Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
205 PM EST Sun Nov 16 2025

Valid 00Z Mon Nov 17 2025 - 00Z Thu Nov 20 2025

...Northeast/Great Lakes...
Days 1-2...

Amplifying closed mid-level low will continue its slow trek into
the Canadian Maritimes tonight through Monday, but wintry
precipitation will continue in earnest across portions of Northern
New England and Upstate New York.

While the primary warm conveyor will pivot well east and away from
the United States, a modest TROWAL will persist into northern Maine
bringing some pivoting/comma head snowfall with rates of 0.5-1"/hr
at times. This combined with increasing northerly winds will bring
moderate impacts to the north and east portions of the state,
especially the first half of the period, before the DGZ dries out
and precip wanes. WPC probabilities for additional snowfall
exceeding 4" in northern/eastern Maine are generally 30-50%.

The more significant snowfall is expected farther west into the
terrain of central ME and across NH, VT, and Upstate NY. This will
be in response to prolonged and impressive upslope flow as NW winds
develop and strengthen behind the departing low pressure system.
The guidance is in good agreement that unidirectional NW flow will
persist D1 and into D2, bringing periods of heavy snow as ascent is
forced into the deepening DGZ and moistening the lower half of the
resultant column. This should provide an environment favorable for
heavy snow into Tuesday, and with Froude numbers indicating
critical flow, the focus should be along the crests of the higher
terrain and just upwind. With snowfall rates from the HREF progged
to exceed 1"/hr at times, this will produce heavy accumulations for
which WPC probabilities are high (>70%) for an additional 6+ inches
of snow in the northern Adirondacks, Greens, and portions of the
Whites of NH and ME, with locally up to 1 foot of snowfall possible
(10-30%). Precipitation will wane on Tuesday with snow likely
ending Tuesday aftn.

Finally, the pronounced CAA on NW flow behind this system will
cross the Great Lakes and produce some narrow bands of lake effect
snow (LES), especially downwind of Lake Superior and Lakes
Erie/Ontario. While this direction is not ideal for elongated fetch
across the lakes, the high-res guidance is in good agreement that
an upstream connection will occur from Superior, to Huron, to Erie,
with an additional upstream band from Georgian Bay to Lake
Ontario. This should produce narrow corridors of heavy snowfall
exceeding 1"/hr on D1, leading to accumulations for which WPC
probabilities indicate have a 50-90% chance of exceeding 4 inches
across the Chautauqua Ridge, the Tug Hill Plateau, and portions of
the eastern Finger Lakes region, with the latter continuing through
D2.


...California to the Central and Southern Rockies...
Days 1-3...

A pair of upper lows embedded within persistent troughing across
the West will bring periods of high-elevation heavy snow to
portions of the region.

The first will be associated with a compact shortwave lifting
across UT/WY/CO that will deepen into a closed feature as it moves
into the Central High Plains Monday afternoon. The amplification of
this wave will result in increased deformation to its west,
aligning with modest upper diffluence as a weak jet streak develops
in the vicinity. This paired ascent occurring within a moist column
(PWs above the 99th percentile) will result in a transition to
heavy snow through dynamic cooling, especially in the higher
terrain of CO and WY above 7000 ft. WPC probabilities for more
than 6 inches of snow are high (>70%) from the Wind Rivers through
portions of the Uintas and into much of the CO Rockies, with
locally as much as 12 inches possible in the Wind Rivers and the
Park Range.

Upstream of this feature, a more pronounced upper low will dig
slowly along the CA coast, tracking into Baja Wednesday before
finally beginning to eject eastward late in the forecast period.
Downstream of this feature, another round of impressive IVT
exceeding the 90th percentile according to NAEFS ensemble tables
will surge eastward into CA and the Great Basin, reaching the Four
Corners states Wednesday. Beneath the upper low, snow levels will
fall to 5000-5500 ft, but within the plume of IVT and accompanying
WAA, which is also where the heaviest precipitation is expected,
snow levels will remain elevated at 7000-9000 ft. This will result
in heavy snowfall, initially in the Sierra, then spreading east
into the Great Basin and Four Corners terrain including the San
Juans, Wasatch, and portions of the Mogollon Rim (White Mountains
of AZ area). WPC probabilities D1-2 are high (>70%) for 6+ inches
in the Sierra and the higher terrain of the Great Basin and into
the Wasatch and San Juans, where locally 1-2 feet of snow is likely
by the middle of the week.


...Central Plains/Upper Midwest...
Day 2...

Compact mid-level shortwave will close off as it crosses from
Nebraska to the Great Lakes Monday night and Tuesday. Significant
mid-level moisture advection ahead of this feature will spread
precipitation northwards into the Dakotas and then eastward through
the Upper Midwest. On the north side of this precipitation shield,
soundings suggest the column will be marginally thermally favorable
for snowfall. However, cross-sections indicate an increasing threat
for CSI where theta-es decreases with height, directly into the DGZ
to support intense snow growth. Additionally a deep isothermal
layer beneath the DGZ indicates a favorable setup for aggregate
maintenance and large snow flakes with efficient accumulation.

The guidance, especially the available high- res, has become more
aggressive with this development, and it is likely that this ascent
will cause rapid dynamic cooling of the column to support a narrow
band of heavy snowfall which will track slowly from near the
Coteau des Prairies through far southeast Wisconsin. While there is
still pronounced latitudinal spread and timing differences,
confidence is increasing that a band of heavy snow, with rates of
at least 1"/hr, will move across the area. This is supported by
increasing snowfall probabilities in both the NBM and the AIFS, and
the updated WPC probabilities for at least 1" of snowfall have
increased to above 70%. Locally much heavier snow is possible, and
future updates will need to be monitored for this potential.


Weiss