Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Fri Nov 05 2021

Valid 12Z Fri Nov 05 2021 - 12Z Mon Nov 08 2021


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


An active pattern will persist across the Pacific Northwest as a
closed mid-level low in the Gulf of Alaska periodically sheds
shortwave energy eastward and onshore. The primary longwave trough
axis is progged to remain just off the coast through early next
week, allowing for persistent moisture advection on low-level WAA
and modest but continuous Pacific jet energy. Deep layer ascent
through PVA/height falls and modest diffluence will combine with
eastward moving cold frontal passages to produce precipitation
through the period. The combination of slow thickness decrease and
the multiple cold fronts will drive snow levels downward, falling
from around 5000-7000 ft today to as low as 1500-3000 ft by
Monday, lowest across WA state. This will bring at least light
snow to many of the valleys and foothills that have yet to see
much snow so far this winter, with modest to significant
accumulations in the Cascade Passes.

The persistent moist advection from the Pacific will lead to
widespread snowfall above these elevations from the Northern CA
ranges, eastward into the mountains of NW WY, and all of the
Pacific Northwest. While no individual impulse appears extremely
impressive, the nearly continuous moisture stream will allow for
the deep layer ascent to produce moderate to at times heavy
snowfall each day. WPC probabilities for 6 inches are above 50% in
the Olympics and WA Cascades D1, spreading into the Northern
Rockies, OR Cascades, and near Yellowstone NP on D2. By D3,
moisture and forcing reload into the Pacific Northwest and high
WPC probabilities for 6 inches focus once again across the
Olympics and WA Cascades, but this time with a broader footprint
due to much lower snow levels. 3-day total snow will likely exceed
3 feet in the higher terrain of the Olympics and Cascades. More
than 6 inches is likely at both Stevens Pass and Snoqualmie pass
through Monday.

For Days 1-3, the probability of significant icing is less than 10
percent.

Weiss