FOUS11 KWBC 090837
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Thu Jan 9 2025

Valid 12Z Thu Jan 09 2025 - 12Z Sun Jan 12 2025

...Great Lakes and Northeast... Day 1...

Cyclonic flow on the backside of a large upper low in Atlantic 
Canada will support another ~day of mainly mountain-based snow over
the northern Green and White Mountains and northern Maine. A few 
inches are possible in the higher elevations, with WPC 
probabilities for at least another 4 inches of snow around 50%. 

...Northwest to the Northern Rockies/Plains... Days 2-3...

Fairly sharp mid-level shortwave and sfc front will enter the 
PacNW with mostly higher elevation snow but lowering snow levels 
(from ~4500 to ~3000ft) as the front moves past the Cascades. The 
shortwave will move southeastward beneath a weakening upper jet, 
but combination of height falls and upslope enhancement will 
maximize snow over the mountains, from the Bitterroots to the 
Tetons and western MT D2 then into the Big/Little Belts into the 
Bighorns and eventually the Black Hills D3 as the stronger height 
falls move through. WPC probabilities for at least 12 inches of 
snow are are >50% above ~5000-6000ft or so (west to east) where 
two-day totals could exceed 18-24 inches above 6000-7000ft (40-70% chance).

Additionally, by D3, Canadian system will move into the northern 
Plains/Upper Midwest with a swath of generally light snow via WAA 
and FGEN on the northern side of an area of low pressure. Amounts 
should generally be on the lighter side of a couple/few inches, but
there are low-end probabilities (10-20%) of at least 4 inches of 
snow over eastern ND.

...Southern Plains, Southeast, southern Mid-Atlantic... Days 1-2...

...*Major winter storm* is forecast to span from west Texas and
southeast Oklahoma starting today before crossing through much of 
the Mid-South and into portions of the Mid-Atlantic by the end of the week...

A potent positively-tilted longwave trough containing plentiful 
upper level energy from an upper level low will support a very 
strong upper level jet that stretches from West Texas east through 
the Carolinas. Surface cyclogenesis will initiate this afternoon 
along the Texas Coast in the RER of that jet, also helping to draw 
in Gulf moisture 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 eastward to the Mid-Atlantic by late 
Friday/early Saturday as multiple jet streaks ride northeastward. 
The tight thermal gradient on the north and west side of the low 
will promote a similarly tight gradient in snowfall amounts. With a
broad area of WAA off the deck, and in the areas where the cold 
air at the surface is shallow but stubborn, an area of freezing 
rain/sleet will develop from the Big Bend of Texas eastward over 
northern Louisiana/southern AR into central Mississippi, Alabama, 
and Georgia, then northeastward through the Carolinas. To the 
north, where the colder air is deeper through the column, snow will
be the dominant or sole ptype across Northwest TX, 
central/northern AR, eastward through the Mid-South/Tennessee 
Valley and into the central/southern Appalachians to the VA 
Beaches. Ptypes will be transitioning from frozen to liquid in some
southern areas where the cold air is shallower and overtaken by 
WAA at all levels. The models still show a fair amount of spread in
the strength of the warm nose and time residence of sub-freezing 
air, resulting in lower confidence in these transition zones. 
Trended a bit warmer overall but will still have a few model cycles
to hopefully gain more certainty. 

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. Additional 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, but even the 00Z runs 
still showed disagreement. This is particularly a concern for 
north-central TX and OK. From Texas through the Lower Mississippi 
Valley, forecast snow amounts have come down nominally along the 
ptype gradient, but have consequently increased for ice 
accumulations -- again, dependent on the strength of any warm air 
and how long colder 2m temps can hang on. WPC probabilities for at 
least 4 inches of snow are >50% near/north of D-FW across 
southeastern OK eastward across northern AR, the Mid-South, into 
Middle TN in a mostly continuous fashion. Orographic lift will also
help the central/southern Appalachians see several inches of snow.
Over AL/GA, initial WAA-driven snow should lead to a changeover to
sleet/freezing rain and then rain for more southern areas, with 
the highest probabilities for at least 2 inches of snow closer to 
the TN border but non-zero probs to I-20/I-85. More uncertainty in 
snow amounts lies east of the Appalachians across southeastern VA 
where the guidance has wavered on the amount of QPF. Low pressure 
should strengthen as it leaves the coast, perhaps allowing some 
enhancement over SE VA early Saturday.

For ice probabilities, there is a >40% chance of at least a tenth 
of an inch of ice around the Arklatex region through southern AR. 
In addition, areas from northern GA through parts of 
Upstate/Midlands SC into eastern NC show >30% probabilities of at 
least a tenth of an inch of ice. 

Fracasso

...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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