Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
318 AM EDT Tue Oct 25 2022

Valid 12Z Tue Oct 25 2022 - 12Z Fri Oct 28 2022

...Pacific Northwest into the Northern & Central Rockies...
Days 1-3...

Generally fast and broadly cyclonic flow will envelop the western
CONUS through late week, within which successive shortwaves will
drop across the region. The first of these is moving across the
Pacific Northwest this morning, and will race E/SE as a positively
tilted low amplitude trough, reaching the Great Lakes on
Wednesday. Quickly following this first feature, a more pronounced
shortwave will dig along the British Columbia coast Tuesday night
moving onshore WA/OR Wednesday morning, before amplifying into a
full latitude trough across the Four Corners/Central Rockies
Thursday. This second wave will have much more amplitude, but will
also remain progressive through the forecast period.

The overlap of PVA and successive height falls will drive
pronounced ascent across the region, becoming most intense
Wednesday into Thursday as the deepening trough drives coupled
upper level jet streaks. At the same time, the subsequent
shortwaves and associated strengthening height falls will lead to
a cooling column, and snow levels will drop to around 3000 ft in
the Pacific Northwest, to around 6000 ft in the CO Rockies. While
the valleys and foothills will generally remain all rain, moisture
moving in from the Pacific being wrung out by this synoptic ascent
will manifest as periods of heavy snow in the terrain from the
Cascades into the Bitterroots and Northern Rockies D1, with the
second shortwave spreading snow across a much larger expanse,
reaching the CO Rockies, Sangre de Cristos, and San Juans D2-3.
WPC probabilities for more than 8 inches of snow are above 70% in
the WA Cascades and highest OR Cascades D1, with more than 4
inches likely farther east into ID/MT. For D2, the heaviest
snowfall will likely occur across the Tetons, and into the CO
Rockies including the Park Range, where WPC probabilities for 6+
inches are 20-40%. As the trough continues to move east D3, the
heaviest snowfall should become confined to the Sangre de Cristos
and possibly even out onto the Raton Mesa.


The probability of significant icing is less than 10 percent
nationwide.

Weiss