Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
226 PM EST Fri Feb 03 2023

Valid 00Z Sat Feb 04 2023 - 00Z Tue Feb 07 2023


...Western U.S....
Days 1-3...

Amplifying longwave trough over the Pacific will shed multiple
shortwaves and associated vorticity impulses eastward into the
western U.S. though early next week. The first of these will be a
low amplitude wave moving across the Pacific Northwest tonight
into Saturday, with subtle height falls and PVA overlapping weak
diffluence from the LFQ of a southward sinking upper jet streak.
Forcing will be transient and moisture is somewhat limited noted
by PWs just around normal values, but WPC probabilities indicate a
moderate to high risk for more than 6 inches of snow, but
generally above 5000ft and confined to the northern WA Cascades.

Late Saturday night into Sunday /beginning of D2/ a more
pronounced vorticity lobe will eject towards the CA coast on the
leading edge of the Pacific trough which will now be a closed low
dropping southeast. This entire longwave trough is progged to move
onshore the coast Sunday evening, and then continue to progress
southeast as positively tilted trough from the Northern Plains
through the Desert Southwest by the end of the forecast period.
Confluent mid-level flow downstream of this trough axis will help
funnel moisture eastward, driving PWs as high as +1.5 standard
deviations above the climo mean into CA/NV, with abundant moisture
available across most of the West. The height falls and PVA will
work in tandem with a zonally oriented and strengthening Pacific
jet streak progged to exceed 130 kts as it surges into the Great
Basin and Four Corners. This deep layer ascent within an
environment of high moisture will result in heavy snowfall across
much of the western terrain. The extension of this Pacific jet
streak will maintain modified Pacific air across the region,
supporting snow levels that will in general be around 4000-5000
ft, rising to as high as 7000 ft in CA and the desert southwest.
This will keep the heaviest snow in the higher terrain, but high
probabilities for more than 6 inches spread into the OR Cascades,
northern CA ranges, and Sierra D2, with locally 12-24 inches
likely across the Sierra. During D3, the forcing becomes more
diffuse and spreads northeastward along the positively tilted
trough axis. WPC probabilities for 6 or more inches are 20-40%
across the Wasatch and parts of the CO Rockies including the Flat
Tops and Park Range. Additional heavy snow is likely above 5000 ft
in the WA Cascades once again and near Bitterroots of ID.

Weiss