Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EDT Fri Mar 19 2021

Valid 00Z Sat Mar 20 2021 - 00Z Tue Mar 23 2021


...Pacific Northwest/California/Great Basin/Rockies/central High
Plains...
Days 1-3...

A sharp longwave trough will move onshore the Pacific Northwest
coast early Saturday and then shift southeast through the weekend
becoming positively tilted into the Southwest. Sunday night into
Monday, this trough is likely to amplify in response to shortwave
energy rotating through its base, becoming neutrally or even
negatively tilted as it lifts into the Central Plains Monday. As
this trough sharpens, it will be accompanied by intense LFQ
diffluence of a subtropical jet streak arcing eastward into the
Southern Plains, with some ascent enhancement through modest jet
streak coupling with the RRQ of another jet lifting towards the
Great Lakes. These features together will drive surface
cyclogenesis in the lee of the Rockies Sunday. As this primary
impulse shifts east into the Plains D3, yet another shortwave and
accompanying jet streak will dive into the Pacific Northwest.

The heaviest snowfall this period is expected D1 and D2 associated
with the leading trough and intensifying synoptic ascent. On D1,
SW 700-500mb flow will spread warm advection and moisture
northeastward producing periods of moderate to heavy snowfall in
the terrain of the Olympics, Cascades, Sierra, and into the Great
Basin/NW WY ranges. Snow levels will climb above 6000 ft as this
airmass is tropically sourced, but will fall rapidly behind the
trough axis as it pivots eastward. WPC probabilities are high for
6 inches across these ranges, with the heaviest snow D1 likely in
the Cascades and Tetons where more than 12 inches is possible. On
D2, a brief respite will occur in the Cascades ahead of the next
feature, and the most intense precipitation will spread into the
Central Rockies. Heavy snow is likely in the Big Horns where
persistent mid-level WAA will produce snowfall that may exceed 6
inches. Further south, mid-level WAA will produce heavy snow in
the CO Rockies, but WPC probabilities above 40% for 6 inches are
limited.

Sunday night into Monday, the developing surface low and continued
mid-level WAA will produce heavy snow in the CO Rockies, with E/NE
low-level flow around the surface low driving upslope enhancement
into the Front Range and Palmer Divide. WPC probabilities on D3
increase for 6 inches and spread northeastward, and are 60-80% for
6 inches in the Front Range, and as high as 40% in the Palmer
Divide. The column is marginally cold enough for snow into the
High Plains, but light snow accumulations are likely Sunday night
along the I-25 urban corridor as well.

Also on D3, the next shortwave and associated Pacific jet streak
will spread moisture and ascent back across the Olympics,
Cascades, and into the Northern Rockies. WPC probabilities are
high for 6 inches in these areas, with subtly lower snow levels
allowing for some moderate snow accumulations into the Blue
Mountains of OR as well.


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

Weiss