FOUS11 KWBC 011911

QPFHSD



Probabilistic Heavy Snow and Icing Discussion

NWS Weather Prediction Center College Park MD

211 PM EST Sun Nov 1 2026



Valid 00Z Mon Nov 02 2026 - 00Z Thu Nov 05 2026



...Sierra Nevada into the Intermountain West & Rockies...

Days 1-2...



An active period of unsettled weather begins today across the

western CONUS as anomalous moisture surges northeast from the

tropical Pacific. The primary mechanism driving this surge of

moisture is an elongated and amplifying longwave trough shifting

into the Intermountain West, with embedded shortwaves and lobes of

vorticity swinging through its base. This amplified trough, with

500mb heights below the 10th percentile of the CFSR database, will

steadily translate eastward, and combine with a modest but

favorable placed subtropical jet streak to drive large scale deep

layer ascent from the Sierra eastward through the Central Plains.



Downstream of this trough, an extreme moisture surge will push IVT

across a large breadth of the region to above the maximum of the 

CFSR climatology, which will support widespread precipitation in

response to the impressive ascent overlapping this moist conveyor.

While the accompanying WAA will keep snow levels elevated, around 8000-9000

ft within the heavier precipitation corridor, some locally lower

snow levels are possible where fgen/upslope flow can drag colder

air down within steep lapse rates beneath the upper trough. Still,

the most significant snowfall is expected above 9000 ft and the

probability for an additional 12+ inches of snow is above 70% in

the San Juans. Big Horns, and highest CO Rockies, with lower

amounts expected elsewhere above 6000 ft across much of the

Intermountain West. Since this snow is likely to feature sub-climo

SLR, snow load impacts are expected in these areas.



As the trough pushes east a surface low is likely to deepen in the

lee of the Rockies and then rapidly intensify during Monday as it

lifts into the Upper Midwest/Great Lakes. This low will produce

only rain for most areas, however, depending on the exact evolution

of the associated upper low, a pivoting axis of deformation may

cross the Dakotas and into WY/CO Monday night and Tuesday. Where

precipitation falls heavily enough, some mixed rain and snow is

possible, leading to light accumulations of snow as far east as the

Black Hills of SD.



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

percent.



Weiss



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