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

Valid 12Z Sun Oct 24 2021 - 12Z Wed Oct 27 2021

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

An anomalous strong mid/upper level low approaching the Pacific
Northwest and a secondary digging shortwave trough rounding the
base of the developing longwave trough will drive a very strong
atmospheric river event across portions of California during the
day 1 period characterized by precipitable water anomalies of +3
to +4. This combination of intense large scale forcing for ascent
and impressive moisture plume will bring widespread precipitation
with high likelihood for intense snow rates and impressive
snowfall accumulations for the highest terrain peaks. The heaviest
snow is expected in the Sierra where orthogonal flow will ideally
upslope into the region, but additional heavy snow maxima are
likely in the Sawtooth and Cascades.

The bulk of the QPF and associated snowfall will fall late tonight
into Monday morning as the moisture transport nose works southward
across northern to central California. A cold front passing
through will drop the snow levels at which snow rates well in
excess of 3"/hr are anticipated. The latest WPC probabilities for
6 inches are greater than 40 percent for the Sierra as well as
portions of the WA Cascades and the highest peaks of the OR
Cascades while the highest peaks of the Sierra should see over 2-3
ft. The central Idaho mountains will also see modest accumulations
during the period with the latest probabilities of 6 inches
greater than 50 percent.

For D2 //ending 12Z Tuesday//, the bulk of the heaviest
precipitation for CA occurs in the early part of the period and
then mid-level trough begins to move eastward toward the Four
Corners and Central Rockies. The highest snow probabilities remain
for the Sierra but moderate probabilities of 4 inches spread
across portions of the Great Basin, the peaks of the Unitas in UT
and other high peaks of central ID to western WY. A favorable
plume of moisture and proximity to the deep surface low will keep
moderate to high probabilities of 4 inches for the WA Cascades.

Finally by D3 //ending 12Z Wednesday//, the trough axis will
position itself across the CO Rockies, reaching the Plains by the
end of the period. This will gradually shift the forcing toward
the Plains but early on, the lower heights, upslope flow, and
available moisture will squeeze some snowfall across the
western/southern CO Rockies where the latest WPC probabilities of
4 inches are moderate. Another shortwave trough skirting the
Pacific Northwest will keep unsettled/active weather for the
northern WA Cascades where moderate to locally high probabilities
of 4 inches remain in place.

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

Taylor