FOUS11 KWBC 210759
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Tue Jan 21 2025

Valid 12Z Tue Jan 21 2025 - 12Z Fri Jan 24 2025

...Texas, Gulf Coast, and Southeast... Days 1-3...

...Significant, potentially historic, winter storm to spread across
the Gulf Coast and portions of the Southeast through Wednesday...

A rare, potentially historic, Gulf-Coast and Southeast Atlantic 
Coast winter storm has begun thanks to a positively-tilted upper 
level trough tracking through the Southern Plains this morning.
Ahead of the upper-trough, a healthy and anomalous SWrly IVT
(>500 kg/m/s, >90th climatological percentile per NAEFS) will be
directed in the direction of an Arctic-airmass that, per NAEFS,
are below the 1st climatological percentile and well below
freezing. This overrunning setup will produce a robust 850-700mb
FGEN signal that results in snow bands that would support 1-2"/hr
snowfall rates per WPC's Snowband Probability Tracker. Lastly, a
250mb SW to NE oriented jet streak will top 150 kts while placing
its diffluent right-entrance region over the Gulf Coast and
Southeast, thus fostering deep layer ascent. All of these
ingredients; lift, moisture, frigid air-mass, and mesoscale
influence, will result in a swath of impactful winter weather from
the Upper Texas Coast to the Carolina coast today and through 
Wednesday morning. In fact, there are low chances (10-30%) for 
localized snowfall amounts in southwest LA that could surpass 8" Tuesday.

In South Texas, an ongoing wintry mix will make for treacherous
travel conditions tonight and into Tuesday morning. Focusing on 
amounts and impacts along the Gulf Coast next, snow will be 
heaviest this morning from the Upper Texas Coast on east along the 
I-10 corridor. This band will move east along I-10 through the New
Orleans on east towards Mobile Bay through Tuesday afternoon with 
>1"/hr snowfall rates possible. WPC probabilities show moderate-to-
high chances (50-70%) for >4" of snowfall from the lower TX/LA 
state border on east towards Lake Pontchartrain. Farther east, 
east of NOLA, WPC probabilities show those same moderate-to- high 
chances (50-70%) for >2" of snowfall in southern MS/AL, the western
FL Panhandle, and southwest GA. 

Farther east into Georgia, the northern Florida Peninsula, and the
Carolinas, guidance shows a similar significant snow/ice event in
these regions Tuesday afternoon and into Tuesday night. Similar to
the central Gulf Coast, snowfall rates along the Southeast coast
could approach 1"/hr in northern FL, southern GA, and along the
Carolina coast. Meanwhile, in the northern FL Peninsula freezing
rain and sleet will make for exceptionally dangerous travel
conditions for areas very unfamiliar with >0.1" of ice. Snow and 
ice should be finished not long after sunrise Wednesday morning. 
WPC probabilities for >2" are 40-60% from Cape Fear to the Outer 
Banks of NC. For freezing rain, WPC probabilities show moderate-to-
high chances for >0.1" of ice accumulation in the northern FL 
Peninsula and far southern GA with even some moderate chances 
(40-50%) for >0.25" of ice in northern FL. Such amounts would make
for incredibly dangerous travel conditions that residents are not 
used to seeing, and could also result in some tree damage and power outages.

This event has prompted the issuance of collaborated Key Messages
(KeyMessage_1) linked below.

...Great Lakes... Days 1-2...

A potent shortwave trough rotating beneath the base of an expansive
longwave trough that stretches from the Great Lakes to the Davis 
Strait will enhance an already favorable LES setup thanks to the
added upper level ascent out ahead of the approaching shortwave
trough. Not only will this trough aid in upper-level ascent, but
strong CAA via 850mb temps plummeting from an already very cold -20C
to an even colder -30C over the Lakes. The coldest of the 850mb 
temps will reside over the Michigan U.P., but the more favored wind
convergence will be located over Lakes Erie and Ontario. This will
allow for robust LES single-banded streamers to form and produce 
prolific snowfall rates today and into Tuesday night. This is due 
to lake-induced instability that tops 500 J/kg at the peak of 
these LES bands intensity, resulting in snowfall rates between 
2-3"/hr. LES bands will taper off down wind of Lakes Erie and 
Ontario by Wednesday morning as a clipper system tracks through the
Upper Midwest, bringing a broader WAA snow setup to the Great 
Lakes. LES streamers off Lake Michigan will produce locally heavy 
snowfall totals in the western lower MI peninsula. By Thursday, 
some LES streamers may return over the eastern Great Lakes as weak 
CAA returns, but are likely to be less intense compared to 
Tuesday's LES bands.

WPC probabilities show, through 00Z Thursday, high chances (>70%)
for additional snowfall totals >18" just east of Lake Ontario and
over the Tug Hill with similar probabilities for >12" in the South
Towns of Buffalo. Along the western coasts of Michigan's Mitten,
there are moderate chances (40-70%) for snowfall totals >8" through
Wednesday night. The WSSI shows Major to even locally Extreme
Impacts in the Tug Hill and South Towns of NY, while the western-
most portions of Michigan's Mitten witness Minor Impacts (locally
Moderate in areas with heavier snowfall totals). 
 
...Northern Rockies/High Plains... Days 1-3...

These regions remain sufficiently cold enough to support snow ahead
of series of upper level troughs that have modest 700-300mb
moisture to work with and divergence aloft to support periods of
snow through Thursday. The mountainous terrain of Montana and
Wyoming, as well as the Black Hills, are favored for locally heavy
snowfall. WPC probabilities shows moderate-to-high chances (50-70%)
for snowfall >8" over the next few days in these ranges with the
Little Belt and Big Snowy of Montana sporting moderate chances
(40-60%) for snowfall amounts >12". Minor accumulations (1-4" are
possible in the High Plains of central Montana and northeast
Wyoming with WPC probabilities showing low-to-moderate chances
(20-40%) for snowfall totals >2". 

Mullinax

...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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