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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

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

Broad upper low churning near Newfoundland and southeast Canada
will continue to help stream arctic air and shortwaves over the
Great Lakes through at least early Thursday. This will cause
additional lake- effect snows south and southeast of each of 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, around
Erie, PA where a strong band originating off of Lake Huron is
reinforced by the brief time the band is over Lake Erie, as well as
off the southeast end of Lake Ontario into the Syracuse area. By
D2/Thursday, high pressure moving overhead of the Great Lakes will
end the lake- effect from northwest to southeast until the next
round of light snow arrives from a central U.S. trough on D3.

...Rockies and West Texas...
Days 1-2...

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. Strong isentropic ascent
will allow for a period of snow showers later today into tonight.
Additionally, increasing moisture will overspread much of Texas as
upper level southwesterly flow increases in response to the upper
trough sharpening and lifting north. WPC PWPF values have over 70%
chance of 2 inches of snow across much of southern New Mexico
peaking over the Sacramento Mts, with lower probs (10-30%) over far
west Texas and nearby southern High Plains northward into New
Mexico. and far west Texas and up to 50% for 4 inches through the
D2/Thursday period for the Sacramento Mountains, with a 10 to 30%
chance of 4 inches over the adjacent Plains and into west and
north-central Texas and far southern Oklahoma. The most widespread
snow will be into D3/Thursday night as a more portent surface low
develops and quickly moves east, taking much of the Gulf moisture
with it, as well as any widespread precipitation.

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
tonight into Thursday. WPC probabilities for at least 4 inches of
snow are highest over the Little Belts, Gallatin Range, and into
the Bighorns, in the 70-90 percent range.


...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.


...Pacific Northwest and Northern Rockies...
Day 3...
Shortwave entering the Pacific Northwest on Friday will spread high
elevation snow to the Cascades and Northern Rockies as it pushes
eastward and snow levels are expected to start around 4500ft and
fall to around 3000ft by the end of the forecast period. WPC
probabilities for at least 8 inches are between 40-70% for the WA
Cascades and northern ID mountains.

Snell


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

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png

$$