Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Wed Dec 20 2023

Valid 00Z Thu Dec 21 2023 - 00Z Sun Dec 24 2023

...Sierra Nevada to the Southern Rockies...
Days 1-3...

An upper low dawdling off the southern California coast will
channel Pacific moisture into the Southwest today and into
Thursday. Diffluent flow ahead of the upper low combined with
upsloping southerly flow will result in periods of heavy snow in
the southern Sierra Nevada, particularly for elevations >8,000ft.
Notable locations that receive heavy snowfall include the Inyo,
Sequoia, and Kings Canyon National Park areas. Latest WPC PWPF
shows moderate probabilities (40-60%) for >8" of snow in the
southern Sierra Nevada, with the highest elevations seeing the
higher end of that range of probabilities. The latest WSSI does
depict Major impacts within some of these higher elevated areas
above 9,000ft with Snow Amount the primary driver in the WSSI
algorithm. Expect considerable disruptions to daily life in those
impacted areas.

By Friday morning, the upper low looks to speed up finally as it
heads for the Lower Colorado River Valley. The upper low will
eventually find itself tracking through southern Arizona Friday
evening with healthy 250-500mb diffluence  over the Gila and White
Mountains of eastern Arizona and eventually the San Juans of
southwest Colorado. In addition, there is no shortage of moisture
present as NAEFS shows IVT values (200-300 kg/m/s) and PWs (up to
0.50") that this time of year surpass the 97.5 climatological
percentile. While there is a lack of sufficient cold air for the
valleys, the strong diffluence aloft, combined with ample moisture
and upslope flow enhancing precipitation rates will lead to heavy
snowfall in these mountains ranges starting Friday afternoon and
lingering into Saturday morning. Latest WPC PWPF shows high
probabilities (60-80%) for snowfall totals >8" near Apache
National Forest and into the San Juans above 9,000ft. The
Probabilistic WSSI depicts high chances (>70%) for Minor Impacts
in these mountains ranges, as well as the Nacimiento Mountains
west of Santa Fe.

...Pacific Northwest to the Northern Rockies...
Days 2-3...

A shortwave trough approaching from the northeast Pacific looks to
provide an influx of Pacific moisture and sufficient vertical
ascent aloft to generate periods of heavy snow in the higher
terrain of the Olympics and Cascades Friday morning. As winds
shift out of the west and prolong the fetch of 850mb moisture flux
in wake of the cold frontal passage, snow levels will drop to as
low as 2,000ft and SLRs will rise. The upslope component will
continue to support topographically induced heavy snowfall in the
Cascades Friday evening. Meanwhile, the upper level disturbance
will work its way into the northern Rockies Friday night, bringing
periods of snow to ranges such as the Boise, Bitterroots,
Absaroka, and Tetons. The heaviest snowfall appears to be in the
Cascades where WPC PWPF sports moderate chances (40-60%) for
snowfall totals >6" on Friday. The heaviest snowfall totals will
likely reside above most major passes in western Washington for
this event. The Probabilistic WSSI shows some moderate to high
(50-80%) probabilities in the higher elevations north of I-90,
implying winter driving conditions are most likely there and
motorists should exercise caution while driving.


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


Mullinax