Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 PM EST Sat Dec 03 2022

Valid 00Z Sun Dec 04 2022 - 00Z Wed Dec 07 2022

...Western United States...
Days 1-3...

An active pattern will continue across the West as a closed
mid-level low drops slowly along the Pacific coast and eventually
opens into a positively tilted longwave trough centered over
central CA. While this feature drops southward, it will repeatedly
shed waves of energy E/NE into the Intermountain West as spokes of
vorticity rotate out of the primary low. This will combine with
persistent southern stream jet energy pivoting out of the Pacific
and racing across the West, placing periods of favorable LFQ
diffluence for ascent along its northern flank. At the same time,
this jet will overlap with confluent mid-level flow downstream of
the closed low to advect abundant moisture eastward, with a modest
AR transporting PWs of +1 to +2 sigma as far east as the Four
Corners. Much of the forcing this period across the west will be
driven then by PVA, jet level diffluence, modest height falls, and
downstream WAA, with snow levels climbing to as high as 8000 ft D1
in the Sierra, but then gradually cooling through the remainder of
the period, becoming more generally 3000-5000 ft across the West,
but just 500-1000 ft near the Canadian Border.

This will result in rounds of heavy snowfall D1 and D2 across much
of the Sierra and into the northern CA ranges of the
Shastas/Trinities/Siskiyous. The heaviest snow accumulations are
expected in the Sierra where the highest moisture will be wrung
out by the synoptic lift but also impressive upslope as 700mb
angles into the terrain. WPC probabilities for more than 8 inches
are above 90% D1 in the Sierra, and 40-60% farther north, with
additional heavy snow shifting northward on D2. Snow totals during
this two day period will likely exceed 3 feet in the higher
terrain, and heavy snow impacting both Donner and Black Butte
Passes.

While the heaviest snow D1-2 is expected in the Sierra and other
northern CA ranges, heavy snow accumulations are also likely to
spread eastward through the Sawtooth of ID and into the NW WY
ranges in response to subtle vorticity impulses and the continued
jet level ascent on the periphery of the best moisture plume. WPC
probabilities D1 are high for 4+ inches as far east as the Wind
River ranges, with locally more than 10 inches of snow possible in
the highest terrain. By D2 snow continues into the Bitterroots,
the NW WY mountains, and farther south into the Uintas and Wasatch
with WPC probabilities for 4 inches or more spiking above 70% in
the highest terrain of many of these ranges and surrounding areas.

Two areas of heavier snow may develop late D2 into D3. The first
is from the Northern Rockies into the High plains of MT as a
shortwave shifts eastward and interacts with a cold front that is
digging southward out of Canada and banking into the terrain. At
the same time, high pressure behind this front sinks southward
providing some impressive upslope flow into the terrain. The
overlap of upslope flow with modest fgen along the edge of the
moisture plume and a shortwave noted in 500mb progs to drive
subtle height falls will result in bands of heavy snowfall from NW
to SE. There may be two rounds of snow, or it is possible that
features combine to produce a longer duration event in this
region. There is also some uncertainty into how much moisture will
be available north of the southward sinking moisture plume.
However, WPC probabilities at this time are high for more than 4
inches from the Northern Rockies into central MT, but these
probabilities may change quite a bit as the event approaches.

Also on D3, the placement of the primary longwave trough axis in
CA leaves downstream divergence across the Four Corners, directly
beneath the best upper jet streak and in the vicinity of shedding
shortwave energy. This could result in surface low development
east of the Rockies in CO, providing additional ascent into the
terrain. The guidance has become a bit more aggressive with this
evolution for D3, resulting in an increase of WPC probabilities
for more than 4 inches which now reach 50-80% in the portions of
the CO Rockies including the Flat Tops, Sawatch, and Elk Mountains.


For Days 1-3, the probability of significant icing exceeding 0.10
inch is less than 10%.


Weiss