Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 AM EDT Wed Oct 27 2021

Valid 12Z Wed Oct 27 2021 - 12Z Sat Oct 30 2021

...Pacific Northwest and North-Central Rockies...
Days 1-3...

A strong upper jet of 130-150 kts across British Columbia will
provide favorable right entrance lift across the Pacific Northwest
through early Thursday. An anomalous amount of moisture,
characterized by PWs of around 1-1.25" (up to +2.5 sigma) will
bring a steady fetch and provide plentiful amounts of QPF to the
favored slopes of the Olympics and Cascades. Snow levels will
vary, starting off initially around 5000 ft but then warm air
advection ahead of the cold front will push levels to 9000 ft by
the end of Day 1 //12Z Thursday//. For Day 1, WPC probabilities
for 6 inches are around 40 percent, where the highest peaks could
see accumulations in excess of 12 inches.

A brief lull in precipitation and cold air for Thursday //D2//
will confine additional snow accumulations to the highest peaks
with only light/minor amounts expected. The next shortwave trough
and associated cold front then is forecast to progress through the
region during D3 //ending 12Z Saturday//. Much of the WA Cascades
will be too warm with high snow levels to see much in the way of
additional accumulations and it's not until the front passes and
snow levels crash that precipitation is expected to change over to
snow for the NW Montana mountains around Glacier NP. WPC
probabilities for 6 inches are 40-60 percent for the highest peaks.

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


Taylor