Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Wed Nov 24 2021

Valid 00Z Thu Nov 25 2021 - 00Z Sun Nov 28 2021


...Pacific Northwest and the Northern Rockies...
Days 1-3...
Dry conditions are expected to prevail across much of the
Northwest into early Thursday as an upper ridge shifts east across
the region. A series of shortwaves will begin to flatten the ridge
with a lead cold front Thursday evening and a warm front on
Saturday, separated by a brief respite on some of Friday. Snow
levels are expected to be high through the event, with WPC
probabilities indicating little to no potential for significant
accumulations for the primary passes in the northern Cascades.
Probabilities show little potential for significant accumulations
in the northern Rockies as well, confined to the highest
elevations above about 5000 ft.

...Great Lakes and the Northeast...
Days 2-3...
A shortwave trough currently centered over the northern Plains is
forecast to move steadily eastward, amplifying as it moves through
the Upper Midwest and northern Great Lakes tomorrow. Strong
northwesterly flow and cold air advection behind an area of low
pressure moving across the U.P. of Michigan and deepening over
southern Ontario will support increasing lake effect snow showers,
with the potential for at least a few inches of snow for portions
of the eastern U.P. and northwestern Lower Michigan on Thursday.
Models show the system continuing to deepen, with a
negatively-tilted upper trough/closed low developing over the St
Lawrence Valley/Northeast on Friday. Surface low pressure will
likely weaken on Friday north of Vermont as a wrapped-up oceanic
system retrogrades northwestward, eventually merging with and
overtaking the approaching system from the west. Result will be a
period of moderate to strong upslope snow around the newly
deepening low, with ample moisture in the lower levels and
potential for at least modest snow totals where lift can maximize
in the DGZ as shown in some of the models. Though the evolution is
a bit complex, the guidance has been mostly steady in their
depiction. Higher elevations of northern NY/VT/NH into
northwestern Maine show the potential for highest totals. Farther
southwest, lake effect off Lake Erie will favor NW PA into SW NY
on NW winds day 2 and into the Tug Hill plateau off Lake Ontario.
WPC probabilities for at least 4 inches of snow ending Saturday
evening are at least 50 percent in these snow belt areas and into
the northern Adirondacks. For northern VT/NH into northwestern ME,
WPC probabilities of at least 6 inches of snow for the 2-day
period are generally 50-80 percent, straddling the U.S. Canadian
border.

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

Fracasso/Pereira