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