FOUS11 KWBC 072054
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Tue Jan 7 2025

Valid 00Z Wed Jan 08 2025 - 00Z Sat Jan 11 2025

...Great Lakes and Northeast...
Days 1-3...

Behind a departing low, Arctic air streaming over the Great Lakes will
cause additional lake-effect snows south and southeast of each of=20
the lakes in D1/Wednesday. Lake surface temperatures from GLERL for
Lake Michigan are around 45F/+6C, with the Arctic air moving over
the lake this evening expected to range between -12C and -15C at
850 mb. This will be ample instability to sustain cellular
convection within the lake-effect. WPC probabilities for at least 4
inches of snow are generally ranging between 30 to 70 percent, with
the highest probabilities occurring along much of the immediate
Lake Michigan shoreline in far western Michigan, as well as around
Erie, PA where a strong band originating off of Lake Huron is
reinforced by the brief time the band is over Lake Erie. Lesser
probabilities of 10 to 40 percent are off the southeast end of Lake
Ontario, extending into the Syracuse area. By D2/Thursday, high
pressure moving overhead of the Great Lakes will end the lake-
effect from northwest to southeast.

On D3/Friday, a potent upper level longwave trough will approach
the western Great Lakes. Ahead of the shortwave, weak surface
cyclogenesis will occur over Wisconsin and the U.P. of Michigan.
Moisture will stream north from the much stronger low to the south
(more on that below). This should cause widespread light snow over
the upper Lakes through Friday evening. Lake-enhancement due to
continued very cold temperatures could locally increase snow totals
in the form of bands embedded within the broader snowfall shield,
though those details have yet to be ironed out, depending on the
track of the low and how much Gulf moisture makes it to the region.

...Rockies and Southwest...
Days 1-3...

A broad vertically stacked cutoff low over the Southwest tonight
contrasting with an expansive area of surface high pressure over
the Northern Plains will create an E-SE wind across the Front=20
Range of Colorado tonight, as this will be the region between the
two weather features. A brief period of upslope flow may cause=20
some light snow across the region. As the polar high rapidly sinks=20
southward, so too will the light snow, moving into the mountains of
New Mexico and Arizona. Here too the duration of snow will be brief
as the flow turns to a much less favorable southerly direction.

The arrival of Arctic air into the Southwest and Southern Plains
will set the stage for a much more significant and widespread
snowfall event as the vertically stacked low merges with a strong
shortwave by the D2 period on Thursday. A plume of Gulf moisture
will overspread much of Texas, which will feed the surface low as
that moisture is hurled headlong into the Arctic air mass. WPC WSSI
values have an up to 80% chance of 2 inches of snow across much of
southern New Mexico and far west Texas and up to 50% for 4 inches
through the D2/Thursday period for the Sacramento Mountains, with=20
a 10 to 30% chance of 4 inches over the adjacent Plains and into=20
west and north-central Texas and far southern Oklahoma. The most=20
widespread snow will be into D3/Thursday night as a more portent=20
surface low develops and quickly moves east, taking much of the=20
Gulf moisture with it, as well as any widespread precipitation.

WPC probabilities have come down a bit from previous forecasts over
west Texas and New Mexico as the guidance struggles to resolve how
the cutoff low energy and the shortwave energy from the northern
stream interact with one another. Small changes in their
interaction would result in big changes in the snowfall pattern,
but for now the axis of heaviest snow has shifted east somewhat.=20

Farther north, a shortwave moving out of Canada through Montana on
the northern side of the aforementioned longwave trough on
Wednesday will touch off some light to locally moderate snow over
much of the state what will punch into Wyoming and the Black Hills
on Thursday. WPC probabilities for at least 4 inches of snow are=20
highest over the Little Belts, Gallatin Range, and into the=20
Bighorns, in the 30-50 percent range.


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

A potent positively tilted longwave trough containing plentiful=20
upper level energy from an aforementioned upper level low will=20
support a very strong upper level jet that stretches from West=20
Texas east through the Carolinas. Surface cyclogenesis will occur=20
Thursday night near the Gulf near the TX/LA border in the RER of=20
that jet, which will bed southward, thus maximizing the upper level
divergence over Louisiana. A plume of Gulf moisture will inject=20
into the low with maximum PWATs along the Texas Gulf Coast rising=20
to around 1.75 inches. While that level of moisture doesn't move=20
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 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
Appalachians by Friday evening. A tight temperature gradient on the
north and west side of the low will promote a similarly tight
gradient in precipitation amounts and in the areas where the cold=20
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 widespread snow
north of that from north-central Texas east through the Tennessee
Valley. There will be some overlap with the snowfall footprint=20
from this most recent winter storm, but much of the greatest=20
impacts will be south of there. Thus, for many this will be a very=20
impactful snowfall event and the first winter storm of the season=20
for areas such as Dallas-Fort Worth north and east through the=20
Ozarks and into the Memphis and Nashville metro areas.

WPC probabilities for Major to locally Extreme impacts extend from
the DFW Metroplex area north and east to the Ozarks, with a thin
stripe of extreme impacts from snow and ice along the Sabine River
from Tyler and Longview, TX east to north and east of Shreveport and
Monroe, LA. Just south of that stripe, moderate impacts from ice
are possible form central Texas north and west of Austin and San
Antonio to these same areas of east-central Texas and far northern
Louisiana. Expect considerable to substantial disruptions of daily
life. Travel is not advised.

The updated set of Key Messages are linked below.

Wegman

...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!4gIUHv-LdivaJrm34s5qP1YkcDpwTyEMTxnhdNNJwgJbv=
npigmJrDHsX7MD3ipL0Vr-PgkAYdaJlun4hJs5x1vtmk-k$=20

$$

=3D =3D =3D
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