Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 PM EDT Mon Apr 10 2023

Valid 00Z Tue Apr 11 2023 - 00Z Fri Apr 14 2023

...Pacific Northwest...
Days 1-2...

An upper trough over the Northwest will direct a pair of shortwave
disturbances across the Pacific Northwest today and through early
Wednesday. The first disturbance approaching the Pacific Northwest
coast this evening will envelope the region with PVA aloft and a
steady stream of 850-700mb moisture flux into the region. As the
upper trough tracks over the region Tuesday morning, a cold
frontal passage and crashing heights aloft will cause snow levels
to plummet to as low as 2,000 feet. Snowfall totals will be
highest in the higher elevations and slopes that are orthogonally
positioned to the 850-300mb southwesterly mean flow. The Olympics
and Cascades for Day 1 feature high WPC probabilities (70-90%) for
>6" of snowfall and moderate probabilities (40-60%) in the
Clearwater and Salmon River Mountains of ID through Day 2.

The second upper level disturbance, in the form of an upper low
Tuesday night, will contain exceptionally cold temperatures for
mid-April. By 06Z Wed, NAEFS shows 500-850mb temps that are below
the 10th climatological percentile. Temperatures are cold enough
to support minor snowfall accumulations <1,000 feet. Even along
the I-5 corridor south of Seattle and north of Portland could see
a coating of snow, but should any of the snowfall accumulate, it
would be more of a nuisance snowfall event for these locations
with accumulations generally confined to grassy surfaces.

...Northern Rockies...
Day 3...

The upper low tracking through the Northwest will make its way
east into the Northern Rockies late Wednesday night and into
Thursday where it will direct ample amounts of 700mb moisture flux
into the region. The same cold front that traversed the Northwest
will stall over MT and orient itself southward along the Tetons,
Uinta, and Wasatch Ranges. As a low forms along the MT/WY border,
easterly 850mb flow will inject low level moisture that prompts
upslope flow into the Big Horns, Absaroka, and both the Little
Belt and Big Belt mountains of west-central MT. Periods of heavy
snow will ensue in these ranges by Thursday with moderate chances
(40-60%) for >6" of snow in the these ranges. Localized amounts in
the higher elevations of the Big and Little Belt mountains could
surpass 10" in the tallest peaks where the best topographical
enhancement is likely to transpire. The Day 3 WSSI does show some
Minor impacts in the mountain ranges of southwest MT.

The probability for icing over 0.1" is less than 5% Days 1-3.

Mullinax