Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
436 AM EDT Sat Nov 06 2021

Valid 12Z Sat Nov 06 2021 - 12Z Tue Nov 09 2021


...Pacific Northwest to the Northern Rockies...
Days 1-3...

A series of shortwaves embedded within a persistent longwave
trough will create an unsettled and active period of weather with
waves of precipitation across the West. Shortwave troughs are
progged to lift onshore the Pacific Northwest both D1 /tonight/
and D2 /Sunday night/, with a more robust system approaching the
coast Tuesday /D3/. Moisture will be plentiful during the period
as low/mid level warm advection ahead of surface fronts combines
with persistent Pacific jet energy to drive PWs to as much as +1
standard deviation above the climo mean. By D3, an AR
characterized by IVT of 500 kg/m/s will move into Northern CA
spreading PWs of more than +2 standard deviations onshore.

With persistent troughing overhead and surface cold fronts moving
eastward, snow levels will be quite low through much of the
period. For D1, snow levels will range from as low as 2000 ft in
the Cascades to 6000 ft in the NW WY ranges, before falling to
more uniform 2000-4000 ft on D2. A general increase in snow levels
are likely D3 as the AR approaches with WAA, but will remain quite
low in the Cascades, Olympics, and northern CA ranges.

For D1 and D2, WPC probabilities for 6 inches are high in the OR
and WA Cascades, the Olympics, and as far east as near Grand Teton
NP. 2-day snowfall in the WA Cascades and Olympics could exceed 4
feet in the higher peaks, with several inches of accumulation
possible each day at pass level. Light accumulations are possible
even into some of the elevated valleys of WA, ID, and MT.

For D3, the more anomalous plume of moisture and intense 700mb WAA
coupled with robust upper diffluence should produce additional
heavy snow in the Cascades, but even more impressive snowfall near
the Shasta/Trinity ranges. WPC probabilities for 6 inches are
modest in WA, but are above 70% near Mt. Shasta where more than 1
foot of snow is possible as upslope flow enhances the synoptic
ascent.

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

Weiss