Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 PM EST Sun Jan 19 2025

Valid 00Z Mon Jan 20 2025 - 00Z Thu Jan 23 2025

...Great Lakes...
Days 1-3...

Broad cyclonic flow encompassing much of the eastern CONUS will be
reinforced by dual shortwaves - one moving east of the Great Lakes
to start the period, and a second more amplified wave Tuesday
night. The overall flow pattern is reflective of a true mid-winter
airmass with cross-polar flow surging extremely cold air into the
eastern 2/3 of the CONUS, with 850mb temps progged to fall from
around -15C to as low as -30C (falling to below the 1st percentile
within the CFSR climatology). This will create persistent CAA
favorable for lake effect snow (LES).

Notably, the water temperatures across the Great Lakes have cooled
considerably thanks to a cold January, and ice is now evident in
satellite imagery and as analyzed by GLERL, which may somewhat
limit the intensity of LES. However, with such an extremely cold
airmass in place, and surges of enhanced ascent associated with the
aforementioned shortwaves, inversion depths to 10,000 ft and lake-
induced instability of 500-1000 J/kg will still support periods of
intense LES, especially downstream of Lakes Erie and Ontario, with
2"/hr rates or more possible.

WPC probabilities D1 are high (70%+) for 4+ inches of snow across
the Western U.P, and northern L.P. of MI, as well as downstream of
Lakes Erie and Ontario. The heaviest LES commences D2 downstream of
Lakes Erie and Ontario where WPC probabilities for 8+ inches reach
above 50%, although upstream snowfall across MI may lessen in
coverage and intensity. Another surge of CAA will enhance LES
across the L.P. of MI once again D3, but the heaviest snow will
continue into the Tug Hill Plateau where event-total snowfall of
2-4 feet is forecast.


...South-Central Plains...
Day 1...

A shortwave digging through the broad cyclonic flow encompassing
much of the eastern 2/3 of the CONUS will dig E/SE from the Great
Basin as a southern stream impulse. This will eventually interact
with a northern stream shortwave to drive a full-latitude trough
development, and while it remains positively tilted, it will drive
downstream jet intensification, with the resulting overlap of PVA, jet
level diffluence, and mid-level isentropic ascent causing an
expansion of snowfall Monday afternoon through Monday night. Total
ascent is modest and the best ascent does not seem to overlap with
the DGZ, but an extremely cold column should still result in
efficient/fluffy SLR where snowfall occurs. This will cause some
modest snowfall accumulations for the TX/OK Panhandles and points
east into OK/KS, where WPC probabilities are as high as 30-50% for
2 inches of snow.


...Mid-Atlantic and Northeast...
Day 1...

Fast moving low pressure will bring significant snow to the Mid-
Atlantic and Northeast through Monday as it tracks northeast just
offshore the coast.

This low is being driven by an overlap of a negatively tilted
shortwave trough currently emerging from the TN VLY, with its track
likely to take it over the Washington, D.C. area before lifting
over Cape Cod and Downeast Maine overnight. A potent 150+ kt jet
streak arcing poleward will overlap with this shortwave, resulting
in a favorable environment for low pressure strengthening just
offshore. The resultant surface wave will deepen at a moderate
pace, but lift rapidly northeast, exiting into Canada by late
Monday morning. Despite the rapid pace of this system, a swath of
heavy snow is likely just inland from the coast, with the heaviest
snowfall accumulations likely just NW of I-95 from Washington,
D.C. to Boston, MA.

The primary driver of this heavy snow will be an overlap of
enhanced moisture rotating around the system on the accompanying
isentropic ascent, and a corridor of strengthening 700-600mb fgen.
While the forecast soundings do not scream intense snowfall due to
very dry air above the DGZ resulting in modest available PWs, the
most intense fgen does appear to overlap efficiently with the DGZ
suggesting potential for banded snow structures, within which the
WPC prototype snowband tool indicates the potential for 1-2"/hr
snowfall. The progressive nature of this system will limit overall
accumulations, and precip may start as a rain/snow mix in many
areas before cooling dynamically to support all snow, but WPC
probabilities feature a high risk (>70%) for more than 4 inches of
snow in a continuous swath from far eastern Upstate NY through much
of southern and central New England from 00Z tonight through 18Z
Monday. Locally, more than 8 inches is possible (30-50% chance) in
Downeast Maine. With snowfall rates exceeding 1"/hr at times, this
will result in hazardous travel in many areas.

The progressive nature of this system will limit more substantial
impacts, but heavy snow and hazardous travel impacting much of the
northeast I-95 urban corridor has prompted the issuance of
collaborative Key Messages linked below (KeyMessage_2).


...Texas, Gulf Coast, and Southeast...
Days 2-3...

...Significant winter storm likely across portions of the Southern
U.S. this week...

A rare Gulf-Coast and Southeast Atlantic Coast winter storm is
becoming more likely beginning Tuesday as an anomalously cold
airmass settles across the eastern 2/3 of the CONUS and sets the
stage for the upcoming low pressure system.

Amplified troughing deepening across the east will spill cold air
all the way to the Gulf Coast as reflected by ECMWF EFI for
minimum temperatures falling below -0.9 on Tuesday, and
expanding/intensifying on Wednesday. This cold air will be
entrenched even before the low develops, and despite some
variability in model solutions among the various ensemble camps,
the colder ECMWF is favored at this time.

As this cold air floods across the region, an intensifying
shortwave will dig out of the Great Basin and then sharpen as it
pivots eastward from West Texas through the Gulf Coast Tuesday into
Wednesday. As this occurs, some interaction with a northern stream
impulse will yield a positively tilted full latitude trough
shifting east, with the resultant downstream jet streak
intensifying and shifting poleward to place the favorable RRQ atop
the Gulf Coast. The overlap of height falls and upper diffluence
will help spawn a wave of low pressure across the Gulf of Mexico,
and as 300K isentropic ascent maximizes to the north, precipitation
will begin to overspread eastern Texas Tuesday morning, before
becoming more expansive across the Gulf Coast and eventually
coastal Southeast Tuesday night into Wednesday.

The challenge with this event is where the snow/sleet/freezing rain
transition will occur, and where the northern cutoff of
precipitation will occur. Some very cold and dry air to the north
will likely cause a sharp precip gradient, but south of there will
be a swath of heavy snow driven by strong WAA, and places across
LA/MS/AL that rarely seen snow could experience 1"/hr snow rates
just north of the mixed transition zone. While there is still some
uncertainty into the placement of the mixed precip axis, especially
farther east into the Carolinas/Georgia/Florida, the guidance has
trended a bit colder and farther south, which is impacting the
recent WPC probabilities.

At this time, WPC probabilities indicate a high chance (50-90%)
for at least 2" of snow from far eastern TX through southern/central
LA and into southern MS. Locally, 4-6" of snow is possible,
especially where any banding sets up which could produce snowfall
rates above 1"/hr. The combination of the heavy snow, very cold
temperatures, and the simply rarity of this event could result in
major impacts and considerable disruptions to daily life. Farther
east, as the event translates downstream, WPC probabilities for 2+
inches of snow are as high as 30-50%, focused in the eastern
Carolinas, however uncertainty here is higher due to latitudinal
discrepancies in the axis of QPF from the available models.
Additionally, southeast of the heaviest snow, especially from the
Florida Panhandle east to the northern Peninsula and parts of
coastal GA/SC, freezing rain is likely, which has a 30-50% chance
of producing at least 0.1" of ice.

This event has prompted the issuance of collaborated Key Messages
(KeyMessage_1) linked below.

Weiss



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

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

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