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

Valid 00Z Sat Nov 06 2021 - 00Z Tue Nov 09 2021


...Pacific Northwest to the Northern Rockies...
Days 1-3...
A series of shortwaves embedded within a broadening upper trough
will support unsettled weather through the weekend.  Locally heavy
snow accumulations are expected across the higher elevations of
the Olympics and northern Cascades, with lighter accumulations
expected across the northern Cascade passes as snow levels drop
during the weekend.  

An upper trough centered over the eastern Pacific will begin to
broaden across the Northwest as a series of shortwaves begin to
lift across the region overnight into Saturday. As snow levels
drop below 2000 ft across western Washington on Saturday, locally
heavy snow accumulations are expected across portions of the
Olympics and northern Cascades.  As a well-defined shortwave lifts
across the region, snow levels are expected to continue to drop
Saturday evening through the overnight across  Washington,
slipping below 1000 ft across much of north-central into
northeastern Washington. This will support the increasing
potential for heavy snow accumulations across portions of the
Olympics and Washington Cascades, with at least some light
accumulations expected across the northern Cascades passes. 

Meanwhile, a low-to-mid level baroclinic zone shifting east will
support the development of moderate to locally heavy mountain
snows along the Idaho-Wyoming border into northwestern Wyoming
Saturday night into Sunday. 

Unsettled weather is expected across the Northwest into Monday,
but with a relative lull as a shortwave ridge briefly builds ahead
of the next shortwave approaching the coast late in the day.

For the three day period ending 00Z Tuesday, WPC guidance shows
locally high probabilities for accumulations of 18 inches or more
for parts of the Olympics and northern Cascades, mainly for
elevations above 4000 ft. 

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

Pereira