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

Valid 00Z Thu Nov 19 2020 - 00Z Sun Nov 22 2020

...Western U.S....
Days 1-3...

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, with some sharpening possible by Saturday. 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, with PWs generally around normal levels, although a
surge is likely into Colorado by the weekend. 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 moderate to 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 tonight accompanied by rather strong upper-level
divergence associated with the LFQ of the upper-level jet-streak
nosing into the region. This ascent within an environment of
confluent mid-level flow to drive moisture eastward 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 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. The pronounced
although modest forcing across the region will produce heavy
snowfall in the terrain, and WPC probabilities are high for 8
inches in the Cascades, Sawtooth, and Teton ranges, with lesser
amounts across the remaining mountain ranges.

After a brief respite as forcing shears out to the east and
moisture remains modest, a more intense impulse is progged to
rotate onshore CA Friday morning and then advect towards the
central Rockies D3. This will be accompanied by more robust
moisture with PWs rising above +2 standard deviations from the
climo mean. The temporal duration of significant forcing through
height falls and PVA as well as mid-level saturation is expected
to be limited, and WPC probabilities are only around 10% for 4
inches in the CO Rockies. Also late D2 into D3 modest moisture and
confluent flow will drive a continuation of moderate snowfall into
the WA Cascades.


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

Weiss