FOUS11 KWBC 100738
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
238 AM EST Sun Nov 10 2024

Valid 12Z Sun Nov 10 2024 - 12Z Wed Nov 13 2024

...Northwest...
Days 2-3...

Dual amplified shortwaves will bring an active period of weather to
the Pacific and Interior Northwest through mid-week. The first of
these will dig out of the Gulf of Alaska and approach the British
Columbia coast Sunday night, before sharpening into a short-
wavelength but amplified open wave advecting onshore WA/OR Monday
evening. This feature will then move quickly eastward into the
Northern Rockies while weakening through Tuesday. Although this
trough will be of modest amplitude and progressive, height falls,
divergence and dual jet streaks (one poleward arcing downstream of
the trough axis, another more zonally oriented upstream) will lead
to widespread deep layer ascent from the Pacific Northwest eastward
through the Northern Rockies. At the same time, this trough will
drive a cold front eastward, surging onshore the WA/OR coasts
Monday morning before racing into the Northern High Plains by the
end of the forecast period.

This synoptic lift will act upon an increasingly moist column noted
by rapidly rising 1000-500mb RH as warm/moist advection occurs
within the aforementioned jet streaks, driving IVT to 250-500
kg/m/s. This will support widespread moderate to heavy
precipitation, and as snow levels collapse from around 6000 ft
early to as low as 3500 ft D2, snow will become widespread in the
terrain of the Olympics, Cascades, Shasta/Trinity region of CA, the
Sierra, and then into the Northern Rockies. WPC probabilities for
at least 6 inches of snow are 70+% along the spine of the Cascades
of WA and OR, with amounts reaching multiple feet likely in the
highest volcanoes. There is also a 30-50% chance of more than 6
inches in the Shasta/Trinity region, the northern Sierra, the Blue
Mountains of OR, and the far Northern Rockies.

After a brief period of shortwave ridging across the Pacific
Northwest, a more pronounced impulse digs into the region during D3,
nearly reaching the coast by the end of the period. This will be
accompanied by more impressive moist advection with ECENS IVT
probabilities reaching above 90% for 500 kg/m/s, with several
members exceeding 750 kg/m/s. The accompanying WAA will surge snow
levels back to around 5000-6000 ft along the immediate coast, but
rise only to around 4000 ft across the Cascades. This will support
more widespread heavy snow, with generally below climo SLRs leading
to high snow load and greater impacts, especially across the WA
Cascades. WPC probabilities are above 9)% in the WA Cascades,
primarily above 5000 ft, with 1-2 feet likely in the higher
elevations. Some pass level impacts are also expected, especially
at Washington Pass, with lesser but still potential impacts at
Stevens Pass.


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

Weiss


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