FOUS11 KWBC 200824
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 AM EST Mon Jan 20 2025

Valid 12Z Mon Jan 20 2025 - 12Z Thu Jan 23 2025

...Great Lakes... Days 1-3...

Broad cyclonic flow courtesy of a massive upper low over eastern 
Canada will direct a series of shortwave troughs over the Great
Lakes and eastern U.S. the first half of the week. An exceptionally
strong shortwave trough on Tuesday (500mb heights as low as the 1st
climatological percentile 12Z Tuesday) will deliver 1000-850mb 
temps that are also below the 1st climatological percentile. While
the Great Lakes are gradually icing over with each passing day, the
exceptionally colder air-mass traversing the Great Lakes paired
with the favorable upper-level synoptic-scale ascent will be more
than enough to support persistent and potent lake effect snow (LES)
bands. In fact, lake-induced instability of >500 J/kg would support
exceptional LES bands, particularly downstream of Lakes Erie and
Ontario where single-banded LES streamers are most likely. LES
bands should finally weaken by Wednesday morning as high pressure
moves over the Mid-Atlantic and a weak Alberta Clipper approaches
from the northwest. This will weaken the pressure gradient and
shift winds our of the southwest. Some light snow is possible in
the Great Lakes region on Wednesday as a result of the Clipper, but
snowfall rates will pale in comparison to the LES bands that will
be around through Tuesday afternoon.

WPC probabilities for Monday and Tuesday depict moderate chances
(40-60%) for snowfall >8" along the western-most portions of
Michigan's Mitten and along the northern coasts of the eastern
Michigan U.P., The heaviest LES bands take shape Monday night and
into Tuesday downstream of Lakes Erie and Ontario. By Wednesday
morning, WPC probabilities show high chances (>70%) for snowfall
>18" in some of the South Towns near Buffalo and in the Tug Hill
Plateau. The event total snowfall in the Tug Hill is likely to
range between of 2-4 feet through Wednesday.


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

...Significant winter storm likely across portions of the Southern
U.S. this week...

A rare Gulf-Coast and Southeast Atlantic Coast winter storm is
becoming more likely beginning Tuesday as an anomalously cold
airmass settles across the eastern 2/3 of the CONUS and sets the
stage for the upcoming low pressure system.

As this cold air floods across the region, an intensifying
shortwave will dig out of the Great Basin and then sharpen as it
pivots eastward from West Texas through the Gulf Coast Tuesday into
Wednesday. As this occurs, some interaction with a northern stream
impulse will yield a positively tilted full latitude trough
shifting east, with the resultant downstream jet streak
intensifying and shifting poleward to place the favorable RRQ atop
the Gulf Coast. The overlap of height falls and upper diffluence
will help spawn a wave of low pressure across the Gulf of Mexico,
and as 300K isentropic ascent maximizes to the north, precipitation
will begin to overspread eastern Texas Tuesday morning, before
becoming more expansive across the Gulf Coast and eventually
coastal Southeast Tuesday night into Wednesday.

Guidance is keying in on the snow/sleet/freezing rain transition 
from south-central TX on east to along and south of the I-10
corridor. The mechanisms for this vary, however, as in south-
central TX, it has more to do with the lack of moisture above 700mb
and an exceptional warm nose between 850-700mb, and from the Upper
Texas Coast to the the FL Panhandle, there is no shortage of
moisture but the warm nose is stronger. Some very cold and dry air
to the north will likely cause a sharp precip gradient as well, 
but to the south of the precip gradient, a swath of heavy snow 
driven by strong WAA and 850mb FGEN will result in eastern TX/LA/MS/AL
witnessing up to 1"/hr snowfall rates just north of the mixed 
transition zone, which is rare for the central Gulf Coast region. 
Guidance has continued to suggest more suppression of the QPF axis
over the last 12 hours, and thus WSO and WSSI-P probabilities have
decreased in parts of northern GA and the central Carolinas 
Tuesday night. That said, from the coastal Carolinas on south to 
northern Florida, a combination of snow and ice will make for 
treacherous travel conditions Tuesday night and likely into 
Wednesday morning stubbornly remaining below freezing. 

WPC probabilities indicate a moderate chances chance (40-60%) for 
at least 4" of snow from far eastern TX through south-central LA 
and into southern MS. Locally, 4-6" of snow is possible in south-
central Louisiana with locations such as Lake Charles and Baton
Rouge seeing such totals. These areas referenced above also ave the
better chances of witnessing up to 1"/hr snowfall rates. The 
combination of the heavy snow, very cold temperatures, and the 
rarity of this event could result in major impacts and 
considerable disruptions to daily life. Elsewhere, WPC 
probabilities for 2+ inches of snow are as high as 40-60% from the
northern Houston suburbs to as far east as just north of Mobile
Bay. Farther east, recent trends in guidance has reduced snowfall
probabilities in parts of GA and the central Carolinas with WPC 
probabilities showing low chances (10-30%) for over 2" of snow. 
Additionally, southeast of the heaviest snow, especially from the 
Florida Panhandle east to the northern Peninsula and parts of 
southern GA, freezing rain is expected with a 40-60% chance of 
producing at least 0.1" of ice. Most icing totals in South-central
Texas should generally remain below 0.1", but these totals are
still highly unusual for these parts of the Lone Star State and are
likely to result in hazardous travel conditions Monday night and
Tuesday morning.

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

Mullinax/Weiss

...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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