FOUS11 KWBC 310640
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
240 AM EDT Sat Oct 31 2026

Valid 12Z Sat Oct 31 2026 - 12Z Tue Nov 03 2026

...Sierra Nevada into the Intermountain West & Rockies...
Days 1-3...

A full latitude trough with dual embedded closed mid-level lows
will haunt the western CONUS, causing increasingly unsettled 
weather into early next week with rain and high elevation snowfall.

A closed mid-level low centered off the OR/WA coast will elongated
southward through Saturday, amplifying the accompanying longwave
trough along the Pacific Coast. At the same time, a secondary
shortwave lifting northeast out of the eastern tropical Pacific
will eject northeast, and these two impulses will phase over the
Southwest and then lift across the Great Basin, reaching the
Central Rockies by Monday. Downstream divergence combined with 
periods of impressive PVA will drive large-scale ascent, aided by 
locally impressive fgen and upslope flow.

The airmass accompanying this system will become increasingly moist
thanks to impressive IVT surging northeast downstream of this
eastward translating upper trough. The West-WRF IVT probabilities
peak above 90% for 250 kg/m/s, and although the core of this IVT
stays south across CA/AZ, it represents an impressive moisture
plume that will flood across the region beginning Saturday evening
and then persisting as it shifts eastward through Monday. As is
typical with tropical IVT, the accompanying WAA will push snow
levels to as high as 8000-9000 ft in the Great Basin/Sierra, and as
high as 11,000 ft farther east into the Central Rockies, and
although snow levels will slowly fall by early next week, the
heaviest precipitation will exist within this elevated snow levels,
and despite elevated instability and steep lapse rates, most snow
accumulation should remain above 8000-9000 ft in all terrain.
Precipitation is expected to end by Sunday night across the Sierra,
followed by Monday across the Great Basin/Northern Rockies and
Central Rockies by Monday night.

Still, impactful snow is likely across these higher elevations,
especially within the Sierra, Uintas, San Juans, and Sawtooth where
NBM probabilities for at least 12" of snow exceed 70%, and locally
as much as 2 feet of snow may fall. This snow will feature 
generally lower-than-climo SLR to create snow load impacts, and the
WSSI-P indicates a 70-90% chance of at least moderate impacts in 
the San Juans, 20-40% chance across the other higher elevations. 
This may lead to isolated areas of tree damage in the San Juans, 
where heavy snow may fall just below the tree line (around 11,000 
ft). Although most of this will be above pass level, some locally 
hazardous travel at the higher passes is expected as well.


The probability of significant ice across the CONUS is less than 10
percent.


Weiss/Snell


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