FOUS11 KWBC 251921
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
221 PM EST Mon Nov 25 2024

Valid 12Z Mon Nov 25 2024 - 12Z Thu Nov 28 2024

...Upper Great Lakes to New England...
Days 1-2...

A negatively tilted trough traversing the Great Lakes this evening
and through Tuesday will generate sufficient upper level diffluence
atop the atmosphere to support a shield of precipitation overt the
Great Lakes. In association with this upper trough, a decaying
frontal system will become an open surface trough tonight that
stretches from the northern most portion of Michigan's Mitten on
north and west into southern Canada. As the initial round of low
level WAA concludes this afternoon (which could support some
moderate-to-heavy snow in parts of Michigan's U.P.), winds will
shift as a dying occluded front tracks east. With high pressure=20
building in over the Canadian Prairies and low pressure heading=20
east into southern Ontario early Tuesday morning, cyclonic flow=20
over Lake Superior will increase along with weak low-level CAA,=20
resulting in periods of snow along the Michigan U.P. and in the=20
northern most portions of Michigan's Mitten. Snowfall rates at=20
their peak could range between 1-2"/hr this afternoon and again
Tuesday morning as CAA increases over the Porcupines and Huron
Mountains. Snow rates should taper off to lighter levels by tuesday
evening. WPC PWPF sports moderate-to-high chances (50-70%) for
snowfall totals >6" in the Porcupine and Huron Mountains, while
the eastern half of the Michigan U.P. have similar probabilities
for >4" of snow. Lighter amounts are possible in the northern most
parts of Michigan's Mitten. The WSSI generally shows Minor Impact
potential in these areas, which indicate some slick roadways could
result in hazardous travel conditions for some affected areas.

Farther east, the surface low tracking into southern Ontario and
divergent upper level flow over the Northeast will work with modest
low-levels to allow for precipitation to fall in the form of a n
icy wintry mix. This is due to some sub-freezing low-level
temperatures being trapped over some parts of interior New England
Tuesday morning. WPC PWPF shows low chances (10-30%) for >0.1" of
ice accumulation in parts of the Adirondacks, Green, and White
Mountains.


...Sierra Nevada...
Days 1-2...

An atmospheric river is preparing to deliver a rich plume of
Pacific moisture into the Sierra Nevada that will result in heavy
mountain snow starting today and lasting through Tuesday evening.
NAEFS shows IVT values topping 500 kg/m/s that by 00Z this evening are
topping observed values in the CFSR climatology over portions of
southern California. Portions of the central
Sierra Nevada can expect heavy snowfall above 7,000ft while the
southern Sierra Nevada witness heavy snowfall above 8,000ft. Given
the generally higher snow levels (some of which will occasionally=20
be as high as 8,500ft in the southern Sierra Nevada) it will be=20
elevations above 9,000ft that see the more prolific snowfall rates=20
(3-4"/hr rates in some cases according to the HREF guidance on=20
WPC's Snowband Probability Tracker for an impressive 24 hour=20
stretch). These higher snow levels are also indicative of a=20
heavy/wet snow with lower SLRs, suggesting not only will there be=20
significant snowfall amounts but there will also be significant=20
snow load component that the Sierra Nevada will contend with. Add=20
in unusually strong 700-500mb winds (>90th climatological=20
percentile on NAEFS) and there could be added stress to some trees=20
and above ground power lines that result in tree damage and power=20
outages.

WPC PWPF shows for elevations >9,000ft have high chances (>70%) for
snowfall totals >30" in the Sierra Nevada. In fact, snowfall could
range between 3-7ft by the time the atmospheric river shuts off
Tuesday night. The WSSI does show Extreme Impacts >9,000ft with the Snow
Amount and Snow Load components being the primary drivers in the
impacts for this event. Given these Extreme Impacts, there could be
some significant disruption to those living at such high elevations
with tree damage and power outages possible.


...Wasatch, CO Rockies...
Days 1-3...

The same atmospheric river responsible for the copious amounts of
heavy snowfall over the Sierra Nevada will direct highly anomalous
Pacific moisture into the Wasatch and Central Rockies. NAEFS=20
between 12Z Tuesday - 12Z Wednesday shows 500-700mb mean specific=20
humidity (g/kg) values topping the 97.5 climatological percentile,
signifying that the mid-levels where the DGZ are more commonly=20
located will be well saturated. Winds will also be quite strong=20
with westerly 200mb, 500mb, and 700mb winds all topping the 90th=20
climatological percentile. Strong upslope ascent into some of these
ranges will be coupled with exceptional synoptic-scale lift atop=20
the atmosphere, resulting in heavy snowfall in the Wasatch and=20
Uinta above 7,000ft and the Central Rockies above 8,000ft. Snow=20
will be measured in feet in these ranges with localized amounts=20
surpassing 4 feet in the tallest peaks of the Wasatch and Colorado=20
Rockies. The WSSI shows swaths of Moderate to Major Impacts in=20
these ranges, with the latter signifying the potential for=20
considerable disruptions to daily life. Widespread closures and=20
disruptions to infrastructure are also anticipated in areas=20
expecting Major Impacts. Some accumulating snow is possible along=20
the Palmer Divide late Tuesday night and into Wednesday, then=20
along the Sangre De Cristo Wednesday into Wednesday night. These=20
areas have moderate-to-high chances for >6" of snowfall through=20
Wednesday night.=20

...Northeast...
Day 3...

Plenty of uncertainty remains in the track of a storm system that
will make for a damp and dreary Thanksgiving along the East Coast.
The ECMWF camp of guidance has favored a more amplified solution
that takes the storm across southeast PA Thursday morning and a
swath of heavy snow would occur as far west as northern Ohio to
across the interior Northeast during the day on Thanksgiving. The
12Z CMC has trended closer to the 12Z ECMWF reflection, while the
GFS is more suppressed and not as far north with the precipitation
shield. WPC PWPF, which takes into account various ensemble
members out through Thanksgiving evening, shows low-to-moderate
chances (30-50%) for snowfall >4" in the Adirondacks, Green, and
White Mountains. The Adirondacks in particular sport low chances=20
(10-30%) for >8" of snowfall with additional heavy snow possible
through Thanksgiving night. The WSSI-P does show low-to-moderate
chances (30-50%) for Minor Impacts from as far south as the
Catskills and northern Allegheny's of PA to the Green/White
Mountains of interior New England. Residents in these areas should
closely monitor the forecast over the next few days as this still
has the potential to be a disruptive event across portions of the
interior Northeast from Thanksgiving into Friday.=20


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

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!7AM6YE-TnX7gziAvAVQ83Puc_oPiYQf-994zzO-t6gO-N=
oabr9_fQB7P5kG3Yuj5JPYQxUFP5e5G4ml2JlPJ4EG8akw$=20



$$

=3D =3D =3D
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