FOUS11 KWBC 092023
QPFHSD

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

Valid 00Z Fri Jan 10 2025 - 00Z Mon Jan 13 2025

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

...Major winter storm has begun from west Texas into Mississippi,
Arkansas, and Missouri. Hazardous winter conditions will continue
across much of the Mid- South and into portions of the Mid-Atlantic
through Saturday morning...

A potent positively-tilted longwave trough, formerly a strong
cutoff low, will continue opening up as the energy rejoins the jet
stream tonight through Saturday. As this process unfolds, a piece
of energy on the east side of the upper low will act as the driving
force of upper level energy supporting cyclogenesis along the Texas
Coast. For a brief time tonight, the shortwave will take on a
slight negative tilt within the broader southwesterly flow
associated with the RER of a hyperactive 150+ kt jet stream. This
will work to locally increase precipitation rates, especially into
the Ozarks of southern Arkansas where upslope will also play a role
at increasing the intensity of precipitation.

Plentiful cold air is in place from north Texas across Arkansas
thanks to the longwave trough and former cutoff low now forming the
base of the upper level trough. As the surface low along the Texas
Coast intensifies tonight due to the aforementioned upper level
interactions, plentiful Gulf moisture will be rapidly drawn north
up the Lower Mississippi Valley. Maximum PWATs along the Texas
Gulf Coast will rise 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 into the
developing winter storm.

For those areas that have winter concerns, the greatest storm total precipitation from this storm will be from northern Louisiana into
southern Arkansas. Hazardous winter conditions will extend well
east (and west) of this area, but the greatest hazards will be in
this area. Expect around 8 inches of snow across central to
northern Arkansas.

On the freezing rain/ice side of the storm, the aforementioned
LLJ of Gulf moisture will also advect plentiful warm air northward.
As it runs into the cold air in place over TX/AR, it will be lifted
above the colder air, resulting in an inversion. Thus, any snow
that falls into this warm air layer will melt into the rain, and
depending on latitude, may either refreeze (sleet) or reach the
sub-freezing atmosphere and ground as liquid, resulting in freezing
rain and ice. The greatest concerns for ice accumulation from
freezing rain will be across southern Arkansas, where around a
quarter inch of ice is expected.

By Friday morning, the upper level shortwave will be shearing
apart, as the LLJ aligns more southwest to northeast, and therefore
diminishes a bit in intensity. This will result in somewhat lesser precipitation rates as the shield moves across the Southeast.
Despite this, cold air will remain in place across all of
Tennessee, while the warm air erodes the top of the cold air layer
and ice accumulations continue from northern Mississippi through
northern Georgia. Amounts will generally range between a tenth and
a quarter inch, which are likely to cause significant travel
disruptions, localized power outages and downed branches from the
weight of the ice. Across central Mississippi, Alabama, and
Georgia, the warm air should erode any shallow cold layer with
daytime heating, so those areas may start the day Friday with ice
but then change over to a cold rain. Due to the depth of cold air
in place and reducing WAA from west to east, the area where
freezing precipitation sets up should remain generally constant,
with any areas where ptypes change staying quite narrow.

On Day 2/Friday Night, a separate northern stream shortwave diving
south over the Great Lakes will attempt to merge with the shearing
upper level energy associated with the
narrowing/amplifying/shearing upper level trough. This will cause
the surface low tracking along the Gulf coast to begin to gain
latitude as it tracks along the Atlantic coast of Georgia and South
Carolina. While this will lead to some deepening of the surface
low, it will also give the low a push towards the northeast,
resulting in a faster forward speed. Meanwhile the plume of Gulf
moisture will be fully southeast of the low, with less moisture
available to convert to frozen precipitation as the precipitation
plume moves into the Carolinas, the Tidewater of Virginia, and the
southern Delmarva. This will make for the stripe of ice to continue
from northeastern Georgia across northern South Carolina, central
North Carolina, and into far southeastern Virginia. Meanwhile 1-4
inches of snow are expected north of this area from the
Appalachians across much of Southern Virginia and the southern
Delmarva. These will be significantly lesser amounts of total
precipitation as compared with areas further west that had a better
fetch of moisture from the Gulf to work with. The center of the low
will move over the Outer Banks early Saturday morning then race out
to sea, leaving only very minor amounts of snow for most of the
northern Mid-Atlantic and New England.

This evolution will occur because while the southern stream energy
associated with the winter storm and the northern stream shortwave
will attempt to phase, they will only be partially successful at
doing so early Saturday morning before they separate again. Due to
the ultimate failure of these pieces of energy to merge, the storm
will remain relatively weak and fail to rapidly intensify and bend
back westward and impact New England more. Rather, the relatively
weak and moisture starved northern stream energy will cause just a
brief period of light snow for the northern Mid-Atlantic and New England.


...Northwest to the Upper Midwest... Days 1-3...

Fairly sharp mid-level shortwave and sfc front will enter the
PacNW starting Friday morning 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
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, on D2, an Alberta clipper 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, with moderate probabilities (50-70%) of at least 2 inches
of snow over northern ND. This low will intensify during the Day
Sunday/D3 when it moves over northern Wisconsin and the U.P. A
reinforcing upper level low will swing a shortwave around its
southern periphery and merge with the existing upper level
shortwave that drove the low through Saturday. With additional
moisture from Lakes Superior and Michigan, localized lake-enhanced
snow is likely with this low. It will be from an unusual southerly
direction on the "warm" side of the low into the central U.P. from
Lake Michigan.

Wegman

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

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

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