Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EST Tue Dec 06 2022

Valid 12Z Tue Dec 06 2022 - 12Z Fri Dec 09 2022

...Great Basin through the Central Rockies...
Days 1-2...

Weakly-closed low (bordering open wave) just offshore NorCal this
morning will finally move inland today and through the Great Basin
into the Four Corners region tomorrow. Southwesterly upper jet
will strengthen with time (>130kts by later tonight into early
Wed) and a moisture plume out of the Eastern Pacific will stream
northeastward across AZ/NM (though focused south of the heavy snow
area). Surface front draped through the region will act as a focus
for enhanced lift aided by orographic upslope flow, especially on
southwest to south-facing slopes. CAMs indicate heavier snow rates
>1"/hr possible over the Flat Tops, Sawatch, and San Juans, where
2-day total snowfall will likely exceed 1 foot above 7000 ft.

...Montana/northern Idaho...
Day 1...

Multi-spoke and expansive upper low will meander near the
southeastern coast of Nunavut the next couple of days before
finally lifting northeastward by Thursday. Strung-out vorticity on
its southwest side over Alberta will slip through Montana later
today as a surface high pressure moves into the Dakotas. This will
provide easterly upslope flow into the terrain over western MT
into northern ID, up and over the cold airmass at the surface,
north of a frontal boundary that will remain stretched across the
state. Periods of light snow are expected Day 1 with heavier
amounts over six inches centered near Glacier NP. Additional
snowfall of a few inches is likely near/south of Havre and
along/south of the MO River Valley toward west-central ND. WPC
probabilities for more than 4 inches are above 80% in the Northern
Rockies and 30-50% farther east.

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

A sharp trough and closed low over northeast Pacific and Gulf of
Alaska, respectively, on Wednesday will move southeastward into
British Columbia and the Pacific Northwest by Thursday. This will
bring a cold front into Washington/Oregon with a narrow band of
modest moisture (IVT ~ 250-400 kg/m-s). Snow levels 2000-3000ft
ahead of the front west of the Cascades (but below 1000ft along
and to the east) will lower by 500-1000ft into Thursday as colder
air moves in behind the front but with lower QPF. WPC
probabilities for more than 8 inches of snow Wed-Thu are high
(>70%) in the WA Cascades and Olympics and 30-60% as far south as
the Shasta/Trinity ranges in northern CA. More than 10 inches of
snow is likely in the higher terrain and significant snow is
possible at pass levels in the Cascades. Additionally, more
widespread and lower-elevation snowfall accumulations are likely
just beyond this forecast period.


For Days 1-3, the probability of significant icing exceeding 0.10
inch is less than 10%.


Fracasso