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

Valid 12Z Wed Oct 06 2021 - 12Z Sat Oct 09 2021

...Central Rockies...

Day 3...
An impressive shortwave moving onshore California will lift
northeast towards the Central Rockies Friday while a second
impulse digs down into the Pacific Northwest to amplify a
full-latitude trough across the West. As this trough deepens, a
subtropical jet streak will amplify, placing impressive downstream
diffluence across the Central and Northern Rockies. 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 heavy precipitation on D3.

The guidance differs considerably in the timing of this trough
moving to the east, with the ensemble clusters indicating the GEFS
creating the fast side of the dipole while the ECENS/CMCE
comprises the slower solutions. The current forecast supports an
evolution slower than the GEFS, leading to slightly less snowfall
as the snow levels fall more slowly from west to east and SLRs are
more compromised. Despite that, WPC probabilities for 4" are as
high as 60% in the Uinta and Wind River ranges where maxima may
approach 8", with lesser probabilities extending into the Tetons
and Absarokas. The greatest accumulation should remain confined
above 9,000 ft.


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


Weiss