Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
302 PM EDT Tue Oct 14 2025

Valid 00Z Wed Oct 15 2025 - 00Z Sat Oct 18 2025

...Sierra through the Northern Rockies...
Days 1-3...

An impressive closed low will reach peak intensity tonight just
west of San Francisco before beginning to advect northeast on
Wednesday. NAEFS height anomalies show 500mb percentiles dropping
to below the 0.5th percentile rank in the CFSR climatology before
slowly filling as the entire longwave trough moves across the Great
Basin and then into the Rockies by Thursday. This trough is then
progged to become an open wave by Friday morning as it exits into
the Northern High Plains.

Despite the slow weakening, impressive deep layer ascent will
expand downstream of this evolution, as height falls, PVA, and a
strengthening jet streak overlap to produce surface cyclogenesis.
This low pressure will likely move steadily northeast across the
Great Basin through Wednesday night before occluding, with
secondary lee-cyclogenesis in CO likely on Thursday.

Accompanying the primary closed low tonight into Wednesday,
impressive moisture advection/IVT will push inland reflected by
moderate probabilities (>60%) for 500 kg/m/s pushing into CA and
NV. With snow levels falling beneath the upper low and behind the
cold front accompanying the surface low, this will result in heavy
snowfall, generally above 5500-6000 ft in the Sierra and into the
terrain of Nevada where WPC probabilities for 8+ inches of snowfall
are as high as 50%, with locally 12" or more possible (30%) in the
Ruby Mountains of Nevada through D1.5. Briefly heavy snowfall
rates exceeding 1"/hr are possible (30-50% chance) in the Sierra
and higher terrain of Nevada D1 thanks to elevated instability and
favorably placed ascent into the DGZ. With SLRs likely to be
modest, this will result in at least some moderate impacts at
elevated mountain passes tonight and Wednesday.

Thereafter, the intensity of the snowfall and the impacts should
decrease in response to weaker overall ascent, but heavy snow is
likely to spread into parts of UT and WY, including the Wasatch,
Uintas, and ranges around Yellowstone NP. Snow levels moderate at
this time to, generally hovering around 7000 ft, but steep lapse
rates beneath the mid-level trough could allow for some lowering to
around 6000 ft during times of heavier snowfall as theta-e lapse
rates fall to 0C/km at times suggesting convective snow shower
potential, especially during periods of pronounced upslope into
terrain features. WPC probabilities for 8+ inches on D2 are
moderate to high in the Tetons, Wind Rivers, and highest terrain
surrounding Yellowstone NP, but pass level snow should remain below
6 inches. On D3, snow wanes quickly as the system ejects to the
east, leaving just minimal additional accumulation across the
higher terrain of NW WY.

The probability of significant freezing rain across CONUS is less
than 10 percent.


Weiss