Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 AM EDT Sun Oct 30 2022

Valid 12Z Sun Oct 30 2022 - 12Z Wed Nov 02 2022

...Pacific Northwest...
Days 1-3...

Combination of incoming troughing (now over the Gulf of Alaska)
and a shortwave in the mid-latitudes out of the North Pacific will
lead to an unsettled period across parts of the Western US over
the next several days. Atmospheric river will focus into
Washington today on a 150kt jet streak into British Columbia.
Additional shortwave energy over Alaska will slide through the
broad trough that will sink southeastward into the Pac NW for the
first part of the week. The associated surge of moisture will have
PW anomalies +1.5 to +2 sigma and IVT of ~300 to 700 kg/m-s
through Monday before the moisture surge weakens. Still, this will
result in multi-inch QPF to the Olympic Peninsula and the
central/northern WA Cascades through Monday morning. Snow levels
around 7000ft today will confine heavy snow totals to the highest
peaks of the northern WA Cascades where accumulations over 6-12
inches are likely on Day 1 (perhaps several feet at the very
highest peaks).

On Day 2, the AR will weaken and shift southeastward through
Oregon as colder air moves in behind the front. With snow levels
progged to drop to below 4000ft, WPC probabilities for 4 or more
inches is moderate across the Washington Cascades. By Day 3, core
of the upper trough (potentially small closed low) will start to
come ashore Washington and drive snow levels even lower to around
2000-3000ft from NW to SE over WA/OR and 3000-5000ft over Idaho
and western MT southwestward into NV and the Sierra. WPC
probabilities for at least 4 inches of snow are moderate (>40%)
generally above 4-5000 ft from the Oregon Cascades across central
Idaho to northwestern Montana and high (>70%) generally above
6-7000ft. Snow levels will be higher over the Sierra.

The probability of significant icing greater than 0.25" is less
than 10 percent.

Fracasso