FOUS11 KWBC 052020
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 PM EST Sun Jan 5 2025

Valid 12Z Sun Jan 05 2025 - 12Z Wed Jan 08 2025

...Central Plains through the Mid-Atlantic... Days 1-2...

...Major winter storm continues from Central Plains and Ohio 
Valley today through the Mid-Atlantic...

The major winter storm which has been advertised for several days 
now is ongoing to start the forecast period. This system is being 
forced by a closed 500mb low which will be positioned over Kansas 
City, MO to start the period. This closed low will move 
progressively east, reaching Indianapolis, IN by 12Z Monday, and 
then northern VA at the start of D2 /00Z Tuesday/. During the 
latter half of D1, however, this feature is progged to open into a 
wave, while still potent, but with subtly less downstream ascent. 
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. 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 97th climatological percentile) supporting an expansion 
of widespread precipitation. At the same time, the accompanying 
theta-e ridge rotating cyclonically into the system will lift into 
a TROWAL, especially early in the period, before the upper low 
opens and the low occludes to the secondary development. Despite 
that, some enhanced elevated instability beneath this TROWAL will 
support heavy precipitation rates, leading to widespread 
significant precipitation amounts of snow, sleet, and freezing 
rain. While confidence in heavy precipitation causing impacts is 
high, the exactly placement of the transition zone and associated 
ice/snow amounts remains uncertain.

There continues to be two primary camps with the track of this 
low. THe GFS/NAM/high-res members remain a bit north of the 
CMC/ECMWF. While latitudinal spread has decreased, it is still 
significant in terms of where the transition zone will occur. 
However, regardless of the exact track, a stripe of very heavy snow
(rates 1-2"/hr) is likely within the WAA, 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 MO eastward to MD and DE. 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 
10-30% chance for 12+ inches of snow from northern WV eastward 
through southern DE, but in general WPC probabilities support a 
high (>70%) risk for 6+ inches along this same axis with a sharp 
N/S gradient on both sides.

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, but WPC 
probabilities after 00Z Tuesday for additional snow accumulations 
exceeding 2" are just around 10%, highest around the Chesapeake Bay.

South of the heavy snow area, this system will also pose a 
significant to damaging ice threat within the warm nose/p-type 
transition zone. Intense WAA over-topping the cold surface layer 
accompanied by persistent E/NE dry-bulb advection is a classic 
setup for damaging freezing rain. Although there is some 
uncertainty into how efficiently the ice can accrete at times due 
to what should be intense precipitation rates, the threat for 
damaging ice remains from central KY into the southern/central 
Appalachians and Foothills where WPC probabilities for 0.25"+ are 
50-90% across northern KY and until WV/VA. The greatest risk for 
damaging ice however continues to focus across eastern KY where a 
10-30% chance exists for 0.5" of ice. This could result in downed 
limbs and power lines leading to widespread long-lasting power 
outages and impossible travel. 

Finally, between the heavy snow and heavy icing, the forecast 
profiles indicate a narrow stripe where the p-type may remain sleet
for a considerable temporal duration. This is due both to a subtly
weaker surge of the warm nose as well as a deeper sub-warm- nose 
cold layer which will allow refreezing of droplets. At the same 
time, cold/dry wet-bulb advection on E/NE winds will maintain this 
cold layer effectively to prevent a more rapid p-type transition. 
Heavy accumulations of sleet are difficult to achieve, but in this 
setup it is possible that up to 1" of sleet will accumulate causing
tremendous driving impacts. The greatest risk for the significant 
sleet accumulations D1 extend west to east from northern KY through
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.

...Western/Southwestern U.S.... Days 1-3...

Pacific shortwave will enter Oregon later this evening with 
another surge in moisture to the OR Cascades into the northern 
Great Basin as it weakens and slips southeastward into Nevada 
overnight. High initial snow levels around 6000-7000ft over the OR 
Cascades only drop to around 5000-6000ft under the trough axis, 
restricting significant snow to the highest OR Cascades well above 
pass level. Snow levels will be closer to 5000ft over the Great 
Basin and 4000ft over the northern Rockies, where 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 
Rockies into the Front Range via an easterly low level flow. 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.

Weiss/Fracasso/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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