FOUS11 KWBC 200651
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
251 AM EDT Mon Apr 20 2026

Valid 12Z Mon Apr 20 2026 - 12Z Thu Apr 23 2026

...California... Days 1-3...

Impressive upper low (500mb heights falling to the 2.5 percentile
level of the CFSR climatology) will approach the CA coast tonight
before swinging onshore Tuesday aftn. Although this feature is
likely to slowly decay with time D2 into D3, pronounced ascent
through height falls, PVA, and the LFQ of a jet streak pivoting
onshore downstream of the primary trough will help expand
precipitation across CA beginning this morning. The heaviest
precipitation is likely late Monday night through Tuesday evening
as the greatest IVT (50-60% chance of exceeding 500 kg/m/s from
both the ECENS and GEFS) pushes into CA within the confluent flow
southeast of the trough. While most of the precipitation will occur
as rain due to elevated snow levels, snow is likely in the highest
terrain of the Sierra and Shasta/Trinity region, especially Tuesday
as snow levels fall from around 6000-7000 ft down to as low as 4500
ft, potentially locally lower as steep lapse rates and
impressive ascent dynamically cool the column (the NBM 10th% snow
level drops to 3500 ft in the Sierra).

This will result in heavy snow accumulations, especially across the
Sierra, but also into the higher elevations of the northern CA
terrain as well. The most significant snowfall is likely today and
Tuesday before the strongest ascent lifts away to the northeast on
Wednesday, bringing an end to CA snow on D3. Before that occurs,
however, WPC probabilities indicate a high risk (>90%) for at least
12 inches of snow in the higher Sierra (above 6000 ft), with
lighter accumulations down to 4500 ft. Heavy snow is also likely in
the Shasta/Trinity region as reflected by WPC probabilities that
are above 70% for at least 8 inches. This will create hazardous
travel across the Sierra passes.

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

The same upper low which will bring heavy snow to California Monday
and Tuesday will continue its trek northeast on Wednesday as a
weakening, but still amplified, trough swinging into the Northern
Rockies. Although moisture will gradually decay (IVT less than 150
kg/m/s spilling into the Intermountain West) a strengthening
surface low across the Northern High Plains will work together with
continued mid-level SW flow (funneling the Pacific moisture) to
produce widespread precipitation across the region beginning
Wednesday morning. Sufficient synoptic ascent into this moistening
column will manifest as areas of heavy snow, initially only above
8000 ft, but then falling to around 5000 ft by the end of the
forecast period as the upper trough swings overhead. WPC
probabilities D3 suggest there is high risk (>70%) for widespread
light to moderate snowfall accumulations exceeding 4 inches from
the Blue Mountains of OR through the Sawtooth/Salmon River ranges
of ID, into the Absarokas, Little Belts, Tetons and other areas
around Yellowstone NP, and as far north as the Lewis Range around
Glacier NP. Locally, more than 8 inches is possible (30% chance)
near the Absarokas, Little Belts, and Tetons.

Although current model snowfall accumulations outside of the
higher terrain are modest, there is some concern that low-level
southerly flow downstream of the surface cyclone will pivot
cyclonically and lift the accompanying theta-e ridge into a TROWAL
across Montana D3. The new guidance is trending in this direction,
which could support some heavier snow banding across western parts
of the state. As of this time the probabilities for heavy snow
remain low, but this will need to be monitored for potentially
impactful snow dropping into lower elevations on Wednesday.

The probability for significant freezing rain exceeding 0.1 inches
across the CONUS is less than 10 percent.

Weiss

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