Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 AM EST Wed Nov 18 2020

Valid 12Z Wed Nov 18 2020 - 12Z Sat Nov 21 2020

Days 1-3...

...Western U.S....

No major changes to the previous thinking. A broad upper trough
moving over the western U.S. over the next few days will continue
to foster a weakly cyclonic upper-level flow pattern. Multiple
embedded shortwaves in this deeper layer flow and associated waves
of low pressure will help to maintain a fetch of mid-level Pacific
moisture across the Intermountain Region going through at least
Thursday. The combination of larger-scale forcing associated with
the shortwave energy and the moist, upslope flow into the west and
southwest facing slopes of the higher terrain should maintain a
threat for heavy accumulating snowfall as a result. By Friday, the
Great Basin will begin to dry out as high pressure builds over the
region, but there will be some shortwave energy/troughing
approaching the central Rockies, and also still a fetch of Pacific
moisture still impacting portions of western WA that will maintain
a threat of some wintry weather precipitation for the higher
terrain.

More specifically, the next shortwave impulse will advance onshore
the OR/WA coast early this morning accompanied by rather strong
upper-level divergence associated with the LFQ of the upper-level
jet-streak nosing into the region. This will spread snowfall
across many of the ranges from the Olympics of WA, southward
through the northern Sierra Nevada, and as far east as the Tetons
of WY. Snow levels will gradually fall going through today and
tonight, with snow levels falling to as low as 2000 to 3000 feet
farther north in the WA Cascades, and down to perhaps as low as
5000 to 6000 feet in the Sierra-Nevada. Snow levels will be a bit
higher farther east over the northern Rockies and generally in the
6000 to 7000 feet range. A second shortwave impulse will arrive on
Thursday with additional height falls and forcing associated with
this which will yield a second round of locally heavy accumulating
snowfall. The combination of these two shortwave impulse is
expected to foster additional snowfall totals going through early
Friday of as much as 1 to 2 feet, with the heaviest accumulations
over the WA Cascades and also locally over the southwest facing
slopes of the Sawtooth where some isolated additional amounts over
2 feet will be possible. Consequently, there are some locally
elevated WPC snowfall probabilities for exceeding 12 inches of
snow over these areas.

On Friday and through early Saturday, a moist Pacific fetch will
still be impacting portions of the WA Cascades which may yield an
additional 6 to 12 inches for this period here, but with higher
snow levels. Elsewhere, expect locally several inches of snow to
be possible over the central Rockies, and especially northern CO
as shortwave energy attempts to amplify toward this region.

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

Orrison