FOUS11 KWBC 270856
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Mon Jan 27 2025

Valid 12Z Mon Jan 27 2025 - 12Z Thu Jan 30 2025

...Great Lakes, Northeast, Mid-Atlantic... Days 1-3...

An active period of winter weather is expected through mid-week as
progressive flow drives multiple impulses with rounds of snow from
the Great Lakes through the Northeast.

The period begins with a lead shortwave exiting New England into
the Atlantic, leaving sharp NW CAA in its wake. THe most impressive
CAA is likely to linger primarily across Lake Ontario and into
Upstate NY, at least early in the period before a subtle surge of
WAA occurs ahead of the next front. This will result in a band of
heavy LES waning right at the start of the period, with minimal
additional LES accumulation.

However, the respite will be short as a potent shortwave,
basically demarcating the edge of the larger cyclonic gyre digging
south from Hudson Bay will drop south Monday night. This will
drive a potent cold front southward, first into the U.P and L.P. of
MI late Monday evening, and then continue to race east through the
interior Northeast and New England Monday night into Tuesday
morning. Impressive CAA along the front will drive an intense
baroclinic boundary, leading to a stripe of impressive low-level
fgen, especially in the 925-850mb layer. This will drive strong
mesoscale lift into a saturating column reflected by high low-level
RH, and as lapse rates steepen, some weak to modest instability
will develop. The high-res guidance has become more aggressive
showing a band/corridor of convective snow showers and snow
squalls, with HREF probabilities showing a 20-30% chance for 1"hr
snowfall rates within these elements. The accompanying steep lapse
rates will help mix down strong winds, encouraging the development
of low visibility during heavy snow showers. The limiting factor to
true snow squalls may be a relative dearth of 0-2km CAPE, and it
is beneficial that the most significant impacts will occur at
night, but still, convective snow showers and isolated snow squalls
are expected Monday night into Tuesday morning from the U.P. of
Michigan through much of the interior Northeast and New England,
and potentially as far southeast as the Mid-Atlantic.
Accumulations during this time will generally be minor, but some
LES and some upslope flow with the front across the Adirondacks has
resulted in WPC probabilities reaching 70-90% for 4 inches on D1,

Some modest LES will follow in the wake of this front, but more
significant and widespread snow will occur D2 into D3, as yet
another fast moving impulse dives through the broad cyclonic flow
across the east. This third impulse will track along the residual
baroclinic gradient left by the potent cold front, and will be
overlapped by a strengthening jet streak downstream of the trough
axis to produce more robust deep layer ascent. This will support
modest cyclogenesis as a clipper-type low swings southeast from
Minnesota Tuesday aftn, to Upstate New York Wednesday morning, and
then across New England, exiting into Atlantic Canada by Wednesday
evening. Brief but impressive WAA downstream of this low will
result in expanding precipitation falling as moderate to heavy snow
across many areas.

Then, in the wake of this system, additional LES will develop as
NW flow causes additional accumulations in the favored NW snow
belts, and with strong winds, some of this could push as far SE as
the coastal Mid-Atlantic states by the end of the forecast period.
WPC probabilities D2 are high (70-90%) for 4+ inches across the
U.P., the NW L.P. near Traverse City, and east of Lakes Erie and
Ontario, with a swath of moderate probabilities for 2+ inches
surrounding those areas for the more synoptically forced snowfall.
For D3, the greatest risk for 4+ inches of snow shifts into the
Adirondacks and Greens where WPC probabilities feature a high risk

70%) while additional modest LES occurs east of Lakes Erie and

Ontario. Locally 1-2 feet is possible in some areas through the period.

Finally, a second round of convective snow showers or snow squalls
is becoming more likely on Wednesday, this time a little south of
the event Tuesday morning. These convective snow showers will occur
during a more favorable time of day to support better CAPE behind
the secondary front, and the SnSq parameter is suggesting a risk
across areas from eastern OH through southern Upstate NY and PA.
This secondary set of squalls, if they occur, will again be
accompanied by strong winds and heavy snow rates leading to the
potential for significant travel impacts despite modest snowfall
accumulations.

The two rounds of potential snow squalls have prompted the issuance
of Key Messages linked below.

...Southwest through the Four Corners... Days 1-3...

The anomalous upper low pivoting slowly across CA will begin to
slip east this week, bringing snowfall across the Southwest US
through Thursday. This cutoff begins the period quite amplified,
with 500-700mb heights as low as -2 sigma over CA according to the
NAEFS ensemble tables. Two distinct spokes of vorticity will dance
around the core of this upper low, helping to elongate it with
time as it becomes stretched NE to SW into the Four Corners by
Tuesday night. The interaction of these vorticity impulses
rotating around the closed center will help keep the low amplified,
but also very slow moving, reaching NM/CO by the end of the
forecast period.

During this synoptic evolution, the upper pattern will alter
considerably as well. Initially, a strong 250mb jet streak will be
arced downstream of the upper low, lifting northeast into the
Southern Plains. However, this feature will weaken D1 as the upper
low elongates, leaving less substantial diffluent-caused ascent
into D2. However, a secondary surge of jet level energy will occur
Wednesday into Thursday as the jet streak re-amplifies and arcs
meridionally from the Gulf of California into the Southern Plains,
placing intense LFQ diffluent ascent over the Four Corners. This
jet evolution, combined with the mid-level closed low, and at least
subtle low-level baroclinicity in the vicinity of a weakening front
will cause waves of low pressure to move eastward from CA to the
Four Corners D1-D2. This will cause periods of snowfall from the
Transverse and Peninsular Ranges of CA eastward through the
Mogollon Rim and Kaibab Plateau where WPC probabilities are
moderate (30-50%) D1 for 4 inches of snow, and high (>70%, but only
in higher elevations of AZ) on D2.

However, the most significant snowfall is likely to begin D3 as a
low pressure develops near the TX Big Bend and moisture advection
surges on isentropic ascent across TX. The accompanying theta-e
ridge is progged to lift cyclonically into a TROWAL, with the most
intense theta-e advection being directed into the DGZ over NM/CO.
There is still quite a bit of uncertainty regarding the placement
and evolution of this system, but snowfall chances are increasing,
especially in the higher terrain above 5000 ft in the Sangre de
Cristos and San Juans. Here, WPC probabilities D3 are as high as
70% for 6+ inches, and impactful snow is becoming more likely for
parts of this area Wednesday into Thursday.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.

Weiss

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

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
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FOUS11 KWBC 280709
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
209 AM EST Tue Jan 28 2025

Valid 12Z Tue Jan 28 2025 - 12Z Fri Jan 31 2025

...Great Lakes, Northeast, Mid-Atlantic... Days 1-2...

Progressive pattern continues across the east as a cyclonic gyre
centered near the Hudson Bay drifts east, but vorticity lobes
shedding around it will maintain/re-amplify the upper trough into Thursday.

The first of these shortwaves will be racing east across Upstate NY
and northern New England this morning, pushing an arctic front
southeast in tandem. Scattered convective snow showers and isolated
snow squalls will remain possible along this front through the
morning hours, with briefly heavy snow rates above 1"/hr and gusty
winds combining to produce scattered impacts as far east as I-95 to
NYC and Boston. However, the intensity of these features is likely
to be much less substantial than occurring overnight into Tuesday morning.

Behind this front, a brief respite in snowfall will occur before a
second impulse digs rapidly from Alberta, Canada towards the Great
Lakes and Northeast. The interaction of this shortwave trough with
the leftover baroclinic boundary behind the cold front will yield
surface cyclogenesis in the form of an "Alberta Clipper" which is
progged to track quickly from the Arrowhead of MN this afternoon
through the Upper Great Lakes tonight, and then across New England
on Wednesday. Weak secondary development is possible east of Massachusetts/Maine Wednesday as well, but should have marginal
impact on the winter impacts. Ahead of this clipper, intensifying
SW flow will drive enhanced WAA and moist isentropic ascent,
especially along 285-290K surfaces, to produce an expanding swath
of moderate to heavy snow. The DGZ becomes exceptionally deep
during this time as reflected by SREF probabilities of 90% for
100+mb of depth during the period of most impressive WAA. This will
create a near iso-thermal layer within the -12C to -18C
temperatures, explaining this deep DGZ. PWs are progged to exceed
the 97th percentile over MN/MI according to NAEFS, which will help
additionally enhance snowfall, and despite PWs falling to the east
with time, a period of heavy snow is likely within this downstream WAA.

Then, in the wake of this system and behind a subsequent cold front
Wednesday morning, lake effect snow (LES) will develop as NW flow
causes additional accumulations in the favored NW snow belts. With
strong winds progged in forecast soundings, some of this could
push as far SE as the coastal Mid-Atlantic states by Wednesday
night. WPC probabilities D1 and D2 for areas that receive both the
WAA snow and then the subsequent LES are moderate to high (50-90%)
for 4+ inches from the western U.P. southeast through Traverse City
area, east of Lakes Ontario and Erie, and into the western
Adirondacks, Greens of VT, and Whites of NH. Storm total snow of
12-18" is possible across the U.P. and the Tug Hill, with 6-12" in
lollipops elsewhere across this swath, especially in the higher terrain.

Finally, a second round of convective snow showers or snow squalls
continues to look likely Wednesday. These convective snow showers
will occur during a more favorable time of day to support increased
CAPE forecast to reach 100-200 J/kg in the 0-2km layer, overlapping
some impressive low-level fgen from central New England, Upstate
NY, and into the northern Mid-Atlantic states. This secondary set
of squalls will again be accompanied by strong winds and heavy
snow rates leading to the potential for significant travel impacts
despite modest snowfall accumulations.

After this second front pushes east and snow squalls wind down
Wednesday evening/night, much of D3 will be quiet across the
region. A larger scale storm system is likely to approach late D3
from the south with increasing moisture and some light mixed
precipitation, but at this time any impacts from that event are
most likely into D4, just beyond this forecast period, and WPC
probabilities for 0.01" of ice peak around 50% across northern PA
before the end of the forecast period.

The two rounds of potential snow squalls continue the issuance of
Key Messages linked below.


...Southwest through the Four Corners... Days 1-3...

Modestly anomalous upper cutoff low (NAEFS 700-500mb heights
falling to below the 10th percentile of the CFSR climatology) will
roll slowly eastward from southern CA this morning. This feature
will move slowly (such is the nature of cutoffs) across the Desert
Southwest and Four Corners Wednesday before pinwheeling into the
Central Plains on Thursday. The exact track and placement of this
upper low remains uncertain due to vorticity lobes which will
periodically dance around the central gyre, tugging it in subtly
different directions at different times, but the large scale
pattern is well agreed upon at this time by the various global models.

This evolution will result in pronounced synoptic forcing across
the region from west to east, primarily due to height falls,
divergence, and periods of PVA. However, other forcing at play
will enhance deep layer ascent. This includes increasing LFQ
diffluence as a subtropical jet streak intensifies downstream of
the closed low, especially Wednesday into Thursday across NM/CO,
and periods of upslope flow embedded within isentropic ascent.
There has also been an increase this morning in post-system
deformation as a surge of wrap-around precipitation develops over
eastern CO Thursday morning and then pivots southward. These
parameters together should produce sufficient lift in an area of
increasing moisture to produce warning-level snow above generally
4000-5000 ft in the Sangre de Cristos, San Juans, and Jemez
Mountains. However, there remains some uncertainty as to the
movement of an accompanying dry slot, which could lower
accumulations in some areas.

WPC probabilities for 4+ inches on D1 are minimal, but by D2 expand considerably and amplify into the San Juans and Sangre de Cristos,
where they reach as high as 70-90% for 6+ inches, with locally over
1 foot possible in the highest terrain. Additionally WPC
probabilities are high (70-90%) for 4+ inches across the Raton Mesa
and higher elevations of I-25 near the NM/CO border. By D3 the
event begins to ramp down, but WPC probabilities for 4+ inches
above 30% continue in the Sangre de Cristos, and expand into parts
of the Palmer Divide as well.


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

A trough moving eastward across the Northern Pacific will generate
an intensifying jet streak downstream, pivoting ascent and moisture
into the region after 00Z Friday. This will manifest as a corridor
of enhanced IVT as high as 500 kg/m/s (above the 90th CFSR
percentile according to NAEFS) reaching as far inland as Idaho
before the end of the period. This will drive an expanding area of precipitation spreading eastward, but with the accompanying WAA
driving snow levels to as high as 5500 ft, much of this will fall
as rain. However, in the Olympics and Cascades, a heavy wet snow is
likely, which could accumulate to more than 4 inches before 12Z
Friday as reflected by WPC probabilities reaching 30-50% in these areas.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.

Weiss

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

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
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