Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 PM EDT Mon Oct 25 2021

Valid 00Z Tue Oct 26 2021 - 00Z Fri Oct 29 2021

...Southern California through the North-Central Rockies...
Days 1-2...

The anomalously deep trough swinging into southern CA this
afternoon will shift east to the southern Rockies through Tuesday.
 Impressive jet dynamics and upper level diffluence will drive
surface low pressure northeast from the Great basin to the
northern High Plains through tonight. Widespread precipitation
with snow in the highest elevations (generally 9000ft) at onset
with snow levels decreasing to around 5000ft prior to precip
cessation will continue.

For Day 1 (ending 00Z Wednesday), moderate to high probabilities
of exceeding 6 inches are from eastern NV ranges through the UT
Ranges (Wasatch and Uinta), and western WY/CO ranges. Precip
quickly pushes east of the Rockies Tuesday night with Day 1.5
(ending 12Z Wednesday) capturing the western CO terrain snow best.


...Pacific Northwest to the Northern Rockies...
Days 1-3...

The North Pacific jet reaches the Pacific Northwest tonight with a
notable shortwave trough passage late Tuesday with a larger trough
approaching on Thursday. Onshore flow through this time maintains
precip through all three days over WA (into Wed for OR) with snow
levels generally around 5000ft through Wednesday, quickly rising
to 9000ft on Thursday as a plume of tropically-sourced moisture
pushes inland ahead of the approaching trough. Day 1 snow
probabilities for 6 or more inches are moderate to high across the
higher WA/OR Cascades and Olympics, are limited to WA on Day 2 and
further limited on Day 3 to just the highest peaks.


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


Jackson