Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Thu Nov 04 2021

Valid 00Z Fri Nov 05 2021 - 00Z Mon Nov 08 2021


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

A parade of shortwave troughs embedded within the broad cyclonic
flow will rotate onshore the Pacific Northwest this weekend,
bringing several rounds of precipitation to the area. A favorable
fetch of Pacific moisture within the favorable forcing for ascent
will bring widespread moderate to locally heavy precipitation. The
ongoing shortwave trough and moisture will lift through the region
through 06Z, followed by a brief reprieve late tonight. The next
defined shortwave trough arrives after 12Z Friday followed by a
stronger surface low and deeper trough on Saturday. Finally,
another wave, a bit weaker, comes through on Sunday. All told, a
very unsettled and wet period. Snow levels are forecast to fall
throughout the weekend as heights fall aloft, gradually dropping
toward the 4000 ft level later this weekend.

D1 WPC probabilities for 4 inches are high for the high peaks of
the WA Olympics and Cascades where accumulations could exceed 8-12
inches. As snow levels drop into D2, moderate probabilities for 4
inches expand to lower elevations of the Olympics and Cascades but
also begin to spread into the central Idaho mountains. By D3,
moderate probabilities for 8 inches show up across the Olympics
and WA Cascades with lower probabilities for 4 inches reaching the
Sawtooth Range and Northern Rockies. 


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

Taylor