Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
407 AM EDT Mon Oct 25 2021

Valid 12Z Mon Oct 25 2021 - 12Z Thu Oct 28 2021

...Pacific Northwest and California to the Central Rockies...
Days 1-3

The anomalously deep closed upper level low off the Pacific
Northwest coast will continue to slowly lift east/northeast,
reaching British Columbia later today. A secondary shortwave
trough quickly passing to the south will act to amplify the
longwave trough over much of the western U.S. with the main PV
anomaly reaching southern CA by early Tuesday. The impressive jet
dynamics and upper level diffluence will drive low pressure across
the Intermountain West to central Rockies. This combination of
favorable forcing for ascent and available moisture will work to
produce widespread precipitation with snow occurring in the
highest elevations. For D1 //ending 12Z Tuesday//, the greatest
snowfall accumulations and highest probabilities will be across
the Sierra early in the period as the atmospheric river moisture
continues to sag south/southeast. There are moderate to high
probabilities of exceeding 12-18 inches today, with some local
maximum above 2 ft. Elsewhere, moderate to high probabilities of 4
inches exist further into the higher elevations of Nevada, Idaho,
the UT Unitas, and the Wind River Range as well as the higher
peaks of the OR and WA Cascades.

Through D2 //ending 12Z Wednesday// the trough axis crosses
through the central/northern Rockies and the associated ascent
will spread snowfall to portions of the Colorado Rockies where
moderate to locally high (40 to 70 percent) probabilities of 4
inches for levels mainly above 7 kft. High probabilities of 6
inches exist for the WA Cascades as another shortwave trough
passes into British Columbia, skirting the far northern areas of
Washington State.

By D3, the upper pattern begins to take on some zonal to ridging
flow as the main weather system reaches the Plains. Lack of
forcing for ascent and available moisture will limit snowfall
potential to only the far northern WA Cascades where slight to
moderate probabilities of 4 inches exist.


For Days 1-3, the probability of significant icing is less than 10
percent.

Taylor