FOUS11 KWBC 220711
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
211 AM EST Sat Feb 22 2025

Valid 12Z Sat Feb 22 2025 - 12Z Tue Feb 25 2025



...Pacific Northwest through the Northern Rockies...
Days 1-3...

A closed mid-level low tracking slowly south of the Aleutians will
shed lobes of vorticity eastward and onshore the Pacific Coast,
spreading precipitation from the Pacific Northwest through the
Northern Rockies into early next week.

Generally W to SW flow will persist across the Pacific during this
time, with enhanced confluence south of the Aleutian Low enhancing
moisture advection and IVT as an atmospheric river (AR). Both ECENS
and GEFS probabilities indicate that IVT will almost certainly
(>90%) exceed 500 kg/m/s, with potentially 3 consecutive days of
IVT exceeding 250 kg/m/s surging onshore beginning this evening.
Within this moisture plume, PWs are progged by NAEFS to exceed the
99.5 percentile of the CFSR database, especially late D1 through
the end of D2 from the WA/OR coasts to as far inland as the Central
Rockies. Shortwaves/vorticity spokes rotating within this flow will
cause periodic enhancement to ascent leading to heavier and more
widespread precipitation. Although the timing of these features is
not fully agreed upon by the guidance, consensus suggests bouts of
heavier precipitation are likely beginning late D1 and again late
D2. While this will result in heavy snow in higher terrain features
across the region through the period, snow levels will climb
steadily within the AR/accompanying WAA. Snow levels begin the
period around 4000-6000 ft, then climb to as high as 7000-9000 ft,
especially during D2. This will keep most significant snow
accumulations above area pass levels.

Although waves of heavy snow are likely in the higher terrain each
day, the heaviest accumulations are forecast on D2 when WPC
probabilities support a high risk (>70%) for more than 6 inches of
snow across the higher WA Cascades, the Northern Rockies near
Glacier NP, the Sawtooth, and near the Grand Tetons/Yellowstone NP.
During D3, the highest probabilities shift back into the WA/OR
Cascades with slightly lower snow levels supporting high
probabilities (>70%) for more than 6 inches, and 3-day snowfall in
the Cascades could be measured in several feet above 5000 ft
elevation. Additionally, some light icing is expected in the
Columbia Basin and parts of eastern OR where WPC probabilities D2
reach 10-30% for up to 0.1" of ice.

Weiss

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