FOUS11 KWBC 070855
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 AM EST Tue Jan 7 2025

Valid 12Z Tue Jan 07 2025 - 12Z Fri Jan 10 2025

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

A large, sprawling, and multi-pronged circulation over 
southeastern Canada will send mid-level shortwaves into the eastern
Great Lakes and Northeast over the next few days before finally
sliding eastward south of Greenland by the end of D3. This will
maintain high chances of at least light snow over the higher 
elevations (NY and northern VT/NH) and also downwind of the Great 
Lakes in general owing to the broad cyclonic flow. 

For D1, a compact closed low over northern VT early this morning will
swing through the Northeast today with some terrain enhancement 
over the northern Adirondacks and into the Green Mountains.
Additionally, another trailing shortwave diving out of Canada 
tonight over the Great Lakes and into the Northeast will contain a 
reinforcing shot of arctic air. Even with already cold 850mb temps
below -12C or so this next surge of cold air will steepen lapse 
rates and reinvigorate lake effect snow off the western Great 
Lakes, especially along the western shore of Lower Michigan, and 
also off Lakes Erie/Ontario on NW flow. WPC probabilities for at 
least 4 inches of snow D1-D2 are 40-70% in the favorable northwest
flow snowbelts, as well as over the northern Adirondacks and northern VT.

By the end of D2 and start of D3, one last shortwave will push 
through the eastern Great Lakes and New England, maintaining 
additional light snow off Lake Ontario on NW flow and additional 
snow for the northern Adirondacks and Green Mountains. WPC 
probabilities for at least an additional 4 inches of snow are 
moderate (30-50%) over these areas.

...Rockies and Southwest... Days 1-3...

Broad height falls moving through the Southwest today will sharpen
into a closed low over northern Baja California tonight. With high
pressure nosing down out of the northern Plains, upslope snow will
continue across the CO Front Range via an easterly low level flow 
along with a relatively low DGZ on D1. However, the snow should 
not last too long as the upper low sinks further south over central
Baja California. This will help to drag the snowfall southward as 
well through the Mogollon Rim, Sangre de Cristos, Sacramento 
Mountains, etc. Models continue to vary on the terrain influence on QPF
(and thus snow) over parts of the Southwest but at least a large 
footprint of light snow is expected. For the D1-3 period, WPC 
probabilities for at least an additional 4 inches of snow are >40%
over the Front Range into the CO Rockies, San Juans, and higher 
elevations in AZ/NM above 8000ft or so. 

Farther north, a shortwave moving out of Canada through Montana on
Wednesday will touch off some light to locally moderate snow over 
much of the state that will push into Wyoming and the Black Hills 
on Thursday. WPC probabilities for at least 4 inches of snow are 
highest (>50%) over the Little Belts, Gallatin Range, and into the 
Bighorns. As this shortwave sinks southward and interacts with the
upper low at the base of the trough, increasing snowfall is
possible across the southern NM ranges. WPC probabilities are low
(10-30%) across this region.

...Southern Plains... Day 3...

As the aforementioned upper low over Baja California begins to
eject northeast on Thursday and merge with the diving shortwave
over the central Rockies, a strong 160-170kt southwest jet stretch
surges north and bulges over ridging in the Gulf of Mexico. This 
will prompt rich moisture and prominent upper divergence from an 
increasingly buckled jet. In the right entrance region of this jet,
surface cyclogenesis is expected along the western Gulf Coast by 
late Thursday night with a precipitation shield blossoming to the 
north. Given the strong high situated to the north over the Central
Plains and a cold low- level airmass in place, light snow and 
mixed precipitation is expected over much of western and north- 
central TX starting Thursday morning, expanding eastward to the 
Ozarks by the end of the period. Thermal uncertainty is high (as 
expected) due to the upstream players (timing, depth, etc.) with 
recent trends suggesting a later start time and warmer low-level 
air. Additionally, the northern precipitation gradient across OK 
and northern AR will likely be sharp, with varying snowfall amounts
across short distances.

All ptypes are in play -- snow/sleet/freezing rain and plain rain 
to the south -- and each will likely not be stagnant in location. 
For now, the highest probabilities of snow lie on the northwest 
side of the precipitation shield in the deeper colder air from
north-central TX (near and north of the D-FW Metroplex) through 
southeast OK and into central AR. There, WPC probabilities for at 
least 4 inches of snow through 12z Friday are around 50-70%. This
area was a slight trend to the northwest, with the Texas Big
Country seeing lower probs in response to a slower ejection of the
upper low. To the southeast, within the zone of maximum 
uncertainty, probabilities for accumulating snowfall decrease while
probabilities for ice increase. Areas from near Austin 
northeastward through the Piney Woods into the Hill Parishes of LA 
show the highest chance of at least 0.1 inches of ice accumulation 
(30-60%). Given this is the first widespread winter storm for this 
region, even light amounts of ice or snow could cause travel disruptions.


This system has prompted the issuance of a new set of Key Messages
which are linked at the bottom of the discussion.

Fracasso/Snell


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

https://www.wpc.ncep.noaa.gov/key_messages/La...

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