Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
318 AM EST Thu Nov 09 2023

Valid 12Z Thu Nov 09 2023 - 12Z Sun Nov 12 2023


...Great Lakes and Northeast...
Days 1-3...

A wintry mix of snow, sleet, and freezing rain is expected this
morning across the interior Northeast, the first wintry event of
the season. Current analysis showed a mid-level shortwave over
northern Minnesota with a downstream area of low pressure near
Detroit. Its associated warm front, draped through the Upper Ohio
Valley, was advancing northeastward as the vorticity piece lifts
northeast through the Great Lakes. Strong warm air advection and
southerly flow is driving enough isentropic lift to produce a
widespread area of precipitation across much of Upstate New York
early this morning. With high pressure anchored to the north, a
residual pool of cold air near the surface will be present to
produce a mix of snow, sleet, and freezing rain. Initially, and
especially across the northern extent of the precipitation area,
the air mass is cold enough to support a burst of heavy snow,
which then will be followed by a transition to sleet and freezing
rain as the column warms. For snowfall, a quick/intense coating of
a couple inches will be possible across the White Mountains of
northern NH through northern ME (WPC probabilities for 2" are
20-40 percent) while further south a glaze of ice accumulation
will be possible, particularly for the higher elevations where the
surface temperatures will be cold enough to offset some of the
latent heat release of freezing. Ice probabilities of 0.01" are
moderate (30-50%) but quickly drop off for any accumulation grater
than 0.1" (less than 10 percent).

In the wake of the system, west to northwest flow will provide a
somewhat favorable setup for lake effect precipitation but the
airmass will only be marginally supportive of snowfall so snowfall
accumulations are expected to be relatively light and confined to
some of the higher elevations of the Michigan Upper Peninsula
where a few inches will be possible.


...Pacific Northwest into the Northern Rockies...
Days 1-3...

Unsettled weather pattern will take shape across the Pacific
Northwest into the Northern Rockies over the next several days as
a pair of atmospheric river events move through. The first system
will arrive later today through tonight with a weak mid-level
shortwave driving the large scale forcing for ascent with modest
amounts of IVT per the CW3E (250 kg/m/s). The system passes
quickly eastward early Friday morning, reaching the Intermountain
West and Northern Rockies by Friday afternoon. Despite its quick
passage, modest amounts of moisture and lift and snow levels
around 4000 ft will support locally moderate to heavy snowfall for
the peaks of the Olympics and Washington Cascades where the WPC
snow probabilities for at least 4 inches are moderate to high
(40-90%) through 00Z Fri.

Another embedded shortwave trough will follow quickly, pushing
through the region Saturday into Saturday night, but is likely to
take a track that is further north than the first, focusing more
across southern British Columbia. Regardless, this system will
have more moisture and lift associated with it and should produce
another round of moderate to heavy rainfall mainly for the
Olympics and northern Cascades Saturday/Saturday night before
reaching the northern Rockies. The latest WPC snow probabilities
are high (>80%) again for 4 inches, especially northern Cascades
where some locally significant totals will be possible (20-30% for
18" totals at the highest elevations). Additionally, WPC
probabilities for more than 4 inches reach 10-30% near Glacier
N.P. in the Northern Rockies.

Taylor