FOUS11 KWBC 051942
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
242 PM EST Thu Dec 5 2024

Valid 00Z Fri Dec 06 2024 - 00Z Mon Dec 09 2024

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

Through the remainder of Thursday afternoon and into Friday, the
main story will be the ongoing lake-effect snow that will unfold
across parts the Great Lakes and even parts of the interior
Northeast down wind of Lakes Erie and Ontario. Through this
afternoon and evening, geopotential heights and temperatures at=20
850mb, 700mb, and 500mb all will all be below the 2.5=20
climatological percentile according to NAEFS. Combined with=20
daytime heating, look for occasionally potent snow showers (maybe=20
a squall or two) to be possible across New England. Meanwhile,=20
brisk cyclonic flow over the warm Great Lakes will keep LES=20
streamers in the forecast this afternoon and through tonight over=20
the eastern half of Michigan's U.P. and along the western half of=20
Michigan's Mitten. Farther east, strong NWrly flow paired with low-
level convergence will support some single-banded LES streamers=20
originating off Huron that poses a heavy snow threat in northwest=20
PA and far northeast OH. Lingering LES snow is also expected in=20
central NY as LES lingers tonight off of Lake Ontario.

By Friday, a 500mb vort max approaching from western Ontario allows
for some upper level divergence aloft that is also supported by the
northern Great Lakes position beneath the diffluent left-exit
region of a 125kt 250mb jet streak. Subtle low-level WAA is also
likely that will prompt the development of a stationary front that
acts as a trigger for additional snowfall through Saturday. This
upper level disturbance will revitalize the westerlies across Lakes
Erie and Ontario, resulting in the redevelopment of steady single
banded LES streamers that could produce 1-2"/hr snowfall rates
Friday evening and linger through Saturday morning. All told
through 12Z Saturday, much of Erie County, PA and south of Buffalo
in western NY (including along the I-90 corridor) have high=20
chances (>70%) for snowfall totals topping 18" with some localized=20
areas surpassing 24". This is also the case east of Lake Ontario=20
to the south of Watertown and along the Tug Hill. WSSI shows
Moderate to Major impacts in these areas, indicating the potential
for dangerous to even impossible travel conditions tonight and=20
into Saturday.

Meanwhile, the next in a series of upper level disturbances will=20
spawn a wave of low pressure that will track along the=20
aforementioned stalled frontal boundary Saturday afternoon to the=20
north of Lake Superior. This storm system will have strong=20
850-700mb WAA and isentropic ascent to work with, resulting in a=20
shield of snow from central NY on east through much of New England=20
by Sunday. WPC PWPF between 00Z Sunday - 00Z Monday shows low=20
chances (10-30%) for snowfall totals >4" in the peaks of the Adirondacks,
Green, and White Mountains.


...Pacific Northwest/Northern Rockies...=20
Day 3...

An upper low in the northeastern Pacific heading into British
Columbia will direct two shortwave troughs into the Pacific
Northwest this weekend. Snow levels will initially be as high as
8,000ft Saturday morning, but fall to as low as 3,000ft by=20
Saturday night as a cold front pushes through the region. An=20
influx of Pacific moisture will push through Saturday afternoon in=20
the Pacific Northwest, then into the Northern Rockies by Saturday=20
night where a progressive precipitation shield will lead to some=20
minor snowfall accumulations. This same moisture plume will overrun
boundary layer temperatures that are sub-freezing, while mid-level
temperatures are above freezing. This setup likely leads to some
sleet/freezing rain mix in parts of the Columbia Basin where cold
air is harder to erode.

The next shortwave trough approaches from the northwest Saturday
night but this time with a better source of cold air in place=20
thanks to the previous cold frontal passage. Snow levels will
generally be around 3,000ft, allowing for those above 3,000ft in
the Cascades and Olympics to receive steady rounds of snow through
Sunday afternoon. Most snowfall in the Northern Rockies will be on
the minor side, although some totals topping 6" in the Lewis Range
are possible through Sunday. WPC PWPF shows high chances (>70%) for
snowfall topping 8" at elevations above 4,000ft, suggesting some
travel disruptions are possible at pass level in the Cascades this
weekend. Note however that the heaviest snowfall is likely to be
confined to elevations above 5,000ft in the Cascades. WPC PWPF=20
also shows moderate-to-high chances (50-70%) for snowfall >4" in=20
parts of the Bitterroots and Lewis Range that could result in some=20
locally treacherous road conditions above 5,000ft this weekend.=20
Lastly, there are low chances (10-40%) for minor ice accumulations
(>0.1" accretion) in localized parts of the Columbia Basin on=20
Saturday that could result in slick travel for motorists.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.=20

Mullinax




...Winter Storm Key Messages are in effect. Please see current=20
 Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=
stKeyMessage_1.png__;!!DZ3fjg!7V1mjkJK88QCrkBHnSbkVhtzIgwzqw0UDfdzj-Xkr__CP=
pQTyLTRkWQ26VUhqXuRiFMDQeKmJIAJUGSVDsOKcaI7LiI$=20

$$

=3D =3D =3D
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