FOUS11 KWBC 111906
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
206 PM EST Mon Nov 11 2024

Valid 00Z Tue Nov 12 2024 - 00Z Fri Nov 15 2024

...Western U.S...
Days 1-3...

Two distinct and anomalous shortwaves will amplify and impact the
Western CONUS through Thursday, bringing an extended period of
unsettled weather to the region, with several days of moderate to
heavy snow expected in the higher terrain (mainly above 5000 ft).

The first of these is currently progressing inland across the
Pacific Northwest today, with downstream SW flow driving robust IVT
approaching 500 kg/m/s onshore into central CA. This IVT will help
surge PWs to above climo directly ahead of a surface cold front
which will be positioned beneath the trough axis. As this
trough/front further slide eastward this evening, heavy
precipitation remains possible in the central/southern Sierra. This
moisture axis is forecast to weaken due to lessening IVT as it
slides east, reaching the Northern Rockies and eastern Great Basin
by Tuesday morning. Snow levels ahead of this front will be
generally around 6000 ft during the period of heaviest
precipitation, but will fall quickly to as low as 3500 ft behind
the front. At the same time, while the primary forcing mechanisms
will shift east, continued onshore moist advection in pinched mid-
level flow will drive persistent moisture onshore, resulting in
snow accumulations lowering across the Cascades/Olympics. WPC
probabilities D1 (through Tuesday evening) for more than 6 inches
of snow are above 70% along the spine of the Cascades in OR and WA,
with up to 2 feet possible in the highest terrain.

During D2 (Tuesday night and Wednesday) the lead trough and
accompanying cold front continue to push east as far as the Central
Plains while weakening, as shortwave ridging briefly builds in
across the Northwest. This suggests that the first half of D2 will
feature waning snowfall with lesser accumulations. However, by
Wednesday, the subsequent trough will approach the Pacific
Northwest once again. This trough is progged to be deeper, noted by
700-500mb height anomalies reaching -2 sigma according to NAEFS,
with the subsequent downstream warm/moist advection in the mid and
upper levels driving IVT to as high as 750 kg/m/s, and ECENS
probabilities suggest even a 30% chance of IVT exceeding that
value. This WAA will occur within a narrow channel ahead of yet
another cold front, driving a corridor of heavy precipitation in
conjunction with snow levels rising once again to 6000-7000 ft.
This will limit the heaviest snowfall on D2 to the highest terrain
of the Olympics and Cascades.

The trough axis responsible for pushing the front eastward and
driving the pronounced deep layer ascent across the Northwest will
struggle to move substantially east as repeated lobes of vorticity
rotate down from the Gulf of Alaska, resulting in little movement
of the trough axis and persistent, although weaker, moist advection
onshore. This suggests that snow levels will only slowly fall,
progged to drop to 4500-5000 ft by the end of the forecast period,
with primarily light to moderate precipitation rates from northern
CA through the Northern Rockies. This will still result in heavy
snow accumulations at the highest elevations, reflected by WPC
probabilities for more than 6 inches exceeding 90% in the northern
WA Cascades, and then expanding east into Selkirk Range, the Blue
Mountains, and the Sawtooth/Salmon River area. The higher passes
above 5000 ft will also likely experience significant impacts from
snow accumulations, but in general most of the passes should be
below the heavier snow accumulation levels.


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


Weiss/Snell





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