Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 AM EST Sun Nov 07 2021

Valid 12Z Sun Nov 07 2021 - 12Z Wed Nov 10 2021

...The West...

Days 1-3...
Two distinct shortwaves will rotate onshore the Pacific Northwest
through Tuesday, as broadening cyclonic flow within a longwave
trough persists across the Western CONUS. The first shortwave is
progged to move into WA state tonight, combining with modest upper
diffluence as a jet streak approaches from the Pacific and a
weakening cold front/trough to lower snow levels across the
Northwest. A brief increase in warm and moist advection ahead of
this trough will spread some above normal PWs as far east as
Wyoming, and the overlap of ascent in this moistening column will
produce periods of moderate to heavy snow in the Cascades,
Olympics, Northern Rockies, and into the Wind River range of WY.
WPC probabilities for 6 inches on D1 are above 70% in the Olympics
and WA Cascades where locally 2 feet of snow is possible. Pass
level snow could be 3-6" Sunday and Sunday night.

Brief shortwave ridging will move across the Northwest on D1.5-D2
before a more impressive shortwave and sharp height falls lifts
into the CA/OR coast Monday night into Tuesday morning. This
feature will be accompanied by impressive upper diffluence within
the LFQ of a potent Pacific jet streak which is progged to exceed
150kts and angle orthogonally onto the coast. This Pacific jet
combined with strong mid-level confluence will drive robust
moisture eastward, with IVT exceeding 500 kg/m/s driving PWs to
more than 2 standard deviations above the climo mean. The heaviest
snowfall D2 is likely in the Shasta/Trinity ranges of northern CA
where upslope flow and more intense snowfall rates should keep
snow-levels to around 3000-4000 ft despite the increasing warm
plume of moisture. WPC probabilities D2 are high for 6 inches in
these ranges, and locally 1-1.5 feet is possible. Elsewhere, D2
probabilities for 6 inches reach 20-30% in the Northern Sierra and
Olympics/WA Cascades.

During D3 /Tuesday and Tuesday night/ the impressive jet streak
and associated atmospheric river push well inland while continuing
to plague the coast with precipitation. Heavy snow is likely again
in the WA and OR Cascades where additional snowfall of over 1 foot
is possible in the highest terrain. With snow levels remaining
just 2000-3000 ft across WA, heavy snow of 6+ inches is becoming
more likely even as low as Snoqualmie Pass. Continued deep layer
ascent within the eastward shifting AR will drive heavy snow
further east Tuesday despite the slowly warming snow levels. WPC
probabilities for 6 inches are above 50% in the Sawtooth Range and
Sierra of CA, and 20-30% as far east as the Uintas and near
Yellowstone NP.

For Days 1-3, the probability of significant icing is less than 10
percent.

Weiss