Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
334 PM EST Sat Nov 27 2021

Valid 00Z Sun Nov 28 2021 - 00Z Wed Dec 01 2021


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

Broad but amplifying cyclonic flow will persist across the Great
Lakes and Northeast through the middle of next week as a longwave
trough sharpens. Within this flow, a parade of shortwaves will
race southeast from Alberta/Saskatchewan, to the Great Lakes, and
then off the Northeast Coast, each producing a round of
precipitation. The first of these will race quickly from the Ohio
Valley to southeast of New England D1 producing a swath of very
light snow but no significant accumulations.

Immediately following this leading shortwave, a more impressive
energy pulse will drop through the Arrowhead of MN and into the
Eastern Great Lakes while closing off at 500mb to begin D2. This
feature will then sharpen and tilt negatively while rotating
across New England on Monday. This impulse will be tailed by a
Pacific jet streak which will place its diffluent LFQ atop the
best mid-level height falls to produce deep layer ascent. This
setup is likely to produce a swath of light snow from the U.P. of
MI southeast through western PA and NY, where WPC probabilities
for 2 inches are above 60% in most of this region. However, there
is likely to be some enhancement to the snowfall in a narrow
corridor where some frontogenetical response to the upper jet is
likely. There continues to be uncertainty into the placement and
intensity of this enhancement, but WPC probabilities show a low
risk for more than 4 inches in eastern MI and western NY. More
likely, enhancement will occur in the favored NW snow belts as LES
ramps up across the U.P., northern L.P., and near the Chautauqua
Ridge. In these regions, intense forcing and steepening lapse
rates should produce efficient and intense snowfall, with rates
exceeding 1"/hr at times likely. The duration of these snow bands
should be somewhat limited, but WPC probabilities for 6 inches are
moderate on D1, and become high on D2 east of Lake Erie where more
than 12 inches of snowfall is possible for the event.

As this same shortwave pivots northeast and becomes negatively
tilted, a downstream jet streak will arc poleward, and the overlap
of ascent should lead to pressure falls and rapid cyclogenesis in
the Gulf of Maine. The guidance continues to feature a wide spread
in timing and positioning of this low, and with the 12Z/27 suite
there exists a dichotomy between the high res models, which are
closer to the coast, and the globals which are further east.
However, the ensembles at this time suggest all solutions are
still possible, which is reflected by snowfall that is low in the
WSE mean, but high in the 90th percentile as moisture and forcing
will be plentiful if the system can be close enough to the coast.
WPC probabilities for 4 inches are less than 20% on D2 from
southern Vermont into Maine, but increase to 40% in northern Maine
D3 as the low makes its likely most intense and closest approach.


...Pacific Northwest...
Day 3...

Moisture plume embedded within confluent mid-level flow will
collapse southward slightly in response to a shortwave moving into
British Columbia on Tuesday. WAA associated with a warm front
moving onshore will spread precipitation into Washington State,
but will also drive snow levels to 8000 ft. Ample moisture
associated with this impulse will produce heavy precipitation, but
WPC probabilities for more than 4 inches of snow are confined to
the highest peaks of the WA Cascades and are less than 30%.


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

Weiss