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=20
Canada will support another ~day of mainly mountain-based snow over
the northern Green and White Mountains and northern Maine. A few=20
inches are possible in the higher elevations, with WPC=20
probabilities for at least another 4 inches of snow around 50%.=20


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

Fairly sharp mid-level shortwave and sfc front will enter the=20
PacNW with mostly higher elevation snow but lowering snow levels=20
(from ~4500 to ~3000ft) as the front moves past the Cascades. The=20
shortwave will move southeastward beneath a weakening upper jet,=20
but combination of height falls and upslope enhancement will=20
maximize snow over the mountains, from the Bitterroots to the=20
Tetons and western MT D2 then into the Big/Little Belts into the=20
Bighorns and eventually the Black Hills D3 as the stronger height=20
falls move through. WPC probabilities for at least 12 inches of=20
snow are are >50% above ~5000-6000ft or so (west to east) where=20
two-day totals could exceed 18-24 inches above 6000-7000ft (40-70%=20
chance).

Additionally, by D3, Canadian system will move into the northern=20
Plains/Upper Midwest with a swath of generally light snow via WAA=20
and FGEN on the northern side of an area of low pressure. Amounts=20
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=20
snow over eastern ND.


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

...Major winter storm is forecast to span from west Texas and=20
southeast Oklahoma starting today before crossing through much of=20
the Mid-South and into portions of the Mid-Atlantic by the end of=20
the week...=20

A potent positively-tilted longwave trough containing plentiful=20
upper level energy from an upper level low will support a very=20
strong upper level jet that stretches from West Texas east through=20
the Carolinas. Surface cyclogenesis will initiate this afternoon=20
along the Texas Coast in the RER of that jet, also helping to draw=20
in Gulf moisture with maximum PWATs along the Texas Gulf Coast=20
rising to around 1.75 inches. While that level of moisture doesn't=20
move too far inland, a large fraction of it will, providing ample=20
moisture for the developing surface low. The associated low level=20
jet will advect much of that moisture up the Lower Mississippi=20
Valley.

Meanwhile, an Arctic air mass will be in place due to a retreating
high pressure system over the Central Plains and Mid-South,=20
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=20
TX/OK and spreading eastward to the Mid-Atlantic by late=20
Friday/early Saturday as multiple jet streaks ride northeastward.=20
The tight thermal gradient on the north and west side of the low=20
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=20
air at the surface is shallow but stubborn, an area of freezing=20
rain/sleet will develop from the Big Bend of Texas eastward over=20
northern Louisiana/southern AR into central Mississippi, Alabama,=20
and Georgia, then northeastward through the Carolinas. To the=20
north, where the colder air is deeper through the column, snow will
be the dominant or sole ptype across Northwest TX,=20
central/northern AR, eastward through the Mid-South/Tennessee=20
Valley and into the central/southern Appalachians to the VA=20
Beaches. Ptypes will be transitioning from frozen to liquid in some
southern areas where the cold air is shallower and overtaken by=20
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=20
air, resulting in lower confidence in these transition zones.=20
Trended a bit warmer overall but will still have a few model cycles
to hopefully gain more certainty.=20

For many this will be a very impactful snowfall event and the=20
first winter storm of the season for areas such as Dallas-Fort=20
Worth north and east through the Ozarks and into the Memphis and=20
Nashville metro areas. Additional forecasting challenges include=20
banding potential on the northern and northwest side of the low=20
increased by strong mid-level FGEN and isentropic ascent through=20
the DGZ. This may lead to mesoscale banding that will become more=20
notable once inside the full suite of CAMs, but even the 00Z runs=20
still showed disagreement. This is particularly a concern for=20
north-central TX and OK. From Texas through the Lower Mississippi=20
Valley, forecast snow amounts have come down nominally along the=20
ptype gradient, but have consequently increased for ice=20
accumulations -- again, dependent on the strength of any warm air=20
and how long colder 2m temps can hang on. WPC probabilities for at=20
least 4 inches of snow are >50% near/north of D-FW across=20
southeastern OK eastward across northern AR, the Mid-South, into=20
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=20
the highest probabilities for at least 2 inches of snow closer to=20
the TN border but non-zero probs to I-20/I-85. More uncertainty in=20
snow amounts lies east of the Appalachians across southeastern VA=20
where the guidance has wavered on the amount of QPF. Low pressure=20
should strengthen as it leaves the coast, perhaps allowing some=20
enhancement over SE VA early Saturday.

For ice probabilities, there is a >40% chance of at least a tenth=20
of an inch of ice around the Arklatex region through southern AR.=20
In addition, areas from northern GA through parts of=20
Upstate/Midlands SC into eastern NC show >30% probabilities of at=20
least a tenth of an inch of ice.=20


Fracasso


...Winter Storm Key Messages are in effect. Please see current=20
 Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=
stKeyMessage_2.png__;!!DZ3fjg!6hPeCUOYI6lqI1KOCsG0p9wSjwtDLoVc9kEmGA7_RB1os=
lQPTZ1sRZbWyw46ALFDmB-H0tihyTyTRsQ-DRPjG9o8VbM$=20

$$

=3D =3D =3D
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