Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
332 AM EST Sun Nov 28 2021

Valid 12Z Sun Nov 28 2021 - 12Z Wed Dec 01 2021


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

Within the large, amplifying cyclonic flow across the Great Lakes
region and Eastern U.S., a pair of fast moving shortwave troughs
embedded within the flow will race across the Great Lakes over the
next 24 hours, acting to sharpen the trough axis over the region.
While the first wave passing through early this morning will bring
light/minor snow accumulations to the region, the second upstream
wave is much stronger and pronounced. As this shortwave trough
tracks from northern MN this morning to upstate NY tonight, it
will deepen and sharpen the trough axis in place and in its wake,
much colder air will surge south/southeast. This combination of
forcing for ascent and steepening lapse rates over the relatively
warm lake waters will lead to a period of intense lake effect
snows, particularly downwind of Lake Erie and Ontario. The timing
of the best forcing with daytime heating destabilization could
yield narrow but intense snow rates exceeding 1"/hr at times,
especially east of Lake Erie. Day 1 WPC probabilities for 4 inches
are moderate /50-60 percent/ across portions of northern Michigan
and exceed 90 percent east of Lake Erie where there is also a
slight signal for 8-12 inches during the day 1 period.

As that shortwave approaches the Northeast, 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
bulk of the 00Z guidance still advertises a solution that is
enough offshore to limit any potential moisture wrapping around
for southern New England, but the low could approach close enough
to coastal Maine to bring some precipitation as depicted by some
of the hi-res guidance including the NAM3km, ARW, and ARW2. For
now, a consensus approach leaned toward the global models was
preferred for the QPF and snow forecasts, but given the larger
model spread and range of solutions, this system will continue to
be watched closely over the next 24-36 hours. For now, the WPC
probabilities depict slight probabilities for 4" across interior
New England while moderate probabilities around 40 percent are
present for eastern Maine.

Finally, another shortwave trough will swing through the flow
during the day 2-3 period, skirting the northern Great Lakes
region Monday into Tuesday. Less pronounced forcing and cold air
advection should keep snowfall accumulations light and minor with
the WPC probabilities for 2" highest confined to the northern
areas of the U.P., northern L.P., and off Lake Erie and Ontario.
The signal for 4" during the day 2-3 period drops considerably to
under 10 percent.


...Pacific Northwest...
Days 2-3...

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.

Taylor