Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
334 AM EST Tue Dec 08 2020

Valid 12Z Tue Dec 08 2020 - 12Z Fri Dec 11 2020

...Pacific Northwest...
Days 1-3...

A strengthening but zonally oriented Pacific jet streak will race
across Southern Canada through Wednesday, gradually leaving the
favorable RRQ for ascent atop the Pacific Northwest and into the
Northern Rockies. This feature will advect enhanced column
moisture to the region, which will be wrung out through ascent via
this RRQ combined with weak height falls as a shortwave moves
overhead. This ascent will lead to a weak surface low pressure
developing, with convergence along the attendant cold front aiding
in precipitation development. Snow levels will initially be high
but gradually lower with the height falls and cold front, but the
system remains progressive so snowfall will be modest. WPC
probabilities for 4 inches are above 50% only in the Northern
Rockies on D2.

As the moisture from the above forcing spreads eastward, it will
encounter a cold surface high stretched east of the WA Cascades
and into the Columbia Basin. This cold air will be slow to scour,
so an area of freezing rain potentially accreting to more than
0.1" is likely across parts of central WA on Tuesday.

The next shortwave approaches the WA Coast Thursday night bringing
renewed chances for snowfall to the Cascades of WA and OR. WPC
probabilities for D3 are generally less than 20% for 4 inches of
new snowfall.


...New York...
Day 2...

A fast moving shortwave diving out of Ontario as a Clipper type
system will race across Upstate New York Wednesday with height
falls and PVA. Mid-level flow will gradually become unidirectional
from the W/NW as this feature drops across the area with a swath
of moderate snowfall. Most of the accumulations across NY and into
New England should be light. However, there is the potential for
some enhancement due to LES off Lake Ontario and into the Tug Hill
Plateau/Adirondacks. Although the column is thermally marginal for
snow, and much of the saturation remains in the lower portion of
the DGZ or lower, WPC probabilities for 4 inches are moderate in a
few areas.


...Southwest...
Day 3...

A cutoff low anchored off the CA coast through Wednesday will
gradually open and eject towards the Four Corners Thursday and
into the Southern Plains Friday morning. Modest forcing through
height falls and broad upper divergence will spread precipitation
northeastward into Arizona and New Mexico on D3. Snow levels are
forecast to be high, but in the highest terrain of the Sangre De
Cristos, White Mountains, and Mogollon Rim, WPC probabilities show
a low risk for 4 inches of snowfall.


Weiss