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

Valid 12Z Mon Nov 11 2024 - 12Z Thu Nov 14 2024

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

Two distinct and anomalous shortwaves will amplify and move
progressive across 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.

The first of these will approach the Pacific Coast Monday morning,
with downstream SW flow driving robust IVT approaching 500 kg/m/s
onshore into CA/OR/WA. 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 advect onshore Monday
aftn, the overlapping ascent into the moistening column will result
in heavy precipitation spreading from central CA through the WA
Cascades, and then pushing east, while weakening due to lessening
IVT, 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 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. Lower probabilities exist into the Shasta/Trinity region,
the northern Sierra, and as far east as the Blue Mountains of OR.

During D2 /Tuesday and Tuesday night/ the lead trough and
accompanying cold front continue to push east as far as the Central
Plains while weakening, while shortwave ridging briefly builds in
across the Northwest. This suggests that much of D2 will feature
waning snowfall with lesser accumulations. However, before D2 ends,
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 snowfall before D2 ends except in 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 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


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