FOUS11 KWBC 060759
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Mon Jan 6 2025

Valid 12Z Mon Jan 06 2025 - 12Z Thu Jan 09 2025

...Ohio Valley through the Mid-Atlantic... Day 1...

...Major winter storm continues from the Ohio Valley through the
Mid-Atlantic...

The major ongoing winter storm which has already dumped over a foot
of snow from central KS to northern MO is forecast to rapidly slide
eastward today while producing additional areas of heavy snow and
freezing rain between the Ohio Valley and Mid-Atlantic. This 
system is being forced by a closed 500mb low which will be 
positioned over the IL-IN border to start the period. This closed 
low will weaken into a potent open wave today while pushing due 
east toward the Mid-Atlantic by this evening. The weakening is 
being aided by a separate stronger upper low diving southward 
towards New England and helping shear out the storm system as it 
exits the East Coast. At the same time, however, a potent jet 
streak rotating around the base of the trough will amplify towards 
150 kts, helping to offset some of the loss of deep layer lift 
resulting from the opening of the trough. At the surface, the 
accompanying low pressure will skirt east across the lower OH 
VLY/TN VLY, with secondary development likely occurring across the 
Carolinas and then pressing east off the Mid- Atlantic Coast by 
Tuesday morning.

Together, this will result in a large swath of heavy 
precipitation, with all p-types likely (at least at the beginning
of the forecast period). Downstream of the primary surface low, 
strong isentropic ascent along the 290K-295K surface will draw 
impressive moisture northward into the system (NAEFS PW above the 
90th climatological percentile) supporting the expansiveness of 
widespread precipitation.

A stripe of very heavy snow (rates 1-2"/hr) is likely within the 
WAA during the first half of D1, and the conceptual model for a 
laterally translating band from west to east fits this system. This
suggests a narrow but intense band of snow, supported by both NBM 
and DESI probabilities, as well as the WPC snowband tool which 
indicates a high potential for 1-2"/hr snow rates from northern VA
and MD eastward to DE following the band weakening across southern
OH right before 12z. Since this band will be translating along its
long/axis (more eastward motion than north) some locally very high
snow totals are possible as reflected by 40-50% chance for an
additional 8+ inches of snow from northern VA eastward through 
southern DE, but in general WPC probabilities support a high (>70%)
risk for 4+ inches along this same axis with a sharp N/S gradient 
on both sides. These similar probabilities are also found in
western MD and WV where an upslope component following the passage
of the low pressure system.

Some of this snow will be additionally enhanced by the post-system
comma head/deformation which will pivot across the Mid-Atlantic 
Monday night. The guidance continues to trend a bit stronger with 
this feature, and while latitudinal placement of the most 
significant forcing remains quite uncertain within this axis, 
renewed moderate to heavy snow, with much fluffier SLRs than 
earlier, will likely regenerate across the region adding up to a 
few more inches before exiting by Tuesday morning.

South of the heavy snow area, this system will also pose a 
continuing ice threat within the warm nose/p-type transition
zone. Damaging ice accretion of 0.5-0.75" has already been realized
along a stretch spanning from eastern KS to KY, but thankfully
precipitation will be coming to an end across this area as light
snow/freezing rain pushes eastward today on the backside of the 
low. Intense WAA over- topping the cold surface layer will continue
this morning across parts of the southern Mid-Atlantic, however.
WPC probabilities for at least an additional 0.1" beginning 12z
today fall between 10-40% across southern VA. 

This system has prompted the issuance of key messages which are 
linked at the bottom of the discussion.

...Great Lakes and Northeast... Days 2-3...

Behind the large system moving off the Mid-Atlantic coast Tuesday 
morning, a push of arctic air will dig into the Great Lakes and 
Northeast, fueled initially by a potent shortwave dropping nearly 
due south from Ontario Tuesday morning. 850mb temps are progged to 
crash to -15C to -20C, steepening lapse rates considerably, and 
resulting in a favorable setup for both lake effect snow (LES) and 
upslope snow into the upwind terrain of the Northeast. 

The greatest potential for upslope snow will occur D2 /Tuesday/ as
the potent shortwave dives south pulling a cold front with it. 
This will mark the leading edge of the coldest air, and cause 
impressive NW flow into a moistening column. The temperatures 
during this time will be very cold, so the DGZ will be quite low, 
resulting in effective upslope into the snow growth region. Light 
and fluffy snow will efficient accumulations, so despite a short 
duration, WPC probabilities for 4+ inches of snow are moderate 
(50-70%), highest in the NW Adirondacks and N Greens.

Although some heavy LES may occur downstream of Lake Michigan 
early D2, the greater coverage and intensity of LES is likely D3 
behind this front. GLERL lake-surface temperatures are still 
sampled to be +5C to +8C, so the very cold air moving overhead will
support lake-induced instability to drive LES. At this time the 
LES appears less intense as the last round, but WPC probabilities 
D3 feature a low risk (10-30%) for 4+ inches of snow in the favored
N to NW snow belts.

...Intermountain West, Rockies, and Central High Plains.... Days 1-3...

Pacific shortwave diving across the Intermountain West today will
surge moisture ahead of it and feature favorable divergence to
promote scattered high elevation upslope snow spreading southward 
across the central Great Basin and Central Rockies today before
reaching the Southern Rockies late D2/early D3. Several inches of 
snow are likely D1 over northern NV, southeastern ID, western 
WY/southwestern MT and then into the CO Rockies. WPC probabilities 
for at least 4" of snow are >50% in these mountainous regions.

Into D2, height falls (and trailing height falls) will rotate 
through the Southwest and sharpen, eventually closing off over the 
lower CO River Valley late Tue. With high pressure nosing down out 
of the northern Plains, upslope snow will expand across the CO Front
Range via an easterly low level flow along with a relatively low 
DGZ. The upper low will likely move into northwestern Mexico by the
end of the period, helping to drag the snowfall southward as well 
through the Mogollon Rim, Sangre de Cristos, Sacramento Mountains, 
etc. For the D2-3 period, WPC probabilities for at least 4 inches 
of snow are 20-50% over the Front Range into the CO Rockies, San 
Juans, and higher elevations in AZ/NM above 8000ft or so.

...Southern Plains... Day 3...

As the aforementioned upper low closes off over the Southwest late
D2 it continues to dig into northwestern Mexico D3 and become
anomalously deep (below the 0.5th climate percentile per the 12z 
GEFS). Meanwhile, a separate shortwave rounding the far eastern 
Pacific ridge is forecast to dive southward and interact with this 
feature and spawn strong southwest flow containing rich moisture 
and prominent upper divergence. Given the strong high situated to 
the north over the Central Plains and cold the low-level airmass in
place, snow and mixed precipitation is expected over much of 
western and north-central TX. Currently, WPC probabilities for at 
least 2 inches are low (10-30%) through Thursday morning but these 
may trend up if precipitation start time trends faster. This storm 
will need to be monitored as 00z ensembles depict heavier snowfall 
impacting parts of the Southern Plains on D4.

Snell

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

https://www.wpc.ncep.noaa.gov/key_messages/La...

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