Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
347 AM EDT Wed Apr 07 2021

Valid 12Z Wed Apr 07 2021 - 12Z Sat Apr 10 2021


...Pacific Northwest and Northern Rockies...
Days 1-3...

An upper level trough moving southeast across Vancouver Island and
then WA State produces deep layer west winds today and tonight. 
This results in an influx of Pacific moisture to the Pacific
Northwest this afternoon and into tonight. Snow levels will fall
in the Olympics and Cascades as the trough tracks overhead
tonight. In addition, steady westerly flow leads to typical
topographic enhancement of snowfall rates in the windward 
locations. The result is accumulating snowfall in the Olympics and
Cascade Range where moderate probabilities of greater than 8
inches of snow are present, and isolated peaks will receive a foot
of snow. The snow tapers on Day 2 as the upper trough moves east,
and ridging aloft causes drier air to descend.

As the upper level disturbance heads east on Thursday, warm air
advection out ahead of it will induce periods of mountain snow in
the Northern Rockies. The Bitterroots and western MT ranges can
expect periods of snow  Thursday morning with the higher
elevations ranging between low and moderate probabilities for
greater than 4 inches of snowfall. Snow is also forecast to reach
parts of the Absarokas, Beartooth Mountains, and Big Horns late
Thursday and into Friday, with rates and accumulations tapering as
the upper trough moves east out of the region and across the
northern Plains.

On Friday, the next upper trough builds slowly south across
Vancouver Island and south British Columbia into WA State.
Moisture deepens and upper divergence maxima and mid level
convergence should lead to snow redeveloping in the WA Olympics
and Cascades.  The long duration moisture fluxes should lead to
several inches of snow in higher elevations of the Olympics and
Cascades Fri night to Saturday morning. Similar timing and
amplitude of the 700 mb trough lead to equal weighting of the
model solutions in the forecast.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen