Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 PM EDT Wed Oct 06 2021

Valid 00Z Thu Oct 07 2021 - 00Z Sun Oct 10 2021

...Intermountain West to North-Central Rockies...

Day 3...
A strong shortwave trough digging south off the West Coast through
Thursday night will lift northeast from southern California to
Wyoming Friday/Friday night while a reinforcing trough digs down
the West Coast to build a full-latitude trough across The West by
Saturday. As the initial trough ejects northeast a subtropical jet
streak will amplify, placing impressive downstream diffluence
across the Central and Northern Rockies Friday night into
Saturday. The accompanying upper divergence maxima will couple
with mid-level divergence ahead of the lead shortwave and combine
with WAA to drive deep layer ascent across the region. SW flow out
of the Pacific will drive PWs to +1.5 standard deviations above
the climo mean, providing ample moisture for moderate to locally
heavy precipitation over terrain from Utah to Wyoming Friday night.

Timing of the initial trough as it ejects northeast is in better
agreement with some timing differences still noted in the
reinforcing trough by Saturday. A preference on thermals in the
Intermountain West/North-Central Rockies is with the 00Z ECENS
with some 06Z GEFS where snow levels will generally be around
9000ft. Day 3 WPC probabilities for 4" or more are 50 to 70
percent for Wind River ranges where maxima should approach 8",
with probabilities for 4" or more closer to 30% for the High
Uinta, Tetons, Absarokas, and the highest Big Horns.


For Days 1-3, the probabilities for significant icing (0.25 inch
or more) are less than 10 percent. 


Weiss/Jackson