Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
313 AM EST Fri Nov 26 2021

Valid 12Z Fri Nov 26 2021 - 12Z Mon Nov 29 2021


...Pacific Northwest and the Northern Rockies...
Days 1-2...

An embedded shortwave trough moving through the fast quasi-zonal
flow will lead to heavy precipitation across portions of
Washington and into the northern Rockies through tonight. The
combination of forcing for ascent and modest moisture will give
way to modest snow accumulations above mostly pass levels for the
northern WA Cascades as well as portions of Northern Rockies. WPC
probabilities for 4" are moderate. Ridging building in late
tonight will lead to a quick end of the precipitation ahead of the
next stronger atmospheric river event. This AR, focused during the
Saturday-Sunday time frame will have high snow levels, confining
the greatest snow accumulations to the peaks of the northern WA
Cascades.


...Great Lakes through the Northeast...
Days 1-3...

Two separate storm systems will bring periods of moderate to heavy
snow through the weekend.

A mid/upper level shortwave trough will become negatively tilted
and eventually close off over upstate NY today while its primary
surface low gradually fills as it moves slowly from near Lake
Huron to the Adirondacks. This low is gradually replaced by a
secondary low off the coast of NH/ME that then deepens as it moves
northeast. As the secondary low develops in the Gulf of Maine,
forcing for ascent will intensify and a weak TROWAL will attempt
to pivot into Maine before the entire system lifts northeast and
crawls across Canada. While the most intense synoptic lift will
displace eastward, strong upslope flow into the terrain of the
Adirondacks, Greens, and Whites will produce a prolonged period of
heavy snowfall within which rates will likely exceed 1"/hr. It is
this upslope that should produce the heaviest snowfall through
tonight into Saturday morning. WPC probabilities for 4+ inches are
high in the NW Adirondacks, the Greens, the NE Kingdom of VT, and
the Whites of NH/ME, with locally more than 12 inches likely in
the highest terrain. Heavy snow is also likely across northern
Maine which will be in closer proximity to the low itself, and WPC
probabilities are high for 4 inches with more than 8 inches
likely. Additionally, favorable flow behind the advancing cold
front will bring a period of enhanced lake effect snow today and
tonight, particularly downwind off Lakes Erie and Ontario. This
should bring a period of locally heavy snowfall to the favored
snow-belt areas of western NY, northwest PA as well as portions of
Upstate NY and the Tug Hill area. WPC probabilities for the LES
areas are above 70-80 percent for 4" with a modest signal for
exceeding 6". 

The second storm system will be a fast moving shortwave clipper
racing out of Saskatchewan toward the Upper Midwest and Great
Lakes this weekend. Favorable jet dynamics and a sharpening
shortwave will likely bring a narrow/focused area of precipitation
that could result in a few inches of snow. The latest trends show
the track favoring far northern/northeast MN, northern WI into
central MI where WPC probabilities for 2" are 40 to 60 percent. As
the system pulls away from the Great Lakes on Sunday/Sunday Night,
another round of favorable winds will bring lake effect snows
downwind of Lake Erie and Ontario where WPC probabilities for 4
inches are up to 30 percent during the D3 time frame.


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

Weiss/Taylor