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QPFHSD

Probabilistic Heavy Snow and Icing Discussion...UPDATED
NWS Weather Prediction Center College Park MD
344 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.

As previously mentioned, the base of the trough crosses northern CA
late Monday. It then remains a potent vort lobe crossing south-
central NV Monday night and then southern UT Tuesday morning. QPF
guidance is bullish on this track bring favorable for the Wasatch
with Day 2 PWPF for >6" 60-70% around Park City with snow levels
dropping from 7000 to 5000 ft with the cold frontal passage.


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


Jackson


$$

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