FOUS11 KWBC 080905
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EST Wed Jan 8 2025

Valid 12Z Wed Jan 08 2025 - 12Z Sat Jan 11 2025

...Southern Plains and Southeast... Days 2-3...

...Major winter storm is forecast to span from north- central 
Texas and southeast Oklahoma beginning on Thursday before crossing through
much of the Mid- South and into northern portions of the 
Southeast by the end of the week... 

A potent positively tilted longwave trough containing plentiful 
upper level energy from an aforementioned upper level low will 
support a very strong upper level jet that stretches from West 
Texas east through the Carolinas. Surface cyclogenesis will occur 
Thursday night near the Gulf near the TX/LA border in the RER of 
that jet, which will bed southward, thus maximizing the upper level
divergence over Louisiana. A plume of Gulf moisture will inject 
into the low with maximum PWATs along the Texas Gulf Coast rising 
to around 1.75 inches. While that level of moisture doesn't move 
too far inland, a large fraction of it will, providing ample 
moisture for the developing surface low. The associated low level 
jet will advect much of that moisture up the Lower Mississippi Valley.

Meanwhile, an Arctic air mass will be in place due to a retreating
high pressure system over the Central Plains and Mid-South, 
resulting in a strong temperature gradient which will both increase
the forcing as well as lift that moisture and effectively wring it
out, resulting in widespread wintry precipitation starting in 
TX/OK and spreading east as far as the Mid-Atlantic Saturday 
morning. The tight thermal gradient on the north and west side of 
the low will promote a similarly tight gradient in snowfall amounts
and in the areas where the cold air at the surface is shallow, an 
area of freezing rain/sleet will develop from east central Texas 
over northern Louisiana and into central Mississippi, Alabama, and 
Georgia, with snow to the north of that from north-central Texas 
east through the Tennessee Valley. For many this will be a very 
impactful snowfall event and the first winter storm of the season 
for areas such as Dallas- Fort Worth north and east through the 
Ozarks and into the Memphis and Nashville metro areas. Potential
forecasting challenges include banding potential on the northern
and northwest side of the low increased by strong mid-level fgen
and isentropic ascent through the DGZ. This may lead to mesoscale
banding that will become more notable once inside the full suite of
CAMs (most likely by the 12z cycle today). This is particularly a
concern for north-central TX and OK on D1 where overall guidance
had recently trended down in amounts. For the DFW metro region,
upper-end amounts have increased as some ensemble members
(particularly the ECENS) pick up on this banding potential which
allows for mixed precip to quickly change to heavy snow Thursday
night, with the grand ensembles 75th percentile up to 9 inches.
Given the mesoscale nature of these snowbands, conditions will
likely drastically change over the course of a tens of miles, with
the heaviest snow north-northwest and lesser amounts to the south.

Spanning farther east a WAA thump of snow is likely across the
Mid-South Friday morning extending toward the Southeast by the
afternoon hours. The southern track of this low pressure system
supports snow potential reaching pretty far south compared to
climatology, with ECMWF EFI values of 0.7-0.95 and an extreme 
shift of tail of +8 stretching from northern MS to central GA.
Additionally, the deep cold air mass in place reinforced by a fresh
snowpack to the north will allow for low-level cold air to hold on
while warm air surges north in the mid-levels. This means that
areas along and just south of the I-20 corridor that begin as snow
will likely change over to sleet and freezing rain for a
potentially extended period of time.

WPC probabilities for moderate to major impacts extend from the 
DFW Metroplex area north and east to the Ozarks, through Memphis
and northern MS before also spreading east to northern GA and the
southern Appalachians. For snowfall amounts, WPC probabilities for
at least 4 inches are high (70-80%) across northern MS, AL, and 
GA, as well as much of the TN and the southern Appalachians. A 
stripe of low (10-30%) > 0.1" ice probabilities stretch from 
central TX through northern LA/southern AR, northern MS, central 
AL, and into central/northern GA and the SC Midlands. However, it 
can be expected for these chances to increase with time.

The updated set of Key Messages are at the bottom of the discussion.

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

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

 ...Continued...

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