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

Valid 00Z Thu Apr 08 2021 - 00Z Sun Apr 11 2021


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

A series of potent shortwaves, the first Thursday morning, and the
second Saturday morning, will spread periods of heavy snowfall
across the Northwest and Northern Rockies into the weekend.

The first vorticity impulse will drop along the coast of of
British Columbia this evening and move onshore WA state very early
Thursday. This will be accompanied by a robust divergence maxima
within the LFQ of an approaching Pacific jet streak, and an short
duration increase in PW ahead of a surface cold front dragging
across the region. The combination of low-level convergence and
upper divergence in the moistened column will produce periods of
snow from the Olympics and Cascades into the Northern Rockies.
Snow levels ahead of the front will be modestly high, but will
crash quickly with the FROPA. The heaviest snowfall is likely in
the Olympics and Cascades where probabilities for 8 inches are
higher than 70%, and more than 12 inches is likely in the highest
terrain.

Brief ridging building behind this shortwave will quickly advect
the forcing eastward late Thursday into D2, bringing lighter snows
on Friday with no further significant accumulations expected.

However, another shortwave will drop across WA state Saturday
morning bringing renewed ascent to the region. While this impulse
looks to be a little shallower than the first, pinched mid-level
flow south of the trough will advect increased moisture and robust
WAA coincident with another Pacific jet streak. This will lead to
a longer fetch of moisture advecting onshore Saturday, so despite
ascent that overall looks weaker than Thursday, snowfall could
again be significant across the terrain of the Olympics and
Cascades where WPC probabilities indicate a moderate risk for 8
inches.

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

Weiss