Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
547 AM EDT Mon Oct 24 2022

Valid 12Z Mon Oct 24 2022 - 12Z Thu Oct 27 2022

...Northern High Plains...
Day 1...

The intense surface low and anomalous associated mid-level trough
will weaken today as it ejects into Manitoba by this evening. An
impressive snow band within the deformation axis NW of the low
currently noted on radar will gradually pivot NE today,
maintaining moderate to heavy snow from eastern MT through western
ND into this evening. The total forcing should gradually wane with
time, and the band is likely to become more transient, suggesting
both rates and duration of this heavy snow should weaken by the
aftn. Despite this, additional moderate snow accumulations are
likely beneath this band, with lighter snows surrounding it. WPC
probabilities suggest 20-30% chance for an additional 4+ inches of
snowfall in western ND before the system winds down this evening.
Otherwise, accumulations after 12Z should be light.


...Pacific Northwest into the Central Rockies...
Days 2-3...

Multiple shortwaves embedded within broad cyclonic flow across the
western CONUS will advect across the region, cooling the airmass
and providing forcing for waves of snowfall, primarily in the
higher terrain. The first shortwave, a low amplitude feature
embedded within confluent mid-level flow out of the Pacific will
shed onshore WA state this evening before decaying as it shifts
into the Northern Plains Tuesday evening. Brief shortwave ridging
will follow this feature, before a more pronounced shortwave with
greater amplitude drops into WA/OR Wednesday morning and
intensifies into a positively tilted full latitude rough from the
Northern High Plains into the Great Basin by the end of the
forecast period. Each of these shortwaves will likely drive a
progressively colder column minus leading WAA, leading to higher
SLRs and lower snow levels.

For D1 /today and tonight/, WPC probabilities for more than 6
inches are above 50% in the WA Cascades and Bitterroots of ID,
with moderate snowfall accumulations even down to pass level at
Stevens and Lolo Passes. For D2-D3, the more significant shortwave
and associated deepening trough will spread moderate to heavy
accumulations from the WA Cascades through the Tetons and into the
CO Rockies, with snow levels falling to 3000-4000 ft in the NW,
and to around 6000 ft in CO. The greatest probabilities for 6
inches or more during the D2-3 period will again be the WA
Cascades where orthogonal flow and higher PW anomalies will
translate to greater snowfall rates and accumulations, but heavy
snow exceeding 6 inches also has a greater than 40% chance of
occurring across parts of the Tetons and CO Rockies as well.


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

Weiss