothermal layer just beneath it.
WHile the DGZ seems a bit elevated, the strong ascent and aggregate
maintenance supported by the sub-DGZ isothermal layer indicates=20
the likelihood for large dendrites and rapid accumulation.=20
Additionally, cross-sections indicate a threat for CSI on Thursday,
indicating the potential for convective snowfall rates as the=20
isentropic ascent maximizes, and this is reflected by the potential
for 1-2"/hr snowfall rates in the WPC prototype snowband tool.=20
Overall, D1 will feature widespread heavy snow across NM and CO,=20
and WPC probabilities for more than 8 inches are above 80% for the=20
Sangre de Cristos, Raton Mesa, and surrounding foothills/High=20
Plains including the I-25 corridor between Pueblo and Santa Fe.=20
Locally 2-3 feet is possible in the higher terrain leading to=20
substantial travel impacts. Farther north, WPC probabilities are=20
70-90% for 4+ inches across the Palmer Divide.

The second phase of this event will begin on Friday as the core of
the upper low begins to slowly weaken as it pivots, still slowly,
northeast into the High Plains of NM by Friday aftn. This will=20
help spawn surface cyclogenesis across northern Texas, with this=20
low shifting northward within the broad southerly flow into=20
Saturday. As this low slowly strengthens, a secondary surge of=20
moist advection will lift northward, spreading snowfall farther=20
north once again into CO and maybe even southern WY, while a=20
pivoting dry slow shuts off precipitation over NM. The=20
strengthening theta-e advection this time will likely result in an=20 impressive mid-level TROWAL pivoting NW into CO, with accompanying=20
upslope flow driving heavy snow rates into CO through D2. This will
lead to some elevated instability, especially on the periphery of=20
the dry slot, so once again snowfall rates could be intense,=20
especially where the DGZ deepens (SPC SREF probabilities for >50mb
of DGZ depth eclipse 70% in eastern CO), so another day of=20
impactful heavy snow is likely.=20

The setup also seems to support a pivoting band of heavy snow
somewhere across eastern/central CO Friday evening. Exactly where
this sets up is still uncertain, but the synoptic environment
appears to match the conceptual model for a pivoting band, and the
high- res models simulated reflectivity all feature something that
looks like this, but with different placement. Will need to=20
monitor this closely as this band could result in much heavier snow
totals and strong snowfall gradients, but at this time, WPC
probabilities for more than 8 inches are again high (>70%) across
the Raton Mesa and Sangre de Cristos, leading to event-total
snowfall of 3-4 feet in the higher terrain. As snow pivots
farther northward D2, WPC probabilities also indicate a 70% chance
for more than 6 inches of additional snowfall across the Palmer
Divide. ALthough the heaviest snow will likely again be in the
higher terrain, where this band pivots, snow fall rates should
overcome any terrain features leading to local enhancements in
snowfall even in lower elevations.

By late D2 and then into D3, the low occludes to the east, shunting
the best moisture fetch into the Plains and bringing a slow wane to
the heavy snow. There is potential that some dynamic cooling could
still produce pockets of heavy snow, and guidance still features a
lot of longitudinal spread in the axis of heaviest precipitation,=20
but in general snow should come to an end, finally, on D3.=20

The probability of significant ice across the CONUS is less than=20
10 percent.


Weiss



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!5kQTiRTOdSGJ27wieqc9aX0oy-Vcr76ISF5hEzd5Pl5qZ= mhDAs0UcsvFCDaEK3WZ53MMTJDu_6qvC0OKs23PP0sEgFY$=20

$$

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FOUS11 KWBC 072022
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 PM EST Thu Nov 7 2024

Valid 00Z Fri Nov 08 2024 - 00Z Mon Nov 11 2024


...Central/Southern Rockies and Adjacent High Plains...=20
Days 1-2...

...Heavy snow and major impacts continue for the Southern=20
Rockies and Interstate 25 corridor through Friday...

Deep low pressure drifting south over eastern AZ this afternoon
will pivot east over southern NM tonight before swinging northeast
up the eastern NM/TX Panhandle border Friday then shifts up the
central Great Plains Friday night/Saturday. Pacific and Gulf of
Mexico sourced moisture will continue to wrap in ahead of this low
center and a developing surface trough extending from the southern
Plains into central CO. Jet dynamics above frontogenesis along with topographical lift will allow renewed heavy precip over the Sangre
de Cristos and adjacent High Plains by early Friday.=20
This lift is through the DGZ which should allow large dendrites and
rapid accumulation. Indeed the 12Z HREF mean 1-hr snowfall product
features 1-2"/hr snow rates centered on the Raton Mesa 12Z to 22Z=20
Friday. Day 1 PWPF for additional >12" is around 90% along the=20
Raton Mesa to near the western OK Panhandle border. Please see
updated Key Messages linked below.

Snow shifts north over the eastern slopes and High Plains of CO on
Friday, while a pivoting dry slow shuts off precipitation over NM.
The strengthening theta-e advection will result in an impressive=20
mid-level TROWAL pivoting NW into CO, which along with elevated
instability, especially on the periphery of the dry slot, and=20
upslope flow will driving heavy snow rates over central/eastern=20
CO, especially where the DGZ deepens. The setup also supports a=20
pivoting band of heavy snow around and perhaps north of the Palmer
Divide south of Denver Friday evening. Day 1.5 PWPF for >8"
are 50-80% over the Palmer Divide, but caution is advised with the
decrease in values in the valley between the Palmer Divide and
Raton Mesa where banding could easily overcome marginal thermal
conditions and produce heavy snow in areas of central/eastern CO=20
regardless of elevation.=20

As the low occludes and shifts over Neb Friday night/Saturday, the
moisture moisture fetch is shunted east and thermals farther east
do not support snow, so the heavy snow threat generally ends in CO.=20


The probability of significant ice across the CONUS is less than=20
10 percent.


Jackson



...Winter Storm Key Messages are in effect...

Southern Rockies and adjacent High Plains: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!7XDq_NvZyxvJRdYbO0XSOEUih6c7hySGUslchv-cGXbAD= G_ZrGNdmS98kIQX-6hBPZI9vAXf7tXDPjpWb9xPzhhbIzc$=20


$$

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FOUS11 KWBC 080737
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 AM EST Fri Nov 8 2024

Valid 12Z Fri Nov 08 2024 - 12Z Mon Nov 11 2024


...Central/Southern Rockies and Adjacent High Plains...=20
Days 1-2...

...Heavy snow and major impacts continue for the Southern=20
Rockies and Interstate 25 corridor into Saturday morning...

Significant winter storm will persist across eastern CO through
very early Saturday morning before ending. The primary driver of
this impressive system is an anomalous upper low which will be
moving across New Mexico this morning before slowly ejecting to the
northeast and into the Central Plains on Saturday. NAEFS height
anomalies reach as low as -3 sigma, indicating the strength of this
feature, and the resultant downstream height falls and divergence=20
will combine with a modestly coupled jet structure to produce
impressive large-scale ascent across the Southern/Central Rockies
and High Plains.

This evolution today will also drive surface low development over
Texas, and this wave will lift northward into Saturday. This low
will be accompanied by enhanced forcing through persistent moist=20
isentropic lift, especially along the 295K surface where mixing
ratios climb to 4-6 g/kg, lifting into a robust TROWAL pivoting
over the region Friday aftn/eve. Additionally, an axis of elevated
instability is likely beneath the TROWAL and collocated with an=20
axis of deformation, which suggests intense snowfall rates within a
pivoting band of snowfall. Omega maxima into the DGZ, although=20
subjectively elevated, will support heavy snow rates which the WPC=20
snowband tool indicates will reach 1-2"/hr. It is possible that=20
some rates may briefly even reach 2-3"/hr as model cross-sections=20
indicate a narrow corridor of theta-es lapse rates <0C/km
collocated with EPV<0 suggesting the potential for upright=20
convection. Regardless, a highly impactful snow is expected again=20
Friday from far northeast NM into much of eastern CO.=20

The heaviest accumulations are likely in the higher elevations of=20
the Raton Mesa, Palmer Divide, and into the Sangre de=20
Cristos/southern Front Range, but intense ascent should allow for=20
snow significant snow accumulations even into the High Plains, at
least until the more intense easterly/WAA develops warming the
column. There will likely be a thermal gradient near the KS/CO=20
border which will be the demarcation between primarily snow and=20
primarily rain, but even all the way to the KS border periods of=20
snow are possible during heavier snow and dynamic cooling.=20
WPC probabilities for more than 8 inches of additional snow on D1
are above 80% across the Raton Mesa and Palmer Divide, as well as
into the Sangre de Cristos and southern Front Range. Elsewhere,
probabilities for more than 4 inches are high across much of
eastern CO except towards the KS border and northeast corner.

As the low occludes and shifts over Neb Friday night/Saturday, the
moisture moisture fetch is shunted east and thermals farther east
do not support snow, so the heavy snow threat generally ends in CO.
Some lingering snowfall is likely, however, especially in the CO
Rockies, but additional accumulations will likely be minimal.


The probability of significant ice across the CONUS is less than=20
10 percent.


Weiss


...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!_Q6n64WBQaITlZWxa2RZrNsgOL99npr9_iRffZWBapcFO= 6sLc-U0XLeUbnnEVhKsaoM6tV108grSUZKyqoS1a-B4kbg$=20

$$

=3D =3D =3D
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FOUS11 KWBC 082018
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
318 PM EST Fri Nov 8 2024

Valid 00Z Sat Nov 09 2024 - 00Z Tue Nov 12 2024


...Colorado Rockies and Adjacent High Plains...=20
Day 1...

...Heavy snow and major impacts continue for portions of the=20
Colorado Rockies and Interstate 25 corridor into Saturday=20
morning...

Strong low pressure system causing this major winter storm will
track north along the CO/KS border tonight, producing final rounds
of heavy snow east from the Front Range over eastern CO into the
overnight before diminishing into Saturday morning.=20
Downstream height falls and divergence will combine with a=20
modestly coupled jet structure to produce impressive large-scale=20
ascent across the Southern/Central Rockies and High Plains as well=20
as a surface low center tracking north over western KS tonight.=20
This low is accompanied by enhanced forcing through persistent=20
moist isentropic lift which will continue to bring TROWAL through=20
eastern CO into the overnight. An axis of elevated instability=20
beneath the TROWAL, collocated with an axis of deformation, will
continue to allow 1-2"/hr snowfall rates in the heaviest TROWAL
bands. This activity will lift from southeast CO through at least
the Palmer Divide through this evening with bands weakening a bit
as they track north from the Denver metro to southern WY.=20

WPC snow probs are mainly terrain based with 30-70% values for >6"
additional after 00Z across the greater Palmer Divide, the Front
Range, and right around the Raton Mesa. Near 10% probs are along
the Cheyenne Ridge in southeast WY. However, the intensity of the
banding should allow heavy snow to overcome more marginal thermals
and accumulate in the valleys, so areas around CO Springs and at
least portions of the Denver metro should see some additional=20
impactful snow tonight.=20

As the low occludes and shifts over Neb on Saturday, the moisture=20
moisture fetch is shunted east and thermals farther east do not=20
support snow, so the heavy snow threat generally ends with the
Cheyenne Ridge. The Black Hills mainly stay below the snow level,
so impactful snow is not expected there, but some flakes should be
seen on Saturday.


...Northwest...
Day 3...

A potent upper trough ejecting from a deep Gulf of Alaska low=20
approaches the Pacific Northwest coast Sunday night and digs down
to the northern Great Basin on Monday. Snow levels in the moisture
surge ahead of the trough rise to around 6000ft in western WA and
7000ft in OR/northern CA before dropping to 4000-5000ft under the
trough on Monday. Day 3 PWPF for >6" are 30-70% over the length of
the Cascades. More impactful snow is likely Monday night into
Tuesday with at least the higher passes in WA seeing accumulating
snow then.


The probability of significant ice across the CONUS is less than=20
10 percent.


Jackson


...Winter Storm Key Messages are in effect...=20

Southern/Central Rockies and High Plains: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!9_n9QFVgwaIkhJLdPvVZONqU-53YTaKPIqRb9VEkyRZ5C= hnyEwRZMz3sCk5oLm8fYzvDlr_oZxTuwF5K0RccR2_mGlY$=20

$$

=3D =3D =3D
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FOUS11 KWBC 090736
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
236 AM EST Sat Nov 9 2024

Valid 12Z Sat Nov 09 2024 - 12Z Tue Nov 12 2024


...Colorado Rockies...
Day 1...

The historic winter storm that has brought multiple feet of snow to
many parts of the Southern Rockies and central High Plains will
wane quickly Saturday morning as the low becomes vertically stacked
and occludes, shutting off the primary moisture feed. Despite this
occurring, the primary upper low will remain intense, and the
strong deformation axis responsible for the heavy snow banding=20
overnight will pivot across north-central CO into southeast WY=20
before slowly eroding through the day. The thermal profile to the=20
northeast is less conducive for snow, but some dynamic cooling due=20
to strong ascent could still result in some modest snowfall through
early Saturday. However, the greatest potential for any additional
significant accumulations will be across the higher terrain of=20
north-central CO, including the Park Range where WPC probabilities=20
for an additional 4+ inches of snow are 50-70%.


...Northwest...
Days 2-3...

A potent upper trough digging from the Gulf of Alaska will dig
towards Washington State, but likely come ashore over British
Columbia Monday night. This will have a two-pronged effect of
causing broad height falls across the Pacific and Interior
Northwest (height anomalies -1 sigma according to NAEFS) while also
causing some pinched broadly cyclonic flow into the Northwest
CONUS. Together, this will drive a corridor of moderate IVT for
which CW3E probabilities from the ECENS and GEFS suggest will
exceed 500 kg/m/s as an AR surges onshore, accompanied by a surface
cold front advecting eastward. This will result in a stripe of
heavy precipitation ahead of the cold front, with a secondary wave
of precipitation following in its wake embedded within the
confluent mid-level flow.=20

Snow levels ahead of the front will be generally 6000-7000 ft, so
the initial precipitation should fall as primarily rain except in
the highest terrain of the Cascades. However, as snow levels crash
behind the front and precipitation persists, accumulating snow will
occur as low as 3500-4000 ft by the end of the period. This will
result in significant snowfall in the WA/OR Cascades and the
Olympics, with increasing pass-level impacts possible.
Additionally, by Tuesday morning /end of D3/ precipitation falling
as snow is expected to spread as far east as the Northern Rockies
and areas around Yellowstone NP.=20

WPC probabilities for more than 4 inches of snow on D2 are confined
to the far northern WA Cascades and the higher terrain around Mt.
Rainier where they are above 70%. By D3, snowfall expands along the
Cascades and as far south as the Sierra, while additionally
pushing inland to the Northern Rockies. WPC probabilities for more
than 6 inches D3 are above 70% along the Cascades and near Mt.
Shasta, where 1-2 feet of snow is likely in the highest terrain.
Elsewhere, WPC probabilities are as high as 30-50%, highest near
the Salmon River and Sawtooth Ranges of Idaho. With snow levels
falling, impactful pass-level snow is also likely, especially at
some of the higher passes like Washington Pass and Stevens Pass in
the WA Cascades.



The probability of significant ice across the CONUS is less than=20
10 percent.

Weiss



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!8KwIpDU7Aq4kHWDMDoRjDrPoB-8-bPQ57othAXp-L28ct= Ikbt5UkMwqN3a07KCibvl3oE3xVtadhMskPBBbFG9f5adc$=20

$$

=3D =3D =3D
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FOUS11 KWBC 091909
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
209 PM EST Sat Nov 9 2024

Valid 00Z Sun Nov 10 2024 - 00Z Wed Nov 13 2024

...Western U.S....
Days 2-3...

A potent upper trough currently rounding a deep low over the Gulf
of Alaska will further amplify as it digs south toward the PacNW
coast Sunday night with the base of the trough crossing northern CA
on Monday. This sharp trough then shifts east over the
northern/central Rockies through Tuesday.

Elevated moisture with a plume of 1-1.25" PW streams into the
Cascades Sunday night ahead of the axis with snow levels generally
6000-7000 ft, before dropping to 4000-5000ft under the trough on
Monday. Moderate precip rates are expected behind the cold front
Monday into Tuesday as moist onshore flow persists, leading to
prolonged snow at and above the higher pass levels in WA/OR
Cascades. Day 2 PWPF for >6" is generally limited to the highest
Cascades, but Day 3 is above 50% at pass level with a couple feet
likely for the highest peaks.

The cold front progresses inland to the northern Rockies Monday
afternoon with a couple rounds of mountain snow (snow levels drop
from around 8000ft to 5000ft with the frontal passage) expected
through Tuesday. Day 3 PWPF for >6" are 40-80% for the Salmon
River, western Sawtooths, Bitterroots, Tetons, and ranges around
Glacier NP.

A band of frontal precip/mountain snow can be expected over
northern CA late Monday with Day 2.5 PWPF 20-60% for >6" for the
Trinity Alps, CA Cascades, and northern Sierra Nevada with snow
levels dropping from 6000ft to 5000ft with the frontal passage.


The probability of significant ice across the CONUS is less than
10 percent.

Jackson


$$

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FOUS11 KWBC 100738
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
238 AM EST Sun Nov 10 2024

Valid 12Z Sun Nov 10 2024 - 12Z Wed Nov 13 2024

...Northwest...
Days 2-3...

Dual amplified shortwaves will bring an active period of weather to
the Pacific and Interior Northwest through mid-week. The first of
these will dig out of the Gulf of Alaska and approach the British
Columbia coast Sunday night, before sharpening into a short-
wavelength but amplified open wave advecting onshore WA/OR Monday
evening. This feature will then move quickly eastward into the
Northern Rockies while weakening through Tuesday. Although this
trough will be of modest amplitude and progressive, height falls,
divergence and dual jet streaks (one poleward arcing downstream of
the trough axis, another more zonally oriented upstream) will lead
to widespread deep layer ascent from the Pacific Northwest eastward
through the Northern Rockies. At the same time, this trough will
drive a cold front eastward, surging onshore the WA/OR coasts
Monday morning before racing into the Northern High Plains by the
end of the forecast period.

This synoptic lift will act upon an increasingly moist column noted
by rapidly rising 1000-500mb RH as warm/moist advection occurs
within the aforementioned jet streaks, driving IVT to 250-500
kg/m/s. This will support widespread moderate to heavy
precipitation, and as snow levels collapse from around 6000 ft
early to as low as 3500 ft D2, snow will become widespread in the
terrain of the Olympics, Cascades, Shasta/Trinity region of CA, the
Sierra, and then into the Northern Rockies. WPC probabilities for
at least 6 inches of snow are 70+% along the spine of the Cascades
of WA and OR, with amounts reaching multiple feet likely in the
highest volcanoes. There is also a 30-50% chance of more than 6
inches in the Shasta/Trinity region, the northern Sierra, the Blue
Mountains of OR, and the far Northern Rockies.

After a brief period of shortwave ridging across the Pacific
Northwest, a more pronounced impulse digs into the region during D3,
nearly reaching the coast by the end of the period. This will be
accompanied by more impressive moist advection with ECENS IVT
probabilities reaching above 90% for 500 kg/m/s, with several
members exceeding 750 kg/m/s. The accompanying WAA will surge snow
levels back to around 5000-6000 ft along the immediate coast, but
rise only to around 4000 ft across the Cascades. This will support
more widespread heavy snow, with generally below climo SLRs leading
to high snow load and greater impacts, especially across the WA
Cascades. WPC probabilities are above 9)% in the WA Cascades,
primarily above 5000 ft, with 1-2 feet likely in the higher
elevations. Some pass level impacts are also expected, especially
at Washington Pass, with lesser but still potential impacts at
Stevens Pass.


The probability of significant ice across the CONUS is less than
10 percent.

Weiss


$$

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FOUS11 KWBC 102034
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
334 PM EST Sun Nov 10 2024

Valid 00Z Mon Nov 11 2024 - 00Z Thu Nov 14 2024

...Western U.S...
Days 1-3...

Back-to-back sharp troughs bring higher elevation snow to the NW
quadrant of the CONUS through midweek. The first trough will
continue to dig south from a parent low over the Gulf of Alaska
before crossing the PacNW coast (with the base of the trough over
northern CA) early Monday before ejecting east over the northern
Rockies through Tuesday. The second trough reaches the PacNW coast
later Wednesday.


Although the first trough will be of modest amplitude and
progressive, height falls, divergence and dual jet streaks (one
poleward arcing downstream of the trough axis, another more zonally
oriented upstream) will lead to widespread deep layer ascent from
the Pacific Northwest eastward through the Northern Rockies. At the
same time, this trough will drive a cold front eastward, surging
onshore the WA/OR/northern CA coasts Monday morning before racing
into the Northern High Plains on Tuesday. This synoptic lift will
act upon an increasingly moist column noted by rapidly rising
1000-500mb RH as warm/moist advection occurs within the
aforementioned jet streaks, driving IVT to 250-500 kg/m/s. This
will support widespread moderate to heavy precipitation, and as
snow levels drop from 6000 to 7000 ft tonight to around 4000 ft
Monday night, snow will become widespread in the terrain of the
Cascades down to the central Sierra Nevada and then the Northern
Rockies by Monday evening. Day 1 WPC snow probabilities for >6" are
50-80% for the higher Cascades and 30-60% for the northern Sierra
Nevada.

Low pressure lingering near Haida Gwaii (ahead of the parent low
exiting the Gulf of Alaska) maintains onshore flow over the PacNW
coast through Tuesday when the moisture surge ahead of the next
wave arrives at the coast. So light to moderate precip continues
over the Cascades to the Northern Rockies, prolonging and
connecting the main accumulating snow events. Day 2 WPC snow
probabilities for >6" are categorical above 4500ft in the Cascades
and 40-70% in the Blue Mtns, Bitterroots, Clearwater, Tetons, and
ranges just NW of Yellowstone (such as the Gallatin and Ranges).

The next wave will be accompanied by more impressive moist
advection with ECENS IVT probabilities reaching above 90% for 500
kg/m/s, with several members exceeding 750 kg/m/s. The accompanying
WAA will surge snow levels back to 5000-6000 ft over the Cascades
Tuesday night. This will support more widespread heavy/wet snow,
with snow load impact concerns. Day 3 WPC probabilities for >6" are
categorical for the WA Cascades and 40-70% for the highest OR/CA
Cascades higher ranges of northeast WA/far northern ID such as the
Selkirk Mtns.

Over the three day span, a few feet are likely above about 6000 ft
in the Cascades and several feet for the highest volcanos.


The probability of significant ice across the CONUS is less than
10 percent.


Jackson

$$

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FOUS11 KWBC 110728
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
228 AM EST Mon Nov 11 2024

Valid 12Z Mon Nov 11 2024 - 12Z Thu Nov 14 2024

...Western U.S...
Days 1-3...

Two distinct and anomalous shortwaves will amplify and move
progressive across the Western CONUS through Thursday, bringing an
extended period of unsettled weather to the region, with several
days of moderate to heavy snow expected in the higher terrain.

The first of these will approach the Pacific Coast Monday morning,
with downstream SW flow driving robust IVT approaching 500 kg/m/s
onshore into CA/OR/WA. This IVT will help surge PWs to above climo
directly ahead of a surface cold front which will be positioned
beneath the trough axis. As this trough/front advect onshore Monday
aftn, the overlapping ascent into the moistening column will result
in heavy precipitation spreading from central CA through the WA
Cascades, and then pushing east, while weakening due to lessening
IVT, reaching the Northern Rockies and eastern Great Basin by
Tuesday morning. Snow levels ahead of this front will be generally
around 6000 ft during the period of heaviest precipitation, but
will fall quickly to as low as 3500 ft behind the front. At the
same time, while the primary forcing mechanisms will shift east,
continued onshore moist advection in pinched mid-level flow will
drive persistent moisture onshore, resulting in snow accumulations
lowering across the Cascades/Olympics. WPC probabilities D1 for
more than 6 inches of snow are above 70% along the spine of the
Cascades in OR and WA, with up to 2 feet possible in the highest
terrain. Lower probabilities exist into the Shasta/Trinity region,
the northern Sierra, and as far east as the Blue Mountains of OR.

During D2 /Tuesday and Tuesday night/ the lead trough and
accompanying cold front continue to push east as far as the Central
Plains while weakening, while shortwave ridging briefly builds in
across the Northwest. This suggests that much of D2 will feature
waning snowfall with lesser accumulations. However, before D2 ends,
the subsequent trough will approach the Pacific Northwest once
again. This trough is progged to be deeper, noted by 700-500mb
height anomalies reaching -2 sigma according to NAEFS, with the
subsequent downstream warm/moist advection in the mid and upper
levels driving IVT to as high as 750 kg/m/s, and ECENS
probabilities suggest even a 30% chance of IVT exceeding that
value. This WAA will occur within a narrow channel ahead of yet
another cold front, driving a corridor of heavy precipitation in
conjunction with snow levels rising once again to 6000-7000 ft.
This will limit snowfall before D2 ends except in the highest
terrain of the Olympics and Cascades.

The trough axis responsible for pushing the front eastward and
driving the pronounced deep layer ascent across the Northwest will
struggle to move east as repeated lobes of vorticity rotate down
from the Gulf of Alaska, resulting in little movement of the trough
axis and persistent, although weaker, moist advection onshore. This
suggests that snow levels will only slowly fall, progged to drop
to 4500-5000 ft by the end of the forecast period, with primarily
light to moderate precipitation rates from northern CA through the
Northern Rockies. This will still result in heavy snow
accumulations at the highest elevations, reflected by WPC
probabilities for more than 6 inches exceeding 90% in the northern
WA Cascades, and then expanding east into Selkirk Range, the Blue
Mountains, and the Sawtooth/Salmon River area. The higher passes
above 5000 ft will also likely experience significant impacts from
snow accumulations, but in general most of the passes should be
below the heavier snow accumulation levels.


The probability of significant ice across the CONUS is less than
10 percent.


Weiss


$$

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FOUS11 KWBC 111906
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
206 PM EST Mon Nov 11 2024

Valid 00Z Tue Nov 12 2024 - 00Z Fri Nov 15 2024

...Western U.S...
Days 1-3...

Two distinct and anomalous shortwaves will amplify and impact the
Western CONUS through Thursday, bringing an extended period of
unsettled weather to the region, with several days of moderate to
heavy snow expected in the higher terrain (mainly above 5000 ft).

The first of these is currently progressing inland across the
Pacific Northwest today, with downstream SW flow driving robust IVT
approaching 500 kg/m/s onshore into central CA. This IVT will help
surge PWs to above climo directly ahead of a surface cold front
which will be positioned beneath the trough axis. As this
trough/front further slide eastward this evening, heavy
precipitation remains possible in the central/southern Sierra. This
moisture axis is forecast to weaken due to lessening IVT as it
slides east, reaching the Northern Rockies and eastern Great Basin
by Tuesday morning. Snow levels ahead of this front will be
generally around 6000 ft during the period of heaviest
precipitation, but will fall quickly to as low as 3500 ft behind
the front. At the same time, while the primary forcing mechanisms
will shift east, continued onshore moist advection in pinched mid-
level flow will drive persistent moisture onshore, resulting in
snow accumulations lowering across the Cascades/Olympics. WPC
probabilities D1 (through Tuesday evening) for more than 6 inches
of snow are above 70% along the spine of the Cascades in OR and WA,
with up to 2 feet possible in the highest terrain.

During D2 (Tuesday night and Wednesday) the lead trough and
accompanying cold front continue to push east as far as the Central
Plains while weakening, as shortwave ridging briefly builds in
across the Northwest. This suggests that the first half of D2 will
feature waning snowfall with lesser accumulations. However, by
Wednesday, the subsequent trough will approach the Pacific
Northwest once again. This trough is progged to be deeper, noted by
700-500mb height anomalies reaching -2 sigma according to NAEFS,
with the subsequent downstream warm/moist advection in the mid and
upper levels driving IVT to as high as 750 kg/m/s, and ECENS
probabilities suggest even a 30% chance of IVT exceeding that
value. This WAA will occur within a narrow channel ahead of yet
another cold front, driving a corridor of heavy precipitation in
conjunction with snow levels rising once again to 6000-7000 ft.
This will limit the heaviest snowfall on D2 to the highest terrain
of the Olympics and Cascades.

The trough axis responsible for pushing the front eastward and
driving the pronounced deep layer ascent across the Northwest will
struggle to move substantially east as repeated lobes of vorticity
rotate down from the Gulf of Alaska, resulting in little movement
of the trough axis and persistent, although weaker, moist advection
onshore. This suggests that snow levels will only slowly fall,
progged to drop to 4500-5000 ft by the end of the forecast period,
with primarily light to moderate precipitation rates from northern
CA through the Northern Rockies. This will still result in heavy
snow accumulations at the highest elevations, reflected by WPC
probabilities for more than 6 inches exceeding 90% in the northern
WA Cascades, and then expanding east into Selkirk Range, the Blue
Mountains, and the Sawtooth/Salmon River area. The higher passes
above 5000 ft will also likely experience significant impacts from
snow accumulations, but in general most of the passes should be
below the heavier snow accumulation levels.


The probability of significant ice across the CONUS is less than
10 percent.


Weiss/Snell





$$

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FOUS11 KWBC 120820
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Tue Nov 12 2024

Valid 12Z Tue Nov 12 2024 - 12Z Fri Nov 15 2024

...Western U.S...
Days 1-3...

An active period of winter weather continues across the Western
CONUS this period as dual shortwave trough cross the region.,

The first of these will be moving from the eastern Great Basin this
morning into the Central Plains by Wednesday evening, with minimal
amplitude gain. The combination of the progressive nature of this
feature with modest PW surge (at least until the Plains when
thermals support only rain) will somewhat limit snowfall potential
D1. However, sharp height falls downstream of the short-wavelength
trough combined with low-level convergence along the accompanying
cold front and post-frontal upslope flow into terrain features will
still support rounds of moderate to heavy snow. The greatest
potential for significant accumulations will be across the Wasatch
and Colorado Rockies, including the Park Range, where WPC
probabilities for more than 6 inches are above 70%, and locally up
to 10 inches is possible, primarily above 5500 ft.

Brief shortwave ridging blossoms across the Northwest behind this
first trough, but is quickly replaced by a more impressive impulse
digging along the Pacific Coast as it drops from the Gulf of
Alaska. Downstream divergence and accompanying WAA/moist advection
will begin to spread precipitation back into the Pacific Northwest
late tonight. Unlike its predecessor, this secondary trough will be
very slow to move east as it gets repeatedly reinforced just off
the coast by renewed lobes of vorticity swinging cyclonically
around it. This will have the two-pronged effect of driving waves
of ascent and persistent moisture onshore, while also keeping the
primary trough axis positioned just west of the region until Friday
when a more pronounced vort max swings through the base and pushes
the trough onshore CA late in the forecast period.

Within the most impressive WAA, a narrow channel of IVT exceeding
750 kg/m/s is likely, focused into the WA/OR and northern CA coast
late D2 into D2 before weakening with inland extent. This channel
/AR/ will also provide the highest increase in snow levels,
reaching as high as 7000 ft. However, dual cold front progged to
push onshore, one Wednesday morning and another Thursday morning,
driven east by periodic shortwaves, will cause gradual lowering of
snow levels to as low as 3000 ft by Friday morning across the
Cascades, and 4500 ft into the Interior Northwest and northern CA.
The lower snow levels will be accompanied by weaker forcing and
more transient moisture however, so heavy pass-level snow is still
not anticipated at this time.

Despite that, heavy snow accumulations are likely, especially on D2
when WPC probabilities for more than 6 inches spike above 90% in
the Cascades, Shasta/Trinity region, the Okanogan Highlands,
Selkirk Mountains, and Blue Mountains. Some of these areas could
receive as much as 2 feet of snow D2, and 6-12 inches is possible
at some of the elevated passes including Washington Pass. By D3,
the snow intensity wanes across most of the NW, but an increase in
WPC probabilities for 6+ inches reaching 30-50% in the Sierra as
the trough finally pushes onshore into CA.

The probability of significant ice across the CONUS is less than
10 percent.


Weiss



$$

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FOUS11 KWBC 121940
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
240 PM EST Tue Nov 12 2024

Valid 00Z Wed Nov 13 2024 - 00Z Sat Nov 16 2024

...Western U.S...
Days 1-3...

An amplified trough crossing the Rockies will continue to produce
high elevation snow showers across the northern and central Rockies
this evening. The greatest potential for significant additional
accumulations centers over the north-central Colorado ranges,
including southern portions of the Park Range, where WPC PWPF
indicates that amounts of 6 inches or more are likely during the
evening and overnight. Snow should diminish as the upper trough
shifts east on Wednesday.

Meanwhile, shortwave ridging over the Northwest will be replaced by
a broad upper trough associated with a low developing northwest of
Vancouver Island and energy digging to its south. Apart from the
northern Cascades, snow levels are expected to remain above 5000 ft
along much the Northwest ranges, limiting the potential for
widespread impacts through Wednesday.

As the leading energy moves east through the northern Rockies
producing some isolated heavy amounts over the high terrain
Wednesday night into Thursday, upstream energy will continue to
amplify the flow along the West Coast, with a defined southern
stream low/trough developing and moving onshore Thursday night into
Friday. Lacking a long fetch of deep onshore flow, moisture with
this system will be limited. Therefore, while at least a few inches
of snow appears probable for areas in the Sierra above 5000 ft,
widespread heavy accumulations are unlikely.

The probability of significant ice across the CONUS is less than
10 percent.

Pereira


$$

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FOUS11 KWBC 130751
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Wed Nov 13 2024

Valid 12Z Wed Nov 13 2024 - 12Z Sat Nov 16 2024

...Western U.S...
Days 1-3...

The weather pattern over the next few days will be driven largely
by a longwave upper level trough with a pair of upper level
disturbances tracking into the West Coast. The first storm system
is already impacting the Pacific Northwest with an IVT up to 750
kg/m/s (topping the 97.5 climatological percentile) escorting
anomalous moisture as far south as northern California and as far
east as the Lewis Range in Montana. Elevations above 5,000ft,
whether it be in the Cascades, Olympics, northern Sierra Nevada, or
inland through the Blue, Sawtooth, Bitterroots, and Lewis Range,
will be favored for witnessing accumulating snowfall that could
result in treacherous travel conditions. The WSSI does depict some
localized areas of Minor to Moderate Impacts in the >5,000ft
elevations of the Olympics, Cascades, Blue, and Shasta Mountains
today.

Onshore flow into the Pacific Northwest will keep high elevation
snow in the forecast through Thursday in the Northwest. By
Thursday night, the next Pacific storm system will track farther
south into California with 500mb heights that below the 2.5
climatological percentile on NAEFS. Heights will be low enough
Thursday night and into Friday morning that snow levels in the
Sierra Nevada will be as low as 4,000ft (5,000ft in the San Gabriel
and San Bernadino of southern California). The heaviest snowfall
will be observed above 6,000ft in the central Sierra Nevada. WPC
PWPF shows moderate-to-high chances (50-70%) for snowfall totals

4" in the central Sierra Nevada between Thursday evening and

Friday morning. By Friday afternoon, the upper trough will track
into the heart of the Intermountain West with strong upper level
ascent over the Northern Rockies. WPC PWPF shows moderate chances
(40-60%) for snowfall totals >4" in the >7,000ft ranges of the
Absaroka.

...Northern New England...
Day 3...

An inverted trough revolving around the northern flank of a
retrograding storm system in the northwest Atlantic will direct a
plume of low level moisture back towards northern New England on
Friday. The air-mass over northern New England (particularly
Maine) will be sufficiently cold and dry enough for wet-bulb
temperatures to remain below freezing within the boundary layer.
Strong easterly mid-level flow with air-mass origins over the
Atlantic will lead to warm air-advection that causes a >0C nose
between 850-750mb Friday morning. This is a setup that will likely
result in periods of sleet/freezing rain over northern Maine and
possibly as far west as northern New Hampshire. WPC PWPF does show
low chances (10-30%) for ice accumulations >0.1" in parts of the
North Woods and Central Highlands of northern Maine. One notable
location with those potential ice accumulations includes Baxter
State Park. These icy conditions could result in Minor Impacts
according to the Probabilistic WSSI with low-moderate chances
(30-50%) through Friday afternoon. The primary impacts would be icy
roads in some areas that could lead to slick travel conditions.

Mullinax


$$

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FOUS11 KWBC 132008
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
308 PM EST Wed Nov 13 2024

Valid 00Z Thu Nov 14 2024 - 00Z Sun Nov 17 2024

...Western U.S....
Days 1-3...

A positively tilted longwave trough will move into the West Coast
Thursday. The trough will become increasingly elongated as the
upper level flow along the Canadian border attempts (and well
eventually succeed) at returning to zonal flow. Meanwhile the
southwestern side of the trough will slow down to a crawl over
southern California by Saturday. As the trough elongates, a strong
jet streak on the west side of the trough will gradually weaken as
it orients in an unusual northeast to southwesterly flow. Meanwhile
the jet streak ahead of the trough will gradually strengthen,
eventually speeding up to peak over 150 kts from the Four Corners
to the Upper Midwest.

The cold air associated with the trough will support low snow
levels over much of the Pacific Northwest, and extending into
central California by Friday. Meanwhile all of the shortwave energy
supporting the longwave trough will locally enhance lift with the
various lows that move into the coast or develop over the Rockies.

The strongest of these lows won't come anywhere near the U.S., but
rather will stay over the coast of British Columbia. The associated
cold frontal precipitation will have a long fetch of Pacific
moisture with it, which will advect northeastward into the Pacific
Northwest. Snow has been ongoing in the Washington and Oregon
Cascades, and will move inland with the longwave trough to put a
fresh blanket of snow over the mountains of Idaho and Montana,
primarily in the Day 1/Tonight-Thursday time frame. The heaviest
snow totals will likely be in the Washington and Oregon Cascades
going forward, but amounts over a foot are probable for far
northern Idaho and into the Bitterroots of Montana, as well as the
Sawtooth of central Idaho. As the parent low north of Vancouver
Island weakens, so too will the attendant front, which will allow
for snow rates to diminish by Thursday night.

Further south, a new low will form over Nevada Thursday night as a
kink in the jet stream develops as a highly energetic shortwave
rounds the base of the longwave trough. This will shift the axis of
heaviest precipitation south into California, Nevada, and
southeastern Idaho. By this point the heaviest snow totals will be
into the Sierras of California as storm total snow into Friday
night will approach a foot for the highest elevations. While truly
heavy snow will likely only be relegated to the favored
upslope/west facing slopes of the Cascades on Thursday and the
Sierras on Friday, snow spreading well inland may still have
localized impacts as cooler air into the Pacific Northwest allows
snow levels to drop enough to potentially cause a few inches to
accumulate in some of the more populated valleys of Nevada and into
Idaho and over to Yellowstone N.P. by Friday night and lingering
into Saturday.


...Northern Maine...
Day 2...

A retrograding upper level and surface low will move westward out
of the Canadian Maritimes towards Maine Thursday night. The low's
occluded front will spread precipitation into Maine starting at the
eastern tip of the state late Thursday night, then overspreading
much of northern Maine through Friday morning. Since this
precipitation will move in during the coldest time of day, ground
temperatures will support a period of frozen precipitation as the
front moves westward. There is good agreement in the guidance that
there will be abundant warm air aloft, which will support mostly
freezing rain with the initial (and likely heaviest) round of
precipitation. Thus, ice accumulations were nudged upward just a
bit, though most areas of northern Maine will see less than a tenth
of an inch of ice accumulation. The freezing rain will change over
to plain rain with solar heating. The low will stop retrograding
during the day Friday and gradually pull away, resulting in rain
shower activity Friday afternoon.

Wegman



$$

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FOUS11 KWBC 140802
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Thu Nov 14 2024

Valid 12Z Thu Nov 14 2024 - 12Z Sun Nov 17 2024

...Western U.S....
Days 1-3...

A pair of Pacific storm systems will produce heavy mountain snow
from the West Coast to the Northern Rockies over the next few days.
Today, broad scale upper level ascent out ahead of a longwave
trough off the West Coast will also direct Pacific moisture at the
Northwest and northern California. Periods of snow will occur for
many of the Northwest U.S. mountain ranges which include (but are
not limited to) the Cascades, Siskiyou, northern Sierra Nevada,
Blue, Sawtooth, Bitterroots, Idaho Panhandle, and northern Montana
Mountains). As a potent upper level trough approaches California
Thursday night (500mb heights below the 2.5 climatological
percentile via NAEFS), snow levels will fall to as low as 4,000ft
in the Sierra Nevada and as low as 6,000ft in the San Gabriel and
San Bernadino of southern California. The heaviest snowfall will
be mainly confined to elevations at/above 6,000ft in the Sierra
Nevada. By Friday morning, the upper level trough will advance
through the Great Basin and into the Rockies by Friday afternoon.
Periods of snow will move through northern Nevada and into the
northern Rockies with the Tetons, Wind River, and Absaroka ranges
seeing the heaviest snowfall Friday evening. Snow will also pick up
in intensity along the Wasatch, Uinta, and Big Horn ranges Friday
night and into Saturday morning. WPC PWPF shows moderate-high
chances (50-70%) for >4" of snowfall along the Sierra Nevada above
6,000ft, the northern Great Basin above 7,000ft, and the Absaroka,
Tetons, and Wind River ranges above 8,000ft.

Meanwhile, the Pacific Northwest will see periods of snow gradually
diminish throughout the day today with a fairly tranquil day
weather-wise on tap for Friday. By Saturday, the next Pacific
trough approaches with a stronger IVT (>500 kg/m/s, or above the
90th climatological percentile via NAEFS) will direct another plume
of Pacific moisture at the region. Strong synoptic-scale support
will also be present as 200-500mb winds over western Washington top
the 90th climatological percentile. As the warm front arrives
Saturday night, snow levels will rise to as high as 5,500ft in some
cases, but once the cold front pushes through on Sunday, snow
levels will crash and result in precipitation changing over to
heavy snow Sunday afternoon in the Olympics and Cascades. Through
12Z Sunday, the northern Cascades and the Idaho Panhandle above
5,000ft sport high chances (>70%) for snowfall >6" with a low
chance (10-30%) for >12" in the tallest peaks of the northern
Washington Cascades.

...Northern Maine...
Days 1-2...

An inverted trough revolving around the northern flank of a
retrograding storm system in the northwest Atlantic will direct a
stream of low-level moisture back west towards northern New England
late Thursday night into Friday. The air-mass over northern New
England (particularly Maine) will be sufficiently cold and dry
enough for wet-bulb temperatures within the boundary layer to
remain below freezing for at least a few hours. Strong easterly
mid-level flow with air-mass origins over the Atlantic will lead to
warm-air advection (WAA) that causes a >0C nose between 850-750mb
Friday morning. This setup will result in periods of sleet/freezing
rain over northern Maine and possibly into far northern New
Hampshire. WPC PWPF does depict some spotty areas with low chances
(10-30%) for ice accumulations >0.1" in portions of the North Woods
and Central Highlands of northern Maine. One notable location with
those potential ice accumulations includes Baxter State Park. These
icy conditions could result in Minor Impacts according to the
Probabilistic WSSI with low chances (10-30%) through Friday
afternoon. The primary impacts would be icy roads in some areas
that could lead to slick travel conditions.

Mullinax



$$

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FOUS11 KWBC 142025
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Thu Nov 14 2024

Valid 00Z Fri Nov 15 2024 - 00Z Mon Nov 18 2024

...Western U.S....
Days 1-3...

A pair of Pacific storm systems will produce heavy mountain snow
from the West Coast to the Northern Rockies over the next few days.
Broad scale upper level ascent out ahead of a longwave trough off
the West Coast will also direct Pacific moisture at the Northwest
and northern California. Periods of snow will occur for many of the
Northwest U.S. mountain ranges which include (but are not limited
to) the Cascades, Siskiyou, northern Sierra Nevada, Blue, Sawtooth, Bitterroots, Idaho Panhandle, and northern Montana Mountains). As
a potent upper level trough approaches California tonight (500mb
heights below the 2.5 climatological percentile via NAEFS), snow
levels will fall to as low as 4,000ft in the Sierra Nevada and as
low as 6,000ft in the San Gabriel and San Bernadino of southern
California. The heaviest snowfall will be mainly confined to
elevations at/above 6,000ft in the Sierra Nevada. By Friday
morning, the upper level trough will advance through the Great
Basin and into the Rockies by Friday night. Periods of snow will
move through northern Nevada and into the northern Rockies with the
Tetons, Wind River, and Absaroka ranges seeing the heaviest
snowfall Friday evening. Snow will also pick up in intensity along
the Wasatch, Uinta, and Big Horn ranges Friday night and into
Saturday morning. WPC PWPF shows moderate-high chances (50-70%) for

4" of snowfall along the Sierra Nevada above 6,000ft, the

northern Great Basin above 6,000ft, and the Absaroka, Tetons, and
Wind River ranges above 8,000ft.

Meanwhile, the Pacific Northwest will see periods of light snow
this evening, with a fairly tranquil day weather-wise on tap for
Friday. By Saturday, the next Pacific trough approaches with a
stronger IVT (>500 kg/m/s, or above the 90th climatological
percentile via NAEFS) will direct another plume of Pacific moisture
at the region. Strong synoptic-scale support will also be present
as 200-500mb winds over western Washington top the 90th
climatological percentile. As the warm front arrives Saturday
night, snow levels will rise to as high as 5,500ft, but once the
cold front pushes through on Sunday, snow levels will crash and
result in precipitation changing over to heavy snow Sunday
afternoon in the Olympics and Cascades. Through 12Z Sunday, the
northern Cascades and the Idaho Panhandle above 5,000ft sport high
chances (>70%) for snowfall >6" with a low chance (10-30%) for >12"
in the tallest peaks of the northern Washington Cascades.

...Northern Maine...
Days 1-2...

An inverted trough revolving around the northern flank of a
retrograding storm system in the northwest Atlantic will direct a
stream of low-level moisture back west towards northern New England
late tonight into Friday. The air-mass over northern New England
(particularly Maine) will be sufficiently cold and dry enough for
wet-bulb temperatures within the boundary layer to remain below
freezing for at least a few hours. Strong easterly mid-level flow
with air-mass origins over the Atlantic will lead to warm-air
advection (WAA) that causes a >0C nose between 850-750mb Friday
morning. This setup will result in periods of sleet/freezing rain
over northern Maine and possibly into far northern New Hampshire.
WPC PWPF does depict some spotty areas with low chances (10-30%)
for ice accumulations >0.1" in portions of the North Woods and
Central Highlands of northern Maine. One notable location with
those potential ice accumulations includes Baxter State Park. These
icy conditions could result in Minor Impacts according to the
Probabilistic WSSI with low chances (10-30%) through Friday
afternoon. The primary impacts would be icy roads in some areas
that could lead to slick travel conditions.

Kebede/Mullinax


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FOUS11 KWBC 150811
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
311 AM EST Fri Nov 15 2024

Valid 12Z Fri Nov 15 2024 - 12Z Mon Nov 18 2024

...Western U.S....
Days 1-3...

This morning, an upper level trough will advance through the Great
Basin and into the Rockies by this evening. Periods of snow
along the Sierra Nevada and northern Great Basin this morning will
move into the northern Rockies tonight with the Tetons, Wind
River, and Absaroka ranges seeing the heaviest snowfall. Snow will
also pick up in intensity along the Wasatch, Uinta, Big Horn,
Little Belt, and Big Snowy ranges tonight and into Saturday
morning. WPC PWPF shows moderate chances (40-60%) for >6" of
snowfall along the Sierra Nevada above 9,000ft. High probabilities

70%) for snowfall >8" are forecast along the ridgelines of the

northern Great Basin above 6,000ft and the Absaroka, Tetons, and
Little Belt, and Big Snowy ranges above 7,000ft.

Elsewhere, the Pacific Northwest will witness a fairly tranquil
day weather-wise today. This changes by Saturday as the next
Pacific trough approaches with a stronger IVT (>500 kg/m/s, or
above the 90th climatological percentile via NAEFS) will direct
another plume of Pacific moisture at the region. Strong synoptic-
scale support will also be present as 200-500mb winds over western
Washington top the 90th climatological percentile. As the warm
front arrives Saturday night, snow levels will rise to as high as
5,500ft, but once the cold front pushes through on Sunday, snow
levels will crash and result in precipitation changing over to
heavy snow Sunday afternoon in the Olympics and Cascades. Heavy
snow will continue to push inland through the interior Northwest
Sunday night while persistent upslope flow leads to a prolonged
stretch of accumulating snowfall above 3,000ft in the Cascades.

Through this weekend, the northern Cascades and the Idaho
Panhandle above 5,000ft sport high chances (>70%) for snowfall >12"
with low-moderate chances (30-50%) for >18" in the tallest peaks.
In the Oregon Cascades and Blue Mountains, WPC PWPF shows
moderate-high chances for >12" of snowfall. WSSI-P depicts
moderate-high chances (50-70%) for Major Impacts in the WA/OR
Cascades above 4,000ft, which includes some notable passes such as
Snoqualmie Pass.

...Northern Maine...
Day 1...

Some light ice accumulations are expected across portions of
northern Maine this morning. Warm air at mid levels overrunning
sub-freezing low level temperatures will support periods of
freezing rain/drizzle, especially from the St. John Valley on south
through the Southern Aroostook. The primary impacts would be icy
roads in some areas that could lead to slick travel conditions. WPC
PWPF shows low-moderate chances for ice accumulations >0.01"
through this afternoon.


Mullinax





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FOUS11 KWBC 152016
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
316 PM EST Fri Nov 15 2024

Valid 00Z Sat Nov 16 2024 - 00Z Tue Nov 19 2024

...Western U.S....
Days 1-3...

A large longwave trough over the Western U.S. with very energetic
jet streaks on both sides of it remains the primary driver of the
active winter weather across the Pacific Northwest this afternoon.

A weak low that will be primarily supported by the left exit region
of the impressive southwesterly jet will cause areas of snow to
form from the Sierra Nevada and northern Great Basin northeast
through Yellowstone and into northeastern Montana and northwestern
North Dakota through Saturday afternoon. The heaviest totals from
this low will generally be across southwestern Montana where the
low tracking across eastern Wyoming will be slower moving and
strengthening. Thus, southwestern Montana will be in the favored
comma-head region of the low for the longest period of time through
tonight. Lesser amounts of snow are expected northeast of there
into northeast Montana and northwestern North Dakota due to the low
moving faster and having fully matured, however localized banding
could form within the broader precipitation shield, which could
result in heavier amounts in the valleys where any bands are the
most persistent. Meanwhile heavier snow further south and west
tonight from the Sierras through southeast Idaho will not be quite
as favored for as long, so most snow totals above 6 inches will be
relegated to the highest peaks in those regions. Once again,
localized banding here as well could cause higher snow amounts in
the valleys.

The arrival of a second much stronger longwave trough into the
Pacific Northwest late Saturday night through Monday will be
characterized by the arrival of much colder air and a full mid-
latitude Pacific fetch of moisture. As usual with this prolonged
period of maritime polar air, the Washington Cascades will receive
by far the most snowfall through Monday, with multi-day totals in
the northern Washington Cascades potentially reaching into the 4 to
6 feet range. As multiple energetic shortwaves also move ashore,
heavy snow will spread south down the Oregon Cascades as well as
well inland across much of the mountains of Idaho, western Montana,
and Wyoming. Expect 2 to 3 feet of accumulation into the Oregon
Cascades and 1 to 2 feet for the Idaho, Montana, and Wyoming
mountains. The arrival of the renewed round of cold air will also
send snow levels crashing right to the valley floors inland, while
west of the Cascades they'll get to 1,000-2,000 ft by Monday.

Expect Major impacts in the WSSI fields for the Washington Cascades
and far northern Idaho primarily due to snow amount. Minor impacts
are expected through Monday for most of Idaho, Montana, and
Wyoming. Up to a 50% chance of extreme impacts are expected for
the Washington Cascades from Snoqualmie Pass north starting Sunday
night. Meanwhile chances for Major impacts have increased above
90% for that same time period. Over 80% probabilities of Major
impacts are expected for the Oregon Cascades starting Monday
morning.

Wegman







$$

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FOUS11 KWBC 160820
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Sat Nov 16 2024

Valid 12Z Sat Nov 16 2024 - 12Z Tue Nov 19 2024

...Western U.S....
Days 1-3...

As one storm system originally over the Intermountain West last
night departs (leaving some light-moderate mountain snow over parts
of the central Rockies in its wake this morning), attention shifts
to the next Pacific storm system that is set to bring an onslaught
of heavy snow to the Pacific Northwest and Northern Rockies this
weekend and into early next week. Precipitation will arrive
Saturday morning as a warm front approaches from the West. Snow
levels start out around 3,000ft Saturday morning, but strong warm
air advection associated with the warm front will cause snow levels
to rise above 7,000ft in the OR Cascades, while they are more
focused around 5,000ft in the northern WA Cascades Saturday night.
This same moisture plume, which is associated with a >500 kg/m/s
IVT that also happens to be above the 90th climatological
percentile according to NAEFS, will advect rich low-mid level
moisture well inland into the northern Rockies Saturday night. By
Sunday, while moisture advection into the Cascades will not be as
strong, a cold front passing through on Sunday will cause snow
levels to plummet to as low as 2,000ft in the WA Cascades and
2,500ft in the Oregon Cascades by Sunday evening.

These falling snow levels will also occur inland across the
Northern Rockies Sunday night and into Monday. NAEFS by 00Z Monday
shows 500mb heights that are below the 10th climatological
percentile throughout the northwestern U.S.. By Monday afternoon,
700mb temperatures are unusually cold across the Pacific NW with
temps as cold as the 2.5 climatological percentile. This unusually
cold air-mass is in place at the same time as steady onshore flow
perpendicular to the Cascades and northern Rockies continues to
support moderate-heavy snow from the Olympics, Cascades, and
northern Rockies to as far south as the Salmon Mountains of
northern California. Heavy snow not only looks to continue through
Monday night in the Cascades and Olympics, but a much stronger
Pacific storm system arrives on Tuesday with more heavy snow and
much stronger winds.

Through 12Z Tuesday, WPC PWPF shows which chances (>70%) for
snowfall >18" in the Oregon Cascades, the northern Bitterroots of
the Idaho Panhandle, and the tallest peaks of the Lewis Range. The
northern Washington Cascades show high chances (>70%) for over 2
feet of snowfall with some of the tallest peaks above 7,000ft
topping 3 feet. Impacts will be the worst in these aforementioned
areas with Moderate to Major Impacts shown on the WSSI, which
includes some of the notable passes in the OR/WA Cascades.
Elsewhere, WPC PWPF shows high chances (>70%) for >12" of snowfall
in the Blue, Boise, and Sawtooth Mountains. Similar high chance
probabilities exist in the Tetons for >8" of snowfall. The WSSI
shows Minor to localized Moderate Impacts in these mountain ranges.

...Eastern MT & Western ND...
Day 1...

Over the past 24 hours, guidance shows the upper level trough
tracking through southern Montana tonight to be more potent and
compact than originally shown. The region remains beneath the
divergent left-exit region of a 250mb jet streak located over the
Four Corners region. As the 700mb low tracks through eastern
Montana this morning, a ribbon of anomalous 700mb moisture aloft
will wrap around the northern and western flank of the low while
strong vertical velocities aloft and dynamic cooling allow for
heavy snow to ensue over northeast Montana. Latest CAMs show
excellent upward vertical velocities within a highly saturated DGZ
around 600-550mb. The 00Z HREF Snowband Probability Tracker (SPT)
does show a swath of snowfall rates that could hover around 2"/hr,
if not higher in some cases. Residents in these areas can expect
rapidly accumulating snowfall this morning with US Routes 2 and 85
to the north and west of Williston likely to feature treacherous
travel conditions. Slick roads and significantly reduced
visibilities are expected where the heaviest snowfall occurs. By
this afternoon, the storm system will be over eastern North Dakota
and tracking to the northeast. Periods of snow will continue across
western North Dakota before finally concluding Saturday evening.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax



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FOUS11 KWBC 162035
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 PM EST Sat Nov 16 2024

Valid 00Z Sun Nov 17 2024 - 00Z Wed Nov 20 2024

...Western U.S....
Days 1-3...

A sharp trough sweeping southeast from the Gulf of Alaska will
reach the PacNW coast Sunday morning before crossing the northern
Rockies through Monday. Deep low pressure develops over the Gulf of
Alaska Sunday night in the wake of the trough and a reinforcing
trough around this new low looks to rapidly develop Tuesday west of
Vancouver island and will go a long ways to maintaining a rather
active pattern over the Northwest through the next week.

A rich plume of moisture streaming ahead of the approaching trough
will quickly raise snow levels tonight above 5,000ft in the north
WA Cascades and over 8,000ft in the OR Cascades. This robust
moisture reaches the northern Rockies late tonight with snow levels
around 5,000ft there through Sunday. Meanwhile, the sharp cold
front will cross WA/OR through Sunday and cause snow levels to
plummet to 2,000ft in the WA Cascades and 2,500ft in the Oregon
Cascades by Sunday evening. Snow levels fall to around 2,000ft in
the northern Rockies Sunday night. Continued onshore flow under
this trough and ahead of the next developing system will maintain
moderate snow rates over this terrain through Monday. Day 1 PWPF
for >6" snow are categorical for all the WA and higher OR Cascades,
Blue Mtns of OR and Salmon River Mtns of ID north through the
Bitterroots. The lower snow levels and more expansive moisture
brings categorical Day 2 PWPF for >8" along the WA/OR Cascades and
Northern Rockies down to the Tetons.

One note about the onshore flow tonight is the moisture overriding
cold surface conditions over the Columbia Basin tonight into Sunday
to cause pockets of freezing rain. Day 1 PWPF for >0.1" ice are up
to 20% in the Columbia Basin.

A lull from brief ridging Monday night quickly gives way to the
next surge of Pacific moisture later Tuesday into the PacNW. Snow
levels will rise on the initial plume of SWly flow and from there
is quite a bit of uncertainty from a wintry weather perspective
with the 12Z suite featuring a stalled low just offshore through
the rest of next week. An active pattern to say the least.


...Northern Plains...
Day 3...

Low pressure closes off tonight/Sunday south of AZ which tracks
over west TX into the central High Plains Monday. This portion is
rather warm and should produce rain where last week had the
historic heavy snow. This low rapidly shifts up the Plains Monday
night as a potent low with comma head/snow banding on the NW side
not developing until over North Dakota later Tuesday. Day 3 PWPF
for >4" are 10-40% over north-central WY terrain, the Black Hills,
and much of western/northern ND. A trend to a wetter comma head was
made today in the forecast, so banding similar to what occurred
this morning over eastern MT may be at stake a bit farther east on
Tuesday.


Jackson


$$

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FOUS11 KWBC 170812
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 AM EST Sun Nov 17 2024

Valid 12Z Sun Nov 17 2024 - 12Z Wed Nov 20 2024

...Western U.S....
Days 1-3...

A developing active pattern is in its early stages as the first in
a series of Pacific storm systems revolves beneath the base of a
longwave trough that will become quasi-stationary in the Gulf of
Alaska to start the period. Then, as a strong ridge builds over the
Aleutians mid-week, the longwave trough will dive south into the
northeast Pacific and direct a steady barrage of rich Pacific
moisture at the region into the second half of the week. Tonight,
the warm front associated with the initial surge in Pacific
moisture is working across the interior Northwest. Meanwhile, a
cold front will push across western WA/OR Sunday morning that will
cause snow levels to plummet to as low as 2,000ft in the WA
Cascades and 2,500ft in the OR Cascades by this afternoon. By
Sunday evening, some locations on the windward side of the Cascades
could see snow levels as low as 1,000ft. It is also worth noting
that there are likely to be some instances of freezing rain today
with low-moderate probabilities for >0.01" of freezing rain in
parts of eastern OR and the northern ID Panhandle.

The falling snow levels will occur well inland across the Northern
Rockies Sunday night and into Monday. NAEFS shows 500-700mb temps
and heights that are below the 10th percentile throughout the
Northwest on Monday. This unusually cold air-mass is in place at
the same time as persistent onshore flow, courtesy of the longwave
trough to the north, strikes mountain ranges such as the Cascades
at an orthogonal angle to maximize the snowfall potential through
Monday. Following a brief lull Monday night, a rapidly
strengthening storm system (both the GFS and ECMWF have the low sub
950mb by Tuesday afternoon!) will direct a stronger atmospheric
river at the Pacific Northwest with not only exceptional moisture
advection (>750 kg/m/s IVT on NAEFS would be above the 97.5
climatological percentile Tuesday afternoon), but intense winds
along the mountainous terrain. Robust warm air-advection late
Tuesday and into Tuesday night will lead to snow levels rising to
above 3,000ft in the WA Cascades and Olympics, around 4,000ft in
the OR Cascades, and 6,000ft in the northern California ranges by
Wednesday morning. The concern would be for the potential for a
heavier/wet snow type to rapidly accumulate, while powerful wind
gusts potential lead to tree damage and power outages Tuesday
night and into Wednesday.

All told, there is no shortage of snow in the forecast for the
Pacific Northwest and Northern Rockies. Snow is likely to be
measured in feet along not just the Cascades above 3,000ft, but the
Olympics, the Salmon/Shasta/Siskiyou of northern California, and
well inland through the Blue, Sawtooth, Boise, Bitterroot, Tetons,
and Lewis Mountain Ranges. The latest WSSI shows Moderate to Major
Impacts in the Cascades and Bitterroots with impacts including not
just dangerous travel conditions (including notable Cascade
passes), but the weight of the snow and strong winds by Tuesday
night could result in tree damage and power outages in some areas.

...Northern Plains...
Day 3...

Confidence is increasing in a powerful late November cyclone to
track from the Southern Plains on Monday and race north into the
Midwest Monday night. Both NAEFS and ECMWF SATs support a highly
anomalous area of low pressure Monday night, but the ECMWF is
farther west and closer to the Missouri River than the NAEFS, which
is located farther east over central Iowa. By Tuesday morning, the
NAEFS (a GFS/CMC ensemble based tool) shows the storm about to
cross the Mississippi River in southeast MN while the ECMWF is over
southwest MN. The primary reason for this is the ECMWF shows a
slightly slower and stronger 500mb solution than the GFS, albeit
they are not overly different in the storm evolution.

Where they differ is that the ECMWF (RGEM as well) phases sooner
with an upper level disturbance over the northern High Plains and
is pulled farther west than the GFS. By 21Z Tuesday, both the ECMWF
and RGEM show a closed 500mb low near Bismarck, while the GFS is
more elongated and does not close off fully until a few hours later
near the ND/Canada border. These details are critical in
determining where the deformation zone (or heaviest snow bands) set
up; over the Dakotas or more over south-central Canada. For the
moment, WPC PWPF shows moderate-high probabilities (50-70%) for

4" of snowfall over north-central ND. It is worth noting that the

WPC PWPF does show low chances (10-30%) for snowfall >8", and near
the North Dakota/Canada border, a small 10% chance area for >12" of
snowfall. Residents in the Dakotas should monitor the forecast
closely over the next 24-48 hours.

While exact amounts are unclear, the exceptionally strong storm
system over Minnesota coupled with high pressure building in over
the Northern Rockies will make for a very windy setup over the
Northern Plains. Snow is still expected to fall across much of
central and western North Dakota and wind gusts of 30-40 mph would
result in whiteout conditions for areas that, in some cases, would
experience their first round of accumulating snowfall this season.
The Probabilistic WSSI (WSSI-P) shows moderate-high chances
(50-70%) for Minor Impacts across much of central North Dakota with
the highest probabilities focused in north-central North Dakota.
The WSSI-P shows Snow Amount and Blowing Snow to be the primary
impacts in these areas. In summary, while exact amounts remain low
confidence at this range, the combination of some accumulating snow
and hazardous visibility reductions due to blowing snow are still
likely to cause some treacherous travel conditions in these areas
late Tuesday and into Wednesday morning.


The probability of significant ice across the CONUS is less than 10
percent.

Mullinax





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FOUS11 KWBC 172042
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 PM EST Sun Nov 17 2024

Valid 00Z Mon Nov 18 2024 - 00Z Thu Nov 21 2024

...Western U.S....
Days 1-3...

Active winter pattern for the Northwest at least through the next
week.

A leading wave over the PacNW coast this afternoon will track east
over the northern Rockies through Monday before rapidly developing
into a deep low over North Dakota on Tuesday. A deep low currently
over the Gulf of Alaska is rounded by a powerful shortwave Monday
night allowing for particularly rapid development to a surface low
well off Vancouver Island on Tuesday. This low then stalls/becomes
a gyre off the PacNW coast through at least Friday which will
result in prolonged moisture-potent onshore flow into the PacNW
starting later Tuesday.

The cold front from the leading wave currently crossing the
Cascades crosses the northern Rockies tonight with low levels
plummeting by this evening to 2,000ft in the WA Cascades and
2,500ft in the OR Cascades. These snow levels then gradually drop
an additional 500ft through Monday with moderate precip rates
persisting from continued onshore flow.

Snow levels fall over the northern Rockies from 5000-7000ft now to
2000-2500ft overnight. The unusually cold air-mass is in place at
the same time as persistent onshore flow, courtesy of the longwave
trough to the north, strikes mountain ranges such as the Cascades
at an orthogonal angle to maximize the snowfall potential through
Monday. Day 1 PWPF for >6" are categorical for the Cascades above
about 3000ft and the northern Rockies from the Tetons to the
Bitterroots above about 4000ft.

Following a brief lull in minor ridging Monday night, a rapidly
strengthening storm system (the 12Z GFS/ECMWF both have a sub 950mb
low by 21Z Tuesday!) will direct a stronger atmospheric river at
the Pacific Northwest with not only exceptional moisture advection

750 kg/m/s IVT on NAEFS would be above the 97.5 climatological

percentile Tuesday afternoon), but intense winds along the
mountainous terrain. Warm air advection will raise snow levels
to above 2,500ft in the WA Cascades, around 4,000ft in the OR
Cascades, and 6,000ft in the northern California ranges by
Wednesday morning. The concern is for a heavier/wet snow type to
rapidly accumulate, while powerful wind gusts potential lead to
tree damage and power outages Tuesday night and Wednesday.

All told, there is no shortage of snow in the forecast for the
Pacific Northwest and Northern Rockies. Snow is likely to be
measured in feet along the Cascades above 3,000ft, the
Salmon/Shasta/Siskiyou of northern California, and well inland
through the Blue, Sawtooth, Boise, Bitterroot, Tetons, and Lewis
Mountain Ranges. The latest WSSI shows Moderate to Major Impacts in
the Cascades and Bitterroots with impacts including not just
dangerous travel conditions (including notable Cascade passes), but
very heavy/wet snow and strong winds by Tuesday night that could
result in tree damage and power outages in some areas.

As of now Day 3 PWPF for >6" are categorical in the Shasta/Siskiyou
of CA (which is the center of the prolonged moisture plume, but
where topographical lift should keep snow levels from rising too
far above 4000ft) and the WA Cascades where a burst of snow is
expected from the arrival of the occluded low.


...Northern Plains...
Days 2/3...

A powerful late November cyclone will track from the Southern
Plains tonight to Minnesota Monday night. The 12Z ECMWF is not as
far east as previous runs and a bit more in line with the 12Z GFS
in the track north over MN. This exceptionally strong storm system
tracking over Minnesota coupled with high pressure building in
over the Northern Rockies will make for a very windy setup over the
Northern Plains. Snow is expected to fall across much of North
Dakota and winds of 30-40 mph would result in whiteout conditions
for areas like central and eastern ND would experience their first
round of accumulating snowfall this season. The Day 2.5 PWPF for

6" is 20-70% over much of ND west of the Red River Basin and north

of I-94.


The probability of significant ice across the CONUS is less than 10
percent.


Jackson


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FOUS11 KWBC 180826
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
326 AM EST Mon Nov 18 2024

Valid 12Z Mon Nov 18 2024 - 12Z Thu Nov 21 2024

...Western U.S...

Days 1-3...

**Multiple days of heavy snow to continue in the mountains of the
Pacific Northwest through mid-week**

An anomalously deep 200-500mb mean trough over the Northwest will
keep snow levels lower than usual as low-mid level westerlies
deliver a persistent fetch of Pacific moisture into the the
Olympics, Cascades, and as far inland as the Northern Rockies
today. Snow levels will be as low as 1,000ft in some parts of the
northern WA Cascades, but 1,500-2,000ft will be more of the norm
throughout the remainder of the Northwest. Through Tuesday morning,
WPC PWPF shows high chances (>70%) for snowfall >6 inches in the
Olympics, WA/OR Cascades above 3,000ft, the peaks of the Lewis
Range, and in the Tetons. Expect hazardous travel conditions in
these areas with roads along complex terrain more difficult to
navigate.

The longwave trough parked to the north off the coast of
British Columbia will dive south Monday night and phase with
another potent vorticity maximum to produce a powerful sub 950mb
low off the Pacific Northwest coast Tuesday afternoon. This storm
system will direct a robust atmospheric river (topping 1,000 kg/m/s
at its peak, or above the 99.5 climatological percentile per
NAEFS) at the northern CA and OR coast. This atmospheric river
will accompany strong warm air advection that will force snow level
to rise above 2,500ft in the WA Cascades, around 4,000ft in the OR
Cascades, and 6,000ft in the northern California ranges by
Wednesday morning. The concern is for a heavier/wet snow type to
rapidly accumulate, while powerful wind gusts potential associated
with the storm cause tree damage and power outages Tuesday night
and Wednesday. This same plume of rich Pacific moisture will spread
well inland with heavy snow set to occur in the Blue, Boise,
Sawtooth, northeast Washington, and Bitterroot mountains. While
snow gradually lessens in the Northern Rockies and Cascades
Wednesday night, heavy snow will continue in the Shasta and
northern Sierra Nevada of California. It is noteworthy that snow
levels will gradually rise across northern California Wednesday
night, ultimately reaching as high as 8,000ft by early Thursday
morning.

In summary, there is no shortage of snow in the forecast for the
Pacific Northwest, Northern Rockies, and northern California. Snow
will be measured in feet (1-3ft in most cases) along the Cascades
above 3,000ft, the Salmon/Shasta/Siskiyou of northern California above
4,000ft, and well inland through the Blue, Sawtooth, Boise,
Bitterroot, Tetons, and Lewis Mountain Ranges. The latest WSSI
still depicts Moderate to Major Impacts in the Cascades, Olympics,
and Bitterroots with impacts including not just dangerous travel
conditions (including notable Cascade passes), but very heavy/wet
snow and strong winds by Tuesday night that could result in tree
damage and power outages in some areas. The WSSI also now shows
some instances for Extreme Impacts in the Salmon/Shasta of northern
California where several feet of heavy/wet snow combined with
strong winds will exacerbate the potential for power outages and
tree damage.

...Northern Plains...
Days 2-3...

Guidance is coming into better agreement on the track of a highly
anomalous area of low pressure (NAEFS and ECMWF shows MSLP values
below the 0.5 climatological percentile over MN midday Tuesday). Snow
is expected to fall across much of northern North Dakota starting
Tuesday morning with the heaviest snowfall occurring Tuesday
evening across much of northern North Dakota and even into parts
of northwest Minnesota. Winds of 30-40 mph would result in whiteout
conditions for areas like central and eastern ND would also
experience their first round of accumulating snowfall this season
in some areas. Snow intensity will gradually diminish through
Wednesday morning, but blustery winds throughout the Red River of
the North may result in reduced visibilities that make for
hazardous travel conditions. WPC's WSSI-P shows moderate chances
(40-60%) for Moderate Impacts (disruptions to daily life, including
hazardous driving conditions and some closures) across central
North Dakota. The highest confidence in those impacts are in north-
central North Dakota. Tuesday's PWPF for >6" is 40-70% over north-
central North Dakota to the west of the Red River Basin and north
of I-94.


The probability of significant ice across the CONUS is less than 10
percent.


Mullinax





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FOUS11 KWBC 182020
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 PM EST Mon Nov 18 2024

Valid 00Z Tue Nov 19 2024 - 00Z Fri Nov 22 2024

...Pacific and Interior Northwest

Days 1-3...

***Impressive and long-lasting atmospheric river event to bring
heavy snow to the Northwest***

Broad cyclonic flow across the Northwest will amplify impressively
as a strong closed low digs off the British Columbia coast and
drifts southward west of WA/OR through Wednesday. This intense
cutoff low is progged to create height anomalies that fall below
anything in the CFSR climatology (in this 3 week period) at both
500 and 700mb heights. This will drive incredible onshore
advection of wind and precipitation as an intense surface low
develops beneath this cutoff and shunts a cold front southeast
towards the coast. This front is progged to stall as the upper low
pivots in place through Wednesday before finally beginning to fill
and retrograde back to the northwest, leaving persistent and
impressive IVT surging onshore. IVT probabilities from the
ECENS/GEFS are above 60%, and from the West-WRF above 90% for
750 kg/m/s lifting onshore near 40N, with plumes suggesting the
potential for IVT reaching 1000, highest on D2. This impressive and long-lasting IVT surge /AR/ will push multiple rounds of heavy
precipitation onshore, especially beginning early D2, and notably
where intense upslope flow will enhance ascent leading to snowfall
rates of 2-3"/hr at times.

Snow levels will begin the forecast period quite low, only around
200 ft east of the Cascades to around 2000 ft in northern CA.
However, the WAA accompanying the AR will raise snow levels
steadily D2 and D3, reaching as high as 4000 ft in northern WA and
the Northern Rockies, and to 8000 ft across northern CA. Despite
this climb, impactful snow is likely at many passes before a slow
transition to rain occurs during D3.

WPC probabilities for 6+ inches D1 are above 70% along the spine of
the OR Cascades and into the Olympics, with additional low
probabilities below 40% encompassing parts of the interior
including the Tetons, Big Horns, Northern Rockies, and even parts
of the CO Rockies. However, the more significant event is D2-D3
when WPC probabilities for more than 6 inches each day are above
90% from the Shasta/Trinity region northward along the OR Cascades
and into the WA Cascades/Olympics, and then east into the Okanogan
Highlands and the Salmon River/Sawtooth/Blue Mountains. 2-day
snowfall will likely reach 1-3 feet in many of these areas, with
snowfall of 6 or more inches likely at many of the passes as well.


...Northern Plains...
Days 1-2...

The mid-level pattern across the CONUS begins to sharpen today,
becoming impressively amplified by mid-week. This begins with a
500mb shortwave trough over southern Alberta sinking southward
through Tuesday while deepening into a closed low over North Dakota
by the end of D1. Throughout D2 this low strengthens further into a
large gyre spinning over the Upper Midwest in response to repeated
vorticity lobes shedding through the flow and phasing into the more
pronounced system. This development will pair with a southward
advancing jet streak out of Canada and a surface cold front moving
eastward to result in deepening cyclogenesis across the Upper
Midwest. Moisture advection reflected by PWs surging to as high as
+4 sigma Tuesday downstream of this low will begin to pivot
northwest as a theta-e ridge lifts into a robust TROWAL, especially
late D1 into D2 across ND, with the greatest theta-e advection
merging into the DGZ.

As the TROWAL pivots southward it will interact with modest
deformation to produce a swath of heavy snow, which despite strong
winds beneath the DGZ to fracture dendrites, will likely result in
snowfall rates above 1"/hr at times. These rates combined with
fluffy SLRs and strong winds will produce significant blowing snow
impacts, regardless of how much snow accumulates. However, at this
time, significant snowfall is appearing more certain, noted by WPC probabilities for more than 4 inches reaching 20-30% in NW ND D1,
and expanding more impressively into eastern ND and far western MN
D2, peaking above 60%. Locally 6-8" of snow is possible across some
of the higher terrain surrounding the Red River Valley of the
North.


...Central Appalachians...
Day 3...

An expansive closed 500mb low will be positioned over the Upper
Midwest/Western Great Lakes at the start of D3, resulting in a
vertically stacked and occluded low pressure. Throughout D3
/Wednesday night into Thursday/ this closed low will wobble
eastward as spokes of vorticity maxima rotate cyclonically around
this feature, tugging it slowly eastward, while also pushing a
secondary surface low, which is extended from the occluded front,
across the Mid-Atlantic and towards the Northeast. This low will be
trailed by dual cold fronts to cause rapid cooling of the column,
and the resultant post-frontal flow will result in heavy snow
developing in the upslope regions of the Appalachians, with
additional enhanced lift occurring in the vicinity of a secondary
wave dropping south out of Michigan Thursday.

There is still some uncertainty into the timing of the secondary
cold frontal passage, and how impressive ascent will be in the
vicinity of the occluded low dropping into the OH VLY. A faster
progression or deeper low may result in more widespread moderate
snow as dynamic cooling can overcome marginal thermals, but a more
south or west low could reduce upslope potential especially across
the higher terrain of PA. At this time the greatest threat for
significant accumulations continues to focus in the high terrain
of WV, the panhandle of MD, and into SW PA. Here, recent WPC
probabilities for more than 4 inches of snow are 50-70%, with more
significant accumulations likely into D4.


The probability of significant ice across the CONUS is less than 10
percent.

Weiss

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FOUS11 KWBC 190807
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 AM EST Tue Nov 19 2024

Valid 12Z Tue Nov 19 2024 - 12Z Fri Nov 22 2024

...Pacific and Interior Northwest...
Days 1-3...

***Impressive and long-lasting atmospheric river event to bring
heavy snow to the Northwest***

This morning, a large ridge of high pressure building over Alaska
will help to force the longwave trough that has been parked off the
British Columbia coast for the last 24 hours southward into the
heart of the Northwest Pacific. Rounding the base of the longwave
trough is a potent 500mb vort max that will phase and lead to the
rapid cyclogenesis of a sub 950mb low off the Pacific Northwest
coast by Tuesday afternoon. The NAEFS SAT page is littered with
atmospheric parameters (wind speeds, heights, MSLP, IVT) that are
approaching or reaching maximum (or minimum) climatological
percentiles in the CFSR climatology. Perhaps the most notable
parameters are the IVT (topping 1,000 kg/m/s at its peak off the
northern California coast) and 850-700mb winds that are above the
97.5 climatological percentiles Tuesday night into Wednesday
morning. This same moisture plume will spill over the West Coast
mountain ranges and into the Northern Rockies

These two parameters are vital in producing what should be prolific
snowfall rates in the Olympics, Cascades, Siskiyou, Salmon, and
Shasta Mountains Tuesday night. WPC's Snowband Probability Tracker
(SPT) utilizing the HREF shows the potential for 2-3"/hr snowfall
rates as the heaviest precipitation arrives Tuesday evening with
some potential for localized 4"/hr rates. Snow levels will begin
the forecast period quite low; only around 200 ft east of the
Cascades to around 2000ft in northern CA. However, the exceptional
WAA accompanying the AR will raise snow levels steadily D2 and D3,
reaching as high as 4000ft in northern WA and the Northern Rockies,
and to 8000ft across northern CA. Despite this rise in snow
levels, hazardous snowfall impacts are likely at many passes before
p-type changes over to rain in the Olympics and Cascades first
(both via WAA and dry slotting aloft), then over the northern
California ranges Wednesday evening. This coincides with both a
thermodynamic profile that will lead to snowfall being a heavy/wet
consistency that when paired with strong wind gusts will cause
stress to the trees and power lines.

WPC PWPF shows high chances (>70%) for snowfall totals >12 inches
along the spine of the WA/OR Cascades, as well as the Blue,
Sawtooth, and Bitterroots Mountains that stretch as far north as
the Idaho Panhandle. In the Salmon/Siskiyou/Shasta and Olympics,
WPC PWPF shows high chances (>70%) for >30" with the highest
elevations seeing anywhere from 3-5 feet in some cases. The impacts
will be exceptional in northern California where the WSSI shows
Extreme criteria being met in portions of the
Salmon/Siskiyou/Shasta. The Snow Amount and Snow Load components
are the primary drivers in these impacts that are likely to include
extremely dangerous to impossible driving conditions, as well as
extensive closures and both tree damage and power outages. Major
Impacts are highlighted along the spine of the Cascades above
5,000ft and the Olympics above 3,000ft.


...Northern Plains...
Days 1-2...

The much anticipated storm system responsible for today's winter
storm across puritans of the Northern Plains is tracking north
through the Upper Midwest this morning with a comma-head shaped
precipitation shield wrapping around the northern and western
flanks of the storm. As the morning unfolds, temperatures will
continue to fall across North Dakota and allow for lingering areas
of rain to changeover to snow later this morning. The heaviest
period of snow arrives this afternoon and into tonight as the
TROWAL on the backside of the storm pivots through with a
deformation axis capable of generating 1-2"/hr snowfall rates
across north-central North Dakota. On top of the heavy snowfall
rates, whipping wind gusts topping 40 mph in many cases means
significant visibility reductions that will lead to whiteout
conditions Tuesday evening.

As a complex 500mb evolution unfolds over Minnesota Tuesday night
and into Wednesday morning, an enormous upper level low gyre will
still have a saturated ribbon of 700-300mb moisture over the
Northern Plains that will spill east over the Red River of the
North and as far south as northeast South Dakota. WPC PWPF does
show low-to-moderate chances (30-50%) for snowfall totals >6"
east of the Red River in northwest Minnesota and in northeast South
Dakota. Periods of snow will persist throughout the day on
Wednesday but coverage and intensity will gradually dissipate
throughout the day until the snow finally comes to an end overnight
Wednesday. When all is said and done, portions of north-central
North Dakota are likely looking at 6-12" of snowfall with the
heaviest amounts likely to occur closest to the north-central North Dakota/Canada border. The latest WSSI shows a fairly large
footprint for Moderate impacts over most of central North Dakota
and northeast South Dakota, which suggests there are likely to be
some disruptions to daily life such as hazardous driving conditions
and potential closures. Minor Impacts are most commonly seen as
far west as eastern Montana and to the east across the Red River of
the North.


...Northern & Central Appalachians...
Day 3...

The aforementioned bowling ball of a 500mb closed low over the
Midwest on Wednesday will aid in the development of a new wave of
low pressure tracking over the northern Great Lakes Wednesday
evening. A strong cold front associated with the storm will push
through the Central Appalachians Wednesday night that will not only
lead to much of thermal profiles in the region being sub-freezing,
but a plume of 850-700mb moisture will reach the Central
Appalachians by Thursday morning. With 300-700mb mean flow
generally out of the west, this is a recipe for the first upslope
snow event of the season for the Central Appalachians. After the
first lobe of dynamical potential vorticity (PVU) swings through
the Mid-Atlantic on Thursday (helping to kick-start the upslope
event), the second PVU lobe will swing through Thursday night. This
will reinforce the ongoing upslope flow into the Central
Appalachians, and with this feature the initial low pressure
system that pushed the strong cold front through Wednesday night,
will introduce another surge in 850-700mb moisture over the region
through Friday morning. WPC PWPF shows high chances (>70%) for
snowfall >4" along the Allegheny Mountains and Laurel Highlands. In
fact, for elevations >3,000ft in the Allegheny Mountains, WPC PWPF
shows some moderate-to-high probabilities (50-70%) for >8" through
early Friday morning with additional snowfall still to come later
in the day on Friday. The latest WSSI-P shows a large swath of
moderate-to-high chances (50-70%) for Minor Impacts from the
Smokey's of North Carolina on north along the central Appalachians
and into the Laurel Highlands through Friday morning.

Farther north, similar to the original low pressure development
evolution over the Great Lakes on Wednesday, another area of low
pressure will form on the nose of the 2PVU rotating through the
Mid-Atlantic Thursday afternoon and deepen over Long Island
Thursday evening. This becomes a highly anomalous closed mid-upper
level low that is placed at the divergent left-exit region of a
strong 250mb jet streak over the Southeast. Geopotential heights
over the Mid-Atlantic (500mb, 700mb, 850mb) are all pegged to be
below the 1st climatological percentile Thursday night. As the
storm deepens, strong frontogenetic forcing on the northwest flank
of the 850mb low will result in a narrow deformation axis that is
currently forecast to set up somewhere from northern PA on east
through Upstate NY, northern NJ, and into southern New England by
Friday morning. This evolution has been present on the EC-AIFS for
over 24 hours. While boundary layer thermals may be questionable
away from the deformation axis, this setup favors snowfall at
elevations above 1,000ft in the Poconos, Catskills, Adirondacks,
and Berkshires. It is worth noting that there remains some
different solutions on snowfall totals and which areas see the
heaviest amounts. Regardless, this would be the first accumulating
snowfall of the season for these higher elevated areas. Should
guidance trend toward a more robust deformation zone, some lower elevated/valley locations in the Northeast could receive measurable
snowfall on Friday. At the moment, the WSSI-P shows moderate
chances (40-60%) for Minor Impacts (generally hazardous driving
conditions) in the Catskills, northern Poconos, and the Adirondacks
through Friday morning.


The probability of significant ice across the CONUS is less than 10
percent.

Mullinax




$$

= = =
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FOUS11 KWBC 190943
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
443 AM EST Tue Nov 19 2024

Valid 12Z Tue Nov 19 2024 - 12Z Fri Nov 22 2024

...Pacific and Interior Northwest...
Days 1-3...

***Impressive and long-lasting atmospheric river event to bring
heavy snow to the Northwest***

This morning, a large ridge of high pressure building over Alaska
will help to force the longwave trough that has been parked off the
British Columbia coast for the last 24 hours southward into the
heart of the Northwest Pacific. Rounding the base of the longwave
trough is a potent 500mb vort max that will phase and lead to the
rapid cyclogenesis of a sub 950mb low off the Pacific Northwest
coast by Tuesday afternoon. The NAEFS SAT page is littered with
atmospheric parameters (wind speeds, heights, MSLP, IVT) that are
approaching or reaching maximum (or minimum) climatological=20
percentiles in the CFSR climatology. Perhaps the most notable=20
parameters are the IVT (topping 1,000 kg/m/s at its peak off the=20
northern California coast) and 850-700mb winds that are above the=20
97.5 climatological percentiles Tuesday night into Wednesday=20
morning. This same moisture plume will spill over the West Coast
mountain ranges and into the Northern Rockies=20

These two parameters are vital in producing what should be prolific
snowfall rates in the Olympics, Cascades, Siskiyou, Salmon, and=20
Shasta Mountains Tuesday night. WPC's Snowband Probability Tracker=20
(SPT) utilizing the HREF shows the potential for 2-3"/hr snowfall=20
rates as the heaviest precipitation arrives Tuesday evening with=20
some potential for localized 4"/hr rates. Snow levels will begin=20
the forecast period quite low; only around 200 ft east of the=20
Cascades to around 2000ft in northern CA. However, the exceptional=20
WAA accompanying the AR will raise snow levels steadily D2 and D3,=20
reaching as high as 4000ft in northern WA and the Northern Rockies,
and to 8000ft across northern CA. Despite this rise in snow=20
levels, hazardous snowfall impacts are likely at many passes before
p-type changes over to rain in the Olympics and Cascades first=20
(both via WAA and dry slotting aloft), then over the northern=20
California ranges Wednesday evening. This coincides with both a=20 thermodynamic profile that will lead to snowfall being a heavy/wet=20 consistency that when paired with strong wind gusts will cause=20
stress to the trees and power lines.

WPC PWPF shows high chances (>70%) for snowfall totals >12 inches
along the spine of the WA/OR Cascades, as well as the Blue,
Sawtooth, and Bitterroots Mountains that stretch as far north as
the Idaho Panhandle. In the Salmon/Siskiyou/Shasta and Olympics,
WPC PWPF shows high chances (>70%) for >30" with the highest
elevations seeing anywhere from 3-5 feet in some cases. The impacts
will be exceptional in northern California where the WSSI shows
Extreme criteria being met in portions of the
Salmon/Siskiyou/Shasta. The Snow Amount and Snow Load components
are the primary drivers in these impacts that are likely to include
extremely dangerous to impossible driving conditions, as well as
extensive closures and both tree damage and power outages. Major
Impacts are highlighted along the spine of the Cascades above
5,000ft and the Olympics above 3,000ft.=20


...Northern Plains...=20
Days 1-2...

The much anticipated storm system responsible for today's winter
storm across puritans of the Northern Plains is tracking north
through the Upper Midwest this morning with a comma-head shaped
precipitation shield wrapping around the northern and western=20
flanks of the storm. As the morning unfolds, temperatures will=20
continue to fall across North Dakota and allow for lingering areas=20
of rain to changeover to snow later this morning. The heaviest=20
period of snow arrives this afternoon and into tonight as the=20
TROWAL on the backside of the storm pivots through with a
deformation axis capable of generating 1-2"/hr snowfall rates=20
across north-central North Dakota. On top of the heavy snowfall=20
rates, whipping wind gusts topping 40 mph in many cases means=20
significant visibility reductions that will lead to whiteout=20
conditions Tuesday evening.=20

As a complex 500mb evolution unfolds over Minnesota Tuesday night
and into Wednesday morning, an enormous upper level low gyre will
still have a saturated ribbon of 700-300mb moisture over the
Northern Plains that will spill east over the Red River of the
North and as far south as northeast South Dakota. WPC PWPF does
show low-to-moderate chances (30-50%) for snowfall totals >6"=20
east of the Red River in northwest Minnesota and in northeast South
Dakota. Periods of snow will persist throughout the day on=20
Wednesday but coverage and intensity will gradually dissipate=20
throughout the day until the snow finally comes to an end overnight
Wednesday. When all is said and done, portions of north-central=20
North Dakota are likely looking at 6-12" of snowfall with the=20
heaviest amounts likely to occur closest to the north-central North Dakota/Canada border. The latest WSSI shows a fairly large=20
footprint for Moderate impacts over most of central North Dakota=20
and northeast South Dakota, which suggests there are likely to be=20
some disruptions to daily life such as hazardous driving conditions
and potential closures. Minor Impacts are most commonly seen as=20
far west as eastern Montana and to the east across the Red River of
the North.=20


...Northern & Central Appalachians...=20
Day 3...

The aforementioned bowling ball of a 500mb closed low over the
Midwest on Wednesday will aid in the development of a new wave of
low pressure tracking over the northern Great Lakes Wednesday
evening. A strong cold front associated with the storm will push
through the Central Appalachians Wednesday night that will not only
lead to much of thermal profiles in the region being sub-freezing,
but a plume of 850-700mb moisture will reach the Central
Appalachians by Thursday morning. With 300-700mb mean flow
generally out of the west, this is a recipe for the first upslope
snow event of the season for the Central Appalachians. After the
first lobe of dynamical potential vorticity (PVU) swings through=20
the Mid-Atlantic on Thursday (helping to kick-start the upslope=20
event), the second PVU lobe will swing through Thursday night. This
will reinforce the ongoing upslope flow into the Central
Appalachians, and with this feature the initial low pressure=20
system that pushed the strong cold front through Wednesday night,=20
will introduce another surge in 850-700mb moisture over the region=20
through Friday morning. WPC PWPF shows high chances (>70%) for=20
snowfall >4" along the Allegheny Mountains and Laurel Highlands. In
fact, for elevations >3,000ft in the Allegheny Mountains, WPC PWPF
shows some moderate-to-high probabilities (50-70%) for >8" through
early Friday morning with additional snowfall still to come later=20
in the day on Friday. The latest WSSI-P shows a large swath of=20 moderate-to-high chances (50-70%) for Minor Impacts from the=20
Smokey's of North Carolina on north along the central Appalachians
and into the Laurel Highlands through Friday morning.

Farther north, similar to the original low pressure development
evolution over the Great Lakes on Wednesday, another area of low
pressure will form on the nose of the 2PVU rotating through the
Mid-Atlantic Thursday afternoon and deepen over Long Island
Thursday evening. This becomes a highly anomalous closed mid-upper
level low that is placed at the divergent left-exit region of a
strong 250mb jet streak over the Southeast. Geopotential heights=20
over the Mid-Atlantic (500mb, 700mb, 850mb) are all pegged to be=20
below the 1st climatological percentile Thursday night. As the=20
storm deepens, strong frontogenetic forcing on the northwest flank=20
of the 850mb low will result in a narrow deformation axis that is=20
currently forecast to set up somewhere from northern PA on east=20
through Upstate NY, northern NJ, and into southern New England by=20
Friday morning. This evolution has been present on the EC-AIFS for=20
over 24 hours. While boundary layer thermals may be questionable=20
away from the deformation axis, this setup favors snowfall at=20
elevations above 1,000ft in the Poconos, Catskills, Adirondacks,=20
and Berkshires. It is worth noting that there remains some=20
different solutions on snowfall totals and which areas see the=20
heaviest amounts. Regardless, this would be the first accumulating=20
snowfall of the season for these higher elevated areas. Should=20
guidance trend toward a more robust deformation zone, some lower=20 elevated/valley locations in the Northeast could receive measurable
snowfall on Friday. At the moment, the WSSI-P shows moderate=20
chances (40-60%) for Minor Impacts (generally hazardous driving=20
conditions) in the Catskills, northern Poconos, and the Adirondacks
through Friday morning.=20


The probability of significant ice across the CONUS is less than 10
percent.

Mullinax





...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!4RrhSWahRz1r_qcEovQEgR7t_E3x6t9R6FvM9z-XNQFXs= hTlRmk0rty80qGvhBtAAAMUbPZ7uZRIXMzaLqN6qQ6OtoQ$=20

$$

=3D =3D =3D
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FOUS11 KWBC 191926
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
226 PM EST Tue Nov 19 2024

Valid 00Z Wed Nov 20 2024 - 00Z Sat Nov 23 2024

...Pacific and Interior Northwest...
Days 1-3...

***Impressive and long-lasting atmospheric river event to bring
heavy snow to the Northwest***

An extremely amplified pattern over the Pacific characterized by an
intense +4 sigma ridge near the Aleutians will help drive a
downstream closed low which is progged by NAEFS to drop to as low
as -6 sigma with respect to 850mb heights off the Washington coast
to start the period. This intense feature will be accompanied by a
strong surface low, and although this surface low will generally
stall and then retrograde back to the north into Friday, the
accompanying ascent and moisture will surge onshore through the
period.

South of this closed low, pinched mid-level flow combined with a
persistent 130kt Pacific jet streak will cause strong warm and
moist advection to push onshore. This will create a potent
atmospheric river /AR/ which both the ECENS and GEFS probabilities
suggest will exceed 750 kg/m/s IVT. As the parent low retrogrades,
the associated frontal structure will stall in a NE to SW
orientation, providing a favorable setup for a long duration of
impressive IVT pushing onshore, focused into northern CA, but with
downstream PW anomalies reaching as high as +2 sigma all the way
into the Great Basin and interior Northwest. This suggests, and is
agreed upon by nearly all global models and ensembles, that
persistent and widespread moderate to heavy precipitation will
occur through the next 48 hours, with dynamic ascent through
upslope flow enhancing the intensity of precipitation rates.

Snow levels initially will be low, 1500-3000 ft from north to south
tonight, suggesting that the heavy precipitation will begin as snow
in many areas outside of the lowlands, but including the passes.
Snowfall rates will likely peak around 3"/hr in the Cascades and
northern CA ranges tonight, which when combined with gusty winds
will produce treacherous and at times impossible travel. During D2,
the focus will shift to be more aligned SW to NE within the
greatest IVT plume from the Shasta/Trinity region of CA through the
Northern Rockies, but at the same time snow levels will steadily
climb, reaching above most pass levels by the end of D2, around
8000 ft in CA and 5000 ft in MT. WPC probabilities D1 are high for
more than 12 inches in the Shasta/Trinity region of northern CA,
along the spine of the Cascades, and into the Olympics. Pass level
impacts are also expected D1. By D2 the highest probabilities shift
east as snow levels rise, reaching above 70% for 12+ inches in the
Salmon River/Sawtooth area. In the highest terrain, especially
around Mt. Shasta, snowfall of 4-6 feet s likely.

During D3 snow levels continue to rise within the IVT plume, but
also more broadly as a secondary surface low lifts near the WA
coast to move onshore British Columbia by the end of the forecast
period. This will enhance moisture and ascent once again, while at
the same time a cold front drops southward from Canada into the
Northern Rockies. Together, this could result in an enhanced swath
of heavy snow across the Okanogan Highlands eastward to the
Northern Rockies where both upslope flow and enhanced fgen will
combine. WPC probabilities D3 are highest along the WA Cascades
eastward through the Northern Rockies where they reach 50-70% for=20
an additional 6+ inches.


...Northern Plains...=20
Day 1...

An impressive closed 500mb low with 500-700mb heights falling below
the 2.5 percentile according to the NAEFS ensemble tables will be=20
positioned across ND to start the forecast period. This low will=20
drift only slowly southeast as the trough continues to amplified in
response to vorticity lobes swinging cyclonically through the base
of the trough, resulting in additional deepening of the trough and
a pull southeast into the Upper Midwest of the primary trough=20
axis. Beneath this, an occluded surface low will track gradually=20
eastward across southern Manitoba, dropping into MN by the end of=20
D1 while filling. Although this won't result in intense synoptic=20
lift as features will be vertically stacked, impressive mesoscale=20
ascent will persist to produce heavy snow across the Dakotas and=20
into Minnesota.

The evolution of this low will result in persistent theta-e=20
advection, driving a pronounced TROWAL within the 600-500mb layer=20
which will rotate cyclonically southward across the Dakotas and=20
into Minnesota Tuesday night into Wednesday. This TROWAL will=20
overlap efficiently with an axis of mid-level deformation and=20
correlated 850-600mb fgen which will drive a narrow corridor of=20
enhanced ascent into a deepening DGZ (probs for 100mb of depth from
the SREF exceed 50% Wednesday morning). In general, ascent will be
waning through D1, but this axis of strong mesoscale lift combined
with strong winds and a cold column should result in moderate to=20
at times heavy snow rates exceeding 1"/hr, which combined with the=20
gusty winds will produce limited visibility and hazardous travel=20
conditions. Total snowfall may me somewhat moderate, with locally=20
heavier amounts likely. WPC probabilities for more than 4 inches of
snow are above 70% from the Turtle Mountains along the ND/Canada=20
border and then stretching as far south as the Sisseton Hills of=20
northeast South Dakota.


...Ohio Valley, Appalachians, & Northeast...=20
Days 2-3...

A complex upper pattern will produce dual interacting surface low
pressures across the region through Friday. Confidence in placement
and evolution is lower than typical, but at least some areas are
likely to receive heavy snowfall beginning Thursday.

The strong closed low, characterized by widespread 500-700mb height
anomalies of -2 to -3 sigma according to NAEFS, will encompass much
of the region from the Great Lakes, Northeast, and Mid-Atlantic,
with spokes of vorticity periodically rotating through the trough.
The strongest of these vorticity maxima will shift across the Mid-
Atlantic to south of New England Thursday aftn, which will drive
secondary cyclogenesis (the primary low beneath the core of the
closed low over Michigan). These dual low pressures will then
pinwheel around each other through Friday producing periods of
intense ascent within a moistening column to drive rounds of mixed precipitation.

The challenge through the period is where will the heaviest snow
accumulate, and, especially outside of higher terrain, how much
snow can accumulate. The high confidence portion of this forecast
involves the Central Appalachians where post frontal NW flow will
produce impressive upslope ascent into a cold column, while at the
same time the primary low from MI rotates southward to enhance
ascent locally across that same area. This will result in a 2-day
period of moderate to heavy snow, and WPC probabilities on D2 reach
30-50% for 4+ inches, with greater probabilities reaching above=20
90% for additional snowfall exceeding 4 inches on D3. Total=20
accumulation may exceed 12 inches in the highest terrain of WV and=20
the MD Panhandle, with locally as much as 4-6" as far south as the=20
NC/TN border.

In other places, the broadly forced ascent results in lower
confidence forecast but with high risk potential for rounds of=20
heavy snowfall rates. In the vicinity of both the primary low=20
dropping from Michigan across the Ohio Valley and into the Central=20 Appalachians, as well as across New England, New York, Pennsylvania
near the secondary low as it retrogrades, periods of intense=20
deformation are likely, and in fact several of the global models=20
indicate rotating plumes of negative theta-e lapse rates which=20
could support CSI in bands of heavy precipitation. Timing and=20
placement of any of these more intense bands is quite uncertain,=20
but with marginal thermal structures in place, it will likely=20
require these heavier rates to drive dynamic cooling sufficient for
significant snowfall accumulations. The most likely regions will=20
be in higher elevations above around 1500 ft including the=20
Catskills and Poconos, where WPC probabilities have increased to
50-70% for 6+ inches, and across portions of the Ohio Valley where
they are 30-50% for 4+ inches.


The probability of significant ice across the CONUS is less than 10
percent.

Weiss



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!6x9i5vOfghMBoXUymLE8NedzgOf8ZMj3ZFI5VZnH4SdQ5= RpMmQEoOm2gPWanMC788RS09vg-2qy13FS35HcHYf_YhKo$=20

$$

=3D =3D =3D
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FOUS11 KWBC 200808
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
308 AM EST Wed Nov 20 2024

Valid 12Z Wed Nov 20 2024 - 12Z Sat Nov 23 2024

...Pacific and Interior Northwest...
Days 1-3...

***Impressive and long-lasting atmospheric river event continues to
produce heavy snow in the Northwest***

The rate of deepening from the storm system off the Pacific
Northwest coast has been breath-taking to watch. The storm's=20
atmospheric parameters (geopotential heights, wind speeds, moisture
advection) are all approaching NAEFS maximum (or in MSLP cases,=20
minimum) values in its CFSR record. Over the course of the next
couple days, the powerful storm system will be one in a series of=20
systems that are embedded within a massive 500mb trough gyre off=20
the Pacific Northwest coast. This is due to an expansive ridge over
Alaska that has effectively cut this upper low off from the=20
mean westerly flow pattern. Even as the current massive cyclone=20
occludes and gradually weakens Wednesday night, the next rapidly=20 strengthening storm will approach west of California on Thursday=20
directing yet another anomalous plume of moisture into the Pacific=20
Northwest and northern California that will persist into Friday.=20
All this to say that the synoptic scale evolution is favorable to=20
sustain a prolonged atmospheric river event that will generate=20
significant impacts across portions of the Pacific Northwest.

In terms of sensible weather, heavy precipitation will engulf much
of the Pacific Northwest and spill over into the Northern Rockies.
Snow levels starting out fairly low overnight will gradually rise
as strong WAA and a dryslot in the 700-300mb layer works its way
across western WA by Wednesday afternoon. Farther inland, colder
air will be harder to scour out in the Columbia Basin and along the
ridge lines of northeast Washington and the Idaho Panhandle.
Snowfall rates initially along the Cascades, Olympics, and on south
to the Shasta/Salmon/Sierra Nevada of northern California will be
on the order of 2-3"/hr. But as snow levels quickly rise through
Wednesday evening and into Thursday, most snow levels will be above
5,000ft in the Cascades and Olympics while the Shasta/Trinity
mountains see snow levels rise to as high as 8,000ft. The heaviest
snowfall by Wednesday night is likely to occur in the Blue and=20
Sawtooth Mountains. As the next storm approaches late Thursday, the
heaviest snowfall will shift farther north into the northern
Bitterroots and the Lewis Range through Friday.

WPC PWPF shows high chances (>70%) for snowfall >12" in the
Shasta/Trinity of northern California, Olympics, Cascades, Blue,
Sawtooth, and Lewis Ranges. Portions of the northern Idaho
Panhandle also show >70% odds for >12" of snowfall. Snow by Friday
will be primarily confined to the tallest peals of the Pacific
Northwest, making rain the primary precipitation type for the vast
majority of the residents impacted by Friday's storm. The WSSI
maintains Major Impacts in parts of central Washington, the
Cascades and Olympics, and even Extreme Impacts in the
Shasta/Trinity of northern California through Wednesday afternoon.
Only the Shasta/Trinity maintain Major Impacts through Thursday and
into Friday.

...Northern Plains...=20
Day 1...

The winter storm that has produced periods of heavy snow throughout
North Dakota will gradually start to wind down on Wednesday as the
upper low over northwest Minnesota gradually weakens. However, a
500mb vort max pivoting around the northwest flank of the low will
still support modest upper level divergence aloft while a narrow
ribbon of modest 700-300mb moisture remains parked over central
North Dakota and eastern South Dakota. This moisture will spill
over into the Red River of the North and parts of northwest
Minnesota and keep periods of snow in the forecast there through=20
Wednesday afternoon. In addition, wind gusts of 30-40 mph will=20
linger throughout the day, leading to drifting snow on roads and=20
poor visibilities for motorists. Snow should finally taper off=20
Wednesday evening as the aforementioned 500mb vort max tracks south
into southern Minnesota and the plume of mid-level moisture aloft=20
dries up. WPC PWPF shows moderate chances (40-60%) for an=20
additional 4"+ in parts of northeast South Dakota and north-central
North Dakota where the favorable vertical ascent and moisture=20
profiles aloft are paired with the best 850-700mb frontogenetical=20
forcing. The WSSI-P still shows moderate chances (40-60%) for Minor
Impacts due to a combination of Snow Amount and Blowing Snow=20
criteria over central North Dakota, western Minnesota, and=20
northeast South Dakota.


...Great Lakes, Appalachians, & Northeast...
Days 2-3...

A complicated yet highly anomalous upper level closed low will
spawn the first winter storm of the season for parts of the
northern Mid-Atlantic. First off, a potent 500mb low over the
northern most portions of Michigan's Mitten will dive south on
Thursday and lead to a strong surge in both NW winds off Lake
Michigan and strong lift aloft ahead of the 500mb low. Periods of moderate-to-heavy snow are possible from the northern Michigan
Mitten on south to the Michiana region. WPC PWPF shows moderate
probabilities (40-60%) for snowfall totals topping 4" in the
Michiana region through Thursday afternoon.=20

Meanwhile, a new wave of low pressure will form along a triple=20
point (occluded front/warm front/cold front convergence) somewhere=20
near the Jersey Shore early Thursday morning. The storm will be=20
located beneath the divergent left-exit region of a 120kt jet=20
streak and at the nose of a vigorous 2PVU lobe that will pivot=20
north towards the Lower Hudson Valley Thursday evening. As the=20
850mb low deepens Thursday afternoon, rich 850mb theta-e air off=20
the Atlantic will be drawn west and wrap around the storm's mid=20
level circulation Thursday evening. Intense 850-700mb WAA will=20
prompt the development of a robust deformation zone of heavy=20
precipitation. Given the strong vertical velocities aloft within=20
the band, precipitation will changeover to all snow and fall at a=20
heavy clip Thursday night and into Friday morning from north-=20
central PA on east to the Poconos and Catskills.=20

Despite the large scale circulation and more guidance coming into
better agreement on track, a storm like this usually contains an=20
intense deformation axis that may result in subsidence in areas
neighboring the band of heavy snow. This can either cut back
available QPF, or given the marginal thermals in the boundary=20
layer, cannot sufficiently cool the atmospheric column enough to=20
keep precipitation as all snow. Still, the signal is growing=20
stronger for heavy snowfall beneath the deformation zone with=20
snowfall rates of 1-2"/hr likely. Also, strong NE-E flow should=20
provide strong orographic lift into portions of the Catskills and=20
Poconos, making these locations more likely to receive heavy=20
snowfall. One spoiler would be any dry slot wrapping in from the=20
east at mid-upper levels that hinders the atmosphere's ability to=20
produce dendrites. Given these factors, there is still room for=20
adjustments in the snow forecast. However, with such a favorable=20
atmospheric setup and strong dynamics at play, this is likely to=20=20
be the first impactful winter storm of the season for parts of the=20
northern Mid-Atlantic. WPC PWPF shows moderate-to-high chances=20
(50-70%) for >8" of snowfall in parts of the Poconos. WSSI-P sports moderate-to-high chances (50-70%) for Minor Impacts throughout=20
much of northeast PA and the southern tier of NY.

Farther south, a multi-day upslope flow event is set to occur from
Thursday and last through Friday night. Thursday's snowfall will=20
be more light-to-moderate but pick up in intensity on Friday as a
500mb vort max rotating on the western flank of the large upper low
brings both an increase in synoptic scale lift, as well as stronger
850-700mb winds and an increase in moisture at those same levels.
Snowfall rates on Friday could range between 1-2"/hr at their peak
with rapid accumulations possible on roads. Areas most heavily
affected are likely to be the Laurel Highlands, the Allegheny
Mountains, and into portions of the Potomac Highlands. WPC PWPF=20
shows high chances (>70%) for >8" of snowfall in these ranges=20
through Friday night with some moderate-to-high odds (50-70%) for=20

12" in east-central WV above 2,000ft in elevation.

The probability of significant ice across the CONUS is less than
10 percent.

Mullinax




...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!-7ngsBVU9SNRkr2tzZUVrFs4dSKOfWBxtmjtY6_-gw6pb= BQsFk_FbuzAaYgLd1e_wiNNkSa0kxeFx589XkJi30Pjqrg$=20

$$

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FOUS11 KWBC 201929
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
229 PM EST Wed Nov 20 2024

Valid 00Z Thu Nov 21 2024 - 00Z Sun Nov 24 2024

...Pacific and Interior Northwest...
Days 1-3...

Persistent moisture advection within an intense atmospheric river
/AR/ will drive widespread precipitation, including high elevation
snow, across the Northwest into the weekend.

The period begins with an amplified closed low positioned off the
British Columbia coast. This low will feature impressive height
anomalies of -2 to -3 sigma according to NAEFS ensemble tables, but
will gradually fill as it retrogrades back to the northwest. As
this occurs, a secondary shortwave pivoting around the base of this
amplified trough will deepen and pivot eastward, deepening to
feature renewed impressive height anomalies and an accompanying
surface low approaching the WA coast by Friday afternoon.

Downstream of this low, mid-level ridging blossoming across the
Rockies will result in pinched/gradient flow to enhance wind speeds
and warm/moist advection onshore, with robust divergence across=20
the Northwest overlapping with periods of strong jet dynamics to=20
produce strong ascent across the region. At the surface, a cold=20
front will be pushed towards the coast, but is expected to stall=20
just offshore until Friday when the secondary low will finally=20
advect it eastward. This indicates that the most impressive IVT=20
plume, for which both GEFS and ECENS feature high probabilities for
exceeding 750 kg/m/s, will persist into northern CA and push as=20
far east as the Great Basin and Northern Rockies as noted by PW=20
anomalies from NAEFS reaching above +1 sigma into Friday. During=20
this time, however, snow levels will rise dramatically, especially=20
within the core of the most intense IVT, surging to 4000 ft in the=20
Northern Rockies and as high as 9000 ft in CA. Although snow levels
will remain more modest in WA, they will still rise to generally=20
above pass level in most areas, turning snow to rain across a lot=20
of the region, and leaving the highest snowfall confined to the=20
higher peaks. WPC probabilities are above 70% for 6+ inches D1 in=20
the Shasta/Trinity region, the OR Cascades, and most impressively=20
from the Blue Mountains through the Salmon River/Sawtooth region.=20
On D2 this pivots to be focused from the highest terrain of the WA=20
Cascades (near Mt. Rainier) into the Northern Rockies near Glacier=20
NP.

During D3, the pattern begins to evolve as a wave of low pressure
develops across the interior NW along the advecting, finally,
baroclinic gradient to the east. This will interact with a cold
front digging out of Canada to enhance frontogenesis along the
Canadian border near the Northern Rockies, which will interact with
an upper level jet streak to push a swath of heavy snowfall from
eastern OR through eastern MT. Here WPC probabilities are renewed
above 50% for 6+ inches in the higher terrain, with some low
probabilities for 4 inches extending into the High Plains of MT
along the international border. Otherwise, snowfall is expected to
wane D3 across the West, at least briefly, as the most robust IVT=20
pivots south and east away from the area, but snowfall will develop
once again in the Sierra.



...Great Lakes, Appalachians, & Northeast...
Days 1-3...

Challenging forecast into the weekend across much of the Great
Lakes, Mid-Atlantic, and Northeast as a complex upper low evolves
across the region.=20

The period begins with a sprawling upper low centered over
Wisconsin forcing a longwave trough across much of the eastern half
of the CONUS. Vorticity lobes spinning around and within this large
gyre will cause the upper low center to wobble gradually eastward
into Friday while deepening to as much as -3 to -4 sigma across the Mid-Atlantic according to NAEFS ensemble tables. Beneath this
trough, dual surface lows are progged to develop and retrograde in
response to the amplification of the upper pattern, one pivoting NW
and then eventually SE into the Ohio Valley, while a secondary low
develop along the triple point south of New England and rotates NW
into the Hudson Valley before finally advecting eastward to the
coast of Maine by Saturday morning. THere remains uncertainty into
the track and intensity of both of these features, but the general
trend in guidance has been for locally deeper lows driving more
intense ascent, with a track a bit farther south/west than previous
model runs.

This will result in two areas of heavy snow. The first, and region
of highest confidence, is across the Central Appalachians,
beginning Thursday aftn and persisting with rounds of snow until
Friday night. The onset of snow across this area will be due to
post-frontal upslope flow which will maintain saturation within the
deepening DGZ on CAA. This will result in heavy snow, especially
above around 1500 ft, from the Laurel Highlands southward along the Appalachians and as far south as the Great Smokey Mountains/Blue
Ridge of NC. As the low from Michigan shifts southward Thursday
aftn into Thursday evening, it will begin to weaken, but additional
ascent, especially with any modest deformation on its south side,
could result in additional areas of heavy snow from Michigan,
through the Ohio Valley, and then enhance ascent into the
Appalachians. WPC probabilities across the Ohio Valley are
generally 10-30% for more than 4 inches, highest near Lake Michigan
where some enhancement may occur. In the Appalachians, WPC
probabilities are high on D2, and moderate on D3, for 6+ inches,
highest in WV where locally close to 2 feet of snow is possible.

The more challenging aspect of this forecast involves the secondary
low development progged to occur off of New England Thursday
afternoon which will then retrograde NW towards Upstate NY before
moving back off the NJ coast Saturday morning. This low will likely
deepen in a region of impressive synoptic ascent, and the setup
supports an intense deformation axis developing on the west side of
this low as it pivots to the NW Thursday evening into Friday. This
deformation will overlap with some impressively sloped 925-700mb
fgen noted in cross sections, driving ascent into the DGZ and into
areas of conditional instability reflected by pockets of low or=20
even negative SEPV to support convective snow rates. The forecast=20
soundings indicate this will be purely a rain/snow event, however,=20
a potent dry slot will attempt to rotate cyclonically around the=20
system as well, which could dry out the DGZ and cutoff the snow,=20
but latest guidance has backed off on the westward extent of this=20
dry slot, so confidence is increasing that periods of intense=20
snowfall will rotate across Upstate NY, PA, resulting in heavy snow accumulations above 1500 ft as reflected by WPC probabilities for=20
more that 4 inches reaching 70% in the Catskills and Poconos, where
above 1500 ft as much as 8-10" of snow could fall. This snow is=20
expected to be heavy and wet as well, (low SLR), which could=20
produce power outages and damage to trees, as reflected by 40%=20
WSSI-P probabilities for moderate impacts, driven primarily by snow
load.

Farther south, as well as in lower elevations, the incoming models
have become a bit more aggressive with snowfall, but the marginal
thermal structure suggests precipitation will be generally a mix of
cold rain and snow, except during periods of more intense rates.
This creates a very low confidence forecast at lower elevations,
and this is additionally reflected by high standard deviations in
the PWPF snowfall. Currently, WPC probabilities for at least 1 inch
are above 10% across much of PA as far south as the MD border and
towards I-95 where some snow may occur as far southeast as
Philadelphia and Baltimore.


The probability of significant ice across the CONUS is less than
10 percent.

Weiss



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!49V5vVCr4UzZ5bTI7Oe5sUgS__wpdK841tT49W9jfbovi= exAUew1-M510l3YTo6BKyzHPKjQalzleVKzr-U20VhVayE$=20

$$

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FOUS11 KWBC 210829
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Thu Nov 21 2024

Valid 12Z Thu Nov 21 2024 - 12Z Sun Nov 24 2024

...Pacific and Interior Northwest...
Days 1-3...

The massive closed upper low over the northeast Pacific will
continue to support a robust atmospheric river into the the West
Coast and as far east as the Northern Rockies through Friday night.
On the heels of the historic cyclone that produced a myriad of
hazards in the Pacific Northwest the past 24-48 hours, another
potent storm system will strengthen rapidly Thursday night off the
Pacific Northwest coast. By 06Z Fri, the ECMWF SAT shows an IVT
topping 1,000 kg/m/s aimed at the northern California coast. While
this would be a recipe for heavy snow more often than not, but the
intense WAA over the past 24 hours as all but left snow levels to=20
mainly above 5,000ft in the Cascades and Olympics, above 6,000ft in
the Shasta/Trinity mountains, and above 7,000ft in the Sierra=20
Nevada. Of the West Coast mountain ranges, the Sierra Nevada will
become the primary focus for heavy snow late Friday and through
Saturday morning as the atmospheric river trajectory becomes=20
positioned farther south into the Golden State. The atmospheric
river finally relents later in the day on Saturday and a lull in
the active pattern should settle in by Saturday night. WPC PWPF
shows high chances (>70%) for snowfall >12" for elevations

7,000ft. In totals, most of the central and southern Sierra Nevada
7,000ft can expect an additional 1-2ft of snowfall with the=20

tallest peaks above 9,000ft possibly approaching 3ft of snow.

Areas with better opportunities for heavy snowfall at lower=20
elevations are farther north into the Columbia Basin and even east
of the Northern Rockies in northern Montana where colder=20
temperatures locked in place by high pressure to the north will=20
allow for precipitation to fall in the form both snow and freezing=20
rain. WPC PWPF shows moderate-to-high chances (50-70%) for >4" of=20
snowfall along the Montana/Canada border through Sunday morning. In
the mountains, however, snowfall jumps up dramatically with=20
portions of the Lewis Range, Sawtooth, Boise, and Blue mountains=20
measuring as much as 1-2 feet of snow with localized amounts=20
approach 30".=20


...Great Lakes & Appalachians...=20
Days 1-2...

This morning, a massive upper low over the Great Lakes containing a
pair of potent 500mb vorticity maxima will be responsible for heavy
snow in the Central Appalachians and across the northern Mid-
Atlantic. Periods of lake enhanced snowfall will continue over the
Michiana area, while the one of the 500mb vort maxima tracking
south from the northern most portion of Michigan's Mitten aids in a
band of moderate-to-heavy snow in southeast Wisconsin and across
the greater Chicagoland metro area.=20

As the leading 500mb vort max tracks over the Central Appalachians
Thursday afternoon, the large 500mb low will continue to deep with
heights at 500mb, 700mb, and 850mb that are near the lowest=20
observed 06Z Nov 22 heights observed in the CFSR climatological=20
record (1979-2009). As the upper low tracks east, brisk and moist=20
westerly 850-700mb winds allows for periods of moderate to heavy=20
snow to break out along the Laurel Highlands, Allegheny Mountains,=20
and even as far south as the Smokeys of NC/TN. Snowfall increases=20
in intensity in the Laurel Highlands and Allegheny Mountains=20
Friday morning when NW flow not only strengthens but advects a=20
greater concentration of 850-500mb moisture into the Central=20
Appalachians. Snowfall rates of 1-2"/hr are possible during the day
Friday. Snowfall will gradually subside Friday night with only=20
some very light snow in parts of east-central WV leftover by=20
Saturday morning. WPC PWPF shows high chances (>70%) for snowfall=20

12" from the Laurel Highlands on south through east-central WV.=20

Some guidance shows low chances (10-30%) for snowfall totals >24"=20
in the highest terrain of east-central WV by the time the event=20
concludes Saturday morning. The WSSI shows Moderate to Major=20
Impacts in these areas driven largely due to the sheer amount of=20
snow in the forecast, as well as a higher Snow Load component given
the more wet/heavy snow-type expected. This could result in some=20
localized power outages and tree damage, especially when paired=20
with 40kt 850 mb winds on Friday. Travel will be hazardous to=20
impossible in areas with Major Impacts.

...Northern Mid-Atlantic...
Days 1-2...

***The first winter storm of the season for the interior Mid-
Atlantic begins this evening and lingers into the day on Friday**

In the northern Mid-Atlantic, a dynamic storm system will take
shape this afternoon as a vigorous 2PVU anomaly working in tandem
with the left-exit region of a 250-500mb jet streak will strengthen
low pressure off along the triple point just off the Jersey Shore.
As this takes shape and an 850mb low consolidates near the NYC
metro area, a plume of rich 850-700mb moisture off the Atlantic
will wrap around the northern and western flanks of the storm. At
the same time, this moisture advection is a byproduct of intense
WAA at those same levels. As the moisture ascends along an area of
850-700mb frontogenesis, a comma head of heavy precipitation will=20
take shape as early as midday today from eastern PA and the=20
Southern Tier of NY to the Lower Hudson Valley and northern NJ.
Once precipitation rates increase via increasing vertical=20
velocities and the cold temperatures above 800mb arrives that=20
afternoon, the profile will be sufficiently cold enough to produce=20
heavy snow initially in the Poconos and Catskills Thursday=20
afternoon, followed by lower elevations across the Southern Tier of
NY through Thursday night.=20

Even 24 hours out from the peak of the event, there are still some
lingering question marks in this setup that will limit the=20
potential for a more expansive snow storm. The first is the air-
mass leading up to this storm has been mild and soil temperatures=20
remain rather warm. The other is these compact and quickly evolving
storm systems tend to generate a mid- level dry slot. This dry=20
slot should make its presence known by 06Z Fri with central NY=20
likely getting influenced by the dry slot first, then possibly as=20
far west as the Binghamton area Friday morning. While this should=20
limit the areal coverage of heavy snow in central NY, the=20
impressive rates along the PA/NY border and on south into the=20
Poconos will have an expansive areas of wrap around moisture to=20
work with. In addition, HREF guidance shows up to a 9 hour window=20
where these areas could see 1-2"/hr snowfall rates late Thursday afternoon
and into Thursday night. Even with warmer soil temps, while the=20
first inch or so may melt or compact quickly, eventually these kind
of rates should win out and quickly cool the surface to freezing.=20
This would then support rapid accumulations, especially in the=20
higher terrain of the Poconos. But even some of the Valleys could=20
see some heavier totals beneath the deformation zone Thursday=20
night.

By Friday morning, the wrap around moisture at 700-300mb will
rotate as far south as the DC/Baltimore metro area. Some latest
CAMs show an almost squall-line type evolution Friday morning that
could bring a burst of snow to parts of northern MD and possibly
into the DC/Baltimore metro areas closer to midday. In PA, moist
westerly flow beneath this behemoth of a 500mb low (NAEFS show=20
700mb heights over southeast PA that are near the lowest observed=20
heights for this time of year in the CFSR database), will keep=20
periods of snow around until the strong vertical velocities=20
subside. Accumulations beyond midday Friday will be difficult with=20
the lone exceptions being the higher terrain of the Poconos and=20
Catskills. WPC PWPF shows moderate-to-high chances (50-70%) for=20
snowfall >8" in the Catskills, Poconos for elevations >1,500ft.=20
Similar probabilities are present for >4" for elevations >1,000ft=20
in Northeast PA and along the NY/PA border. Note that these totals=20
and probabilities are tied strongly to elevation, with the one=20
disclaimer that some lower elevations in northeast PA could=20
overperform as a result of this deformation band Thursday evening.

Lastly, there is the potential for the first sight of snow in parts
of the NYC/Philly/Baltimore/DC metro areas. While most snow in the
immediate metro areas will be primarily conversational, there are=20 low-to-moderate chances (30-50%) for >1" of snowfall to the west=20
of these metro centers across northern MD, the Lower Susquehanna=20
Valley, and into northern NJ.

...Northern New England...
Days 2-3...

Along the Adirondacks, Green, and White Mountains, the
aforementioned upper low will draw moist easterly flow northward
and into these ranges Thursday night and into Friday. Only the
peaks of these ranges are likely to see measurable snowfall (Mt.
Washington included) but by Saturday, a new wave of low pressure
well off the East Coast will send another plume of moisture towards
northern New England. Uncertainty in amounts is high given the
spread in guidance, but the WPC PWPF currently shows low-to-
moderate chances (30-50%) for snowfall totals >4" from the White
Mountains to the western most border of Maine. Travel conditions
could be hazardous in some areas at minimum, but dependent upon
trends in guidance the next 24-36 hours, it is possible for=20
snowfall to come up in these areas.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax




...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!9XYMFtjW_r-Hf4p9-utAYgVfpGciYXvvMQbJruKR24n_F= v10p6NZMnX0PgBG82KKCGykt6BS2DyR4YgcNhQ8m8014GM$=20

$$

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FOUS11 KWBC 211901
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
201 PM EST Thu Nov 21 2024

Valid 00Z Fri Nov 22 2024 - 00Z Mon Nov 25 2024

...Pacific Coast through the Interior Northwest and Great Basin...
Days 1-3...

Persistent closed low centered off the Pacific Northwest coast will
become re-energized by vorticity maxima swinging through the
associated trough this period. This will result in amplification=20
of the gyre, noted by an expansive region of 500-700mb height=20
anomalies reaching below -1 sigma, and approaching -3 sigma in the=20
core of the trough. Beneath this trough, deepening surface low=20
pressure will approach the WA coast Saturday, but likely get pulled
back to the west before landfall in response to secondary=20
shortwave energy digging to its south. This will result in=20
persistent onshore and divergent mid-level flow, accompanying=20
upper level jet energy to spread moisture onshore as a continuation
of the ongoing atmospheric river (AR) through Saturday. While=20
probabilities for IVT exceeding 500 kg/m/s wane and pivot southward
through D2, this will still result in plentiful moisture and=20
widespread moderate to at times heavy precipitation pushing onshore
the Pacific Coast into the interior Northwest.

Snow levels within this AR will be quite high, generally 6000-8000
ft, except east of the WA Cascades on D1 when they will remain
entrenched around 4000 ft before rising. This indicates that most
of the heavy snow will be above pass levels, except in the higher
Cascade Passes D1, and the Sierra Passes D2. WPC probabilities D1
are high for more than 6 inches of snow in the WA Cascades, the
Salmon River Range, and portions of the Northern Rockies near
Glacier NP. On D2, snowfall is more robust and expansive, with a
greater than 90% chance of 6 inches again in the Northern Rockies
and Salmon River/Sawtooth Ranges. More impressively, the Sierra
will be favorably located to be impinged upon by moisture D2,
leading to pronounced upslope snow. WPC probabilities are above 80%
for 12+ inches on D2, with 2 feet possible above 8000 ft.

By D3, snow levels crash again as a cold front pushes onshore, but
this is accompanied by reduced moisture advection as the column=20
begins to dry out. Some moderate snow will persist through onshore
flow, especially in the Sierra where an additional 6-12 inches is
possible, but otherwise WPC probabilities for more than 6 inches
are between 30-50% and focused across the Wasatch, Uintas, and
ranges of NW WY.


...Great Lakes, Appalachians, Northeast...=20
Days 1-3...

Impressive upper low over the Great Lakes will feature a complex
evolution through the weekend resulting in multiple surface low
pressure and varying areas of heavy snow.

The first surface low will track south across the Ohio Valley and
towards the Central Appalachians tonight into Friday. Immediately
downstream of this feature, a band of moderate to at times heavy
snow is likely, but the fast progression and marginal thermal
structure of the antecedent column will limit overall
accumulations.=20

The exception to this will be across the Central Appalachians
especially from the Laurel Highlands southward across WV and as far
south as the Great Smokey Mtns of TN where prolonged upslope flow=20
ahead of this wave, combined with the wave itself, and then=20
secondary upslope flow on Friday night will result in waves of=20
heavy snowfall. WPC probabilities in this area are high (>70%) on=20
D1 for more than 8 inches of snow, with probabilities for an=20
additional 4+ inches on D2 reaching 30-50%. Event total snowfall=20
could peak around 2 feet in central WV. Otherwise, snowfall at=20
lower elevations and into the Ohio Valley should be 1-3" or less.

A second, and likely stronger area of low pressure developing south
of New England today will retrograde back to the NW while deepening
as it remains embedded within the large 500mb gyre overhead. As
this occurs, an impressive plume of low-level moisture will wrap
northwest into the system, driving rich theta-e advection into a
TROWAL to support some elevated instability and enhance available
moisture for heavy precipitation. During this evolution,
deformation will increase on the W/SW side of the system as it
becomes exceptionally wrapped, with CAA on the southern side
leading to improved fgen response beneath this deformation axis.
This setup will support intense mesoscale ascent and bands of heavy precipitation rotating W/SW as the low retrogrades. Although the
column will be marginally supportive of accumulating snow outside
of higher elevations (above about 1500 ft), the intense lift should
dynamically cool the column and allow efficient snowfall
accumulation at times even in lower terrain. SLRs for this event
will likely be below climo due to near freezing temps and warm soils/compaction, but still, rates of 1-2"/hr will promote heavy
accumulations, especially in the Poconos and Catskills and
surrounding areas. WPC probabilities D1 are high (>70%) for 8
inches, with 2-day snowfall potentially reaching above 12" in a few
spots. This will likely result in dangerous travel and impacts to infrastructure including scattered power outages.

In the lower elevations of the Mid-Atlantic, WPC probabilities are
high for more than 2 inches as far west as near BUffalo, NY and
central PA, but locally higher totals are likely where these bands
pivot. Additionally, the guidance has become more aggressive
pinwheeling some snow bands as far SE as the I-95 corridor, so
while accumulations in the major urban areas from Washington, D.C.
to New York City are expected to be minimal, if any, these cities
could see their first "falling" snow of the season Friday
morning/aftn.

Along the Adirondacks, Green, and White Mountains, the
upper low will draw moist easterly flow northward and into these=20
ranges Friday night through Saturday while yet a third surface low
develops east of Maine and pivots northwest. Snow levels should be
high enough that only the highest terrain will receive significant
snow, but WPC probabilities have increased and now feature a 30-50%
chance on D2 and D3 of more than 4 inches, primarily in the White
Mountains of NH and the peaks of northern ME.


...Northern Rockies through the Northern Plains...
Days 2-3...

A shortwave ejecting through the base of an anomalous trough off
the Pacific Coast will swing eastward into the Northwest Saturday
and then begin to amplify into a potent but fast moving impulse
across the Northern Rockies and into the Northern Plains Sunday.
This deepening system will drive downstream ascent through height
falls, PVA, and divergence. At the same time, a zonally oriented
jet streak will pivot south of the region, placing its favorable
LFQ atop the High Plains, interacting with increasing baroclinicity=20
as a front wavers near the international border. The overlap of=20
synoptic lift with the enhanced mesoscale ascent through fgen will
result in weak cyclogenesis, leading to an expanding area of
precipitation falling as snow. The column will be plenty cold for
wintry precipitation thanks to high pressure from Canada extending
southward, and as the WAA along the warm front lifts north, it will
result in an exceptionally deep DGZ to support fluffy aggregate
dendrites which should accumulate efficiently.=20

The heaviest snowfall is likely in the terrain near the Northern=20
Rockies where upslope flow will enhance ascent, and WPC=20
probabilities support a high risk (>70%) for at least 8 inches near
Glacier NP both D2 and D3. However, heavy snowfall is also likely=20
farther east across northern MT and into ND where the most robust
synoptic ascent will materialize. The guidance has trended a bit
farther north today, but current WPC probabilities suggest around a
30% chance for more than 4 inches along the international border as
far east as central ND through D3.


The probability of significant ice across the CONUS is less than
10 percent.

Weiss



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!-bg4qMZApFANi5aI4LOp1gLRAWvjj6xu6mo7ij0oJQkqs= BMxdRDaVjHO-eXmTr0QjBSAl_c55CLD_MAc23HLEFUVE0A$=20

$$

=3D =3D =3D
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