Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 PM EDT Sun Oct 24 2021

Valid 00Z Mon Oct 25 2021 - 00Z Thu Oct 28 2021

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

The deep closed low which is clearly evident on satellite imagery
today will continue to lift ENE into British Columbia Monday,
while an impressive longwave trough shifts onshore the West Coast.
This trough will then move rapidly eastward towards the Central
Plains by Wednesday, with a subtle negative tilt occurring late in
the forecast period in response to amplification of a shortwave
rotating through the base. As this feature moves eastward, ascent
will remain impressive through height falls and PVA, in addition
to intense upper diffluence as a robust Pacific jet streak,
progged to reach 150 kts, shifts onshore downstream of the primary
trough axis, with a secondary jet max following in its wake.
Pronounced low-level warm and moist advection will drive IVT to
above the climatological maximum as far east as Utah by D2, and
the overlap of intense ascent and anomalous moisture will lead to
widespread precipitation across the West, with snow occurring in
the higher elevations.

The heaviest snow through the period is likely D1 /Sunday night
and Monday/ across the Sierra where the most intense moisture
plume (atmospheric river) pivots orthogonally into the terrain.
This will drive intense upslope enhancement in addition to the
deep layer synoptic ascent, and with PWs progged to be more than 3
standard deviations above the climo mean, snowfall rates of 3"/hr
are likely which will accumulate heavily above 6000 ft. The WSE
10th percentile on D1 is over 20" in the highest peaks above
10,000 ft, and WPC probabilities for 18 inches are more than 90%.
It is likely that some of the higher peaks will exceed 3 feet of
snow D1, with widespread 1-2 ft. Additional heavy snow is likely
in some of the other favored upslope regions including the
Sawtooth, Wind Rivers, and WA Cascades where WPC probabilities for
8 inches are more than 60%.

By D2, the strongest ascent shifts eastward into the Great Basin
and Central Rockies as the trough axis pivots eastward and the
accompanying shortwave amplifies. With still pronounced IVT in
place, this strong ascent has led to an increase in guidance
snowfall, generally above 7000 ft. WPC probabilities have
increased for 6 inches D2, now reaching 50% in parts of the
Wasatch, Uintas, Wind Rivers, Tetons, and San Juans.

With the secondary /upstream/ jet streak moving into the Pacific
NW D2, renewed moist advection and robust diffluence will produce
heavy snow in the WA and OR Cascades where WPC probabilities for 6
inches are above 70%, and locally more than 12 inches of snowfall
is likely. By D3 /Tuesday night and Wednesday/ the snow shuts down
across the Central Rockies, but continued moisture and upper level
ascent in the WA Cascades will create another day of moderate to
heavy snow possibly exceeding 6 inches above 5000 ft. A few inches
of snow is also possible at Washington Pass and Stevens Pass
during the forecast period.


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

Weiss