Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
433 PM EDT Tue Oct 26 2021

Valid 00Z Wed Oct 27 2021 - 00Z Sat Oct 30 2021

...North-Central Rockies...
Day 1...

Longwave trough and front-side vort max will exit Colorado this
evening with rising heights in its wake early Wednesday. Lingering
snow will wind down over elevations generally above 8000-9000 ft
as surface low pressure moves into the Plains. WPC probabilities
for 4 inches or more are around 30-50% for the highest peaks in
the CO Rockies, especially early on Day 1.


...Pacific Northwest...
Days 1-3...

Active North Pacific Jet will maintain a rather steady fetch of
moisture into the Pacific Northwest in quasi-zonal flow. 130+ kt
jet will lift northward Wednesday into early Thursday bringing in
anomalous precipitable water values of +1 to +2.5 sigma ahead of a
cold front. Combined with favorable onshore flow and broad-scale
forcing for ascent, widespread precipitation is expected over the
region for the next few days. Snow levels will start modestly low
around 5000 ft day 1 but then rise to around 8000-9000 ft as
milder air moves in from the WSW Thursday. As the cold front moves
ashore by Thursday afternoon, snow levels will drop back to near
or below 5000 ft as the precipitation moves southeastward out of
the Cascades.

The highest peaks of the Washington Cascades will likely see
significant snow accumulations of several feet owing to plentiful
QPF. WPC probabilities for at least 8 inches of snow are 30-80% in
higher elevations day 1 but 10-50% day 2 and below 10% day 3.
Farther east, day 1 probabilities of at least 4 inches of snow are
30-60% over parts of the Bitterroots and northwestern Montana as
moisture with the lead system today streams eastward.

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


Fracasso