Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
311 PM EST Sat Nov 13 2021

Valid 00Z Sun Nov 14 2021 - 00Z Wed Nov 17 2021


...Pacific Northwest to the Northern Rockies...
Days 1-3...
Another atmospheric river event is forecast to impact the region
beginning overnight, with heavy precipitation expected across the
orographically favored regions of western Washington.  Snow levels
are forecast to rise, climbing to at or above 8000 ft for most
regions overnight.  Snow levels are expected to remain high
through the remainder of the weekend, before a gradual decrease
early Monday as a weak shortwave begins to suppress the prevailing
ridge.  This will be followed by a more pronounced lowering as an
amplified shortwave and its associated cold front push through the
region later in the day.  By late Monday, diminishing onshore flow
and moisture advection will support decreasing heavy precipitation
potential.

Meanwhile, moisture spreading east of the Cascades is expected to
support some locally heavy mountain snows for the northern Idaho
Panhandle into northwestern Montana.  Snow levels are expected to
begin at or below 4000 ft this evening, but gradually increase
overnight.  Snow level are forecast to remain high until Monday
night when the previously noted upper shortwave and surface cold
front push through the region, supporting periods of heavy snow
and strong, gusty winds.

...Upper Midwest, Great Lakes, and Northeast...
Days 1-2...
A clipper system will continue to slide southeast across the
northern Plains into the upper Mississippi Valley this evening and
overnight.  Higher probabilities for snowfall accumulations of 2
inches or more are a reflection of the 12Z model consensus -- with
most models trending wetter than the overnight guidance across
north-central Minnesota to southern Wisconsin.  WPC probabilities
now indicate that widespread amounts of 2 inches or more likely,
with mesoscale banding supporting the potential localized amounts
of 4 inches or more.  While accumulating snows are likely, weaker
dynamics along daytime heating are expected to help limit snowfall
accumulations as the system moves across Lower Michigan during the
day on Sunday. 

Lake effect showers are expected to continue east of lakes Erie
and Ontario into the overnight.  Then synoptic snows are forecast
to spread across the eastern Great Lakes and Allegheny Plateau
into central and northern New York late Sunday-early Monday.  This
will be followed by additional lake effect activity in the wake of
the wave that is forecast to continue through Monday, before a
shortwave ridge shifts east across the Great Lakes by early
Tuesday.  Two-day snowfall totals, ending late Monday, are likely
to exceed 4 inches in the western New York and northwestern
Pennsylvania snow belts, as well as the Tug Hill and western
Adirondack regions.  


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

Pereira