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=20
Valley today through the Mid-Atlantic...

The major winter storm which has been advertised for several days=20
now is ongoing to start the forecast period. This system is being=20
forced by a closed 500mb low which will be positioned over Kansas=20
City, MO to start the period. This closed low will move=20
progressively east, reaching Indianapolis, IN by 12Z Monday, and=20
then northern VA at the start of D2 /00Z Tuesday/. During the=20
latter half of D1, however, this feature is progged to open into a=20
wave, while still potent, but with subtly less downstream ascent.=20
At the same time, however, a potent jet streak rotating around the=20
base of the trough will amplify towards 150 kts, helping to offset=20
some of the loss of deep layer lift resulting from the opening of=20
the trough. At the surface, the accompanying low pressure will=20
skirt east across the lower OH VLY/TN VLY, with secondary=20
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=20
precipitation, with all p-types likely. Downstream of the primary=20
surface low, strong isentropic ascent along the 290K-295K surface=20
will draw impressive moisture northward into the system (NAEFS PW=20
above the 97th climatological percentile) supporting an expansion=20
of widespread precipitation. At the same time, the accompanying=20
theta-e ridge rotating cyclonically into the system will lift into=20
a TROWAL, especially early in the period, before the upper low=20
opens and the low occludes to the secondary development. Despite=20
that, some enhanced elevated instability beneath this TROWAL will=20
support heavy precipitation rates, leading to widespread=20
significant precipitation amounts of snow, sleet, and freezing=20
rain. While confidence in heavy precipitation causing impacts is=20
high, the exactly placement of the transition zone and associated=20
ice/snow amounts remains uncertain.

There continues to be two primary camps with the track of this=20
low. THe GFS/NAM/high-res members remain a bit north of the=20
CMC/ECMWF. While latitudinal spread has decreased, it is still=20
significant in terms of where the transition zone will occur.=20
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=20
system. This suggests a narrow but intense band of snow, supported=20
by both NBM and DESI probabilities, as well as the WPC snowband=20
tool which indicates a high potential for 1-2"/hr snow rates from=20
northern MO eastward to MD and DE. Since this band will be=20
translating along its long/axis (more eastward motion than north)=20
some locally very high snow totals are possible as reflected by=20
10-30% chance for 12+ inches of snow from northern WV eastward=20
through southern DE, but in general WPC probabilities support a=20
high (>70%) risk for 6+ inches along this same axis with a sharp=20
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=20
Monday night. The guidance continues to trend a bit stronger with=20
this feature, and while latitudinal placement of the most=20
significant forcing remains quite uncertain within this axis,=20
renewed moderate to heavy snow, with much fluffier SLRs than=20
earlier, will likely regenerate across the region adding up to a=20
few more inches before exiting by Tuesday morning, but WPC=20
probabilities after 00Z Tuesday for additional snow accumulations=20
exceeding 2" are just around 10%, highest around the Chesapeake=20
Bay.

South of the heavy snow area, this system will also pose a=20
significant to damaging ice threat within the warm nose/p-type=20
transition zone. Intense WAA over-topping the cold surface layer=20
accompanied by persistent E/NE dry-bulb advection is a classic=20
setup for damaging freezing rain. Although there is some=20
uncertainty into how efficiently the ice can accrete at times due=20
to what should be intense precipitation rates, the threat for=20
damaging ice remains from central KY into the southern/central=20
Appalachians and Foothills where WPC probabilities for 0.25"+ are=20
50-90% across northern KY and until WV/VA. The greatest risk for=20
damaging ice however continues to focus across eastern KY where a=20
10-30% chance exists for 0.5" of ice. This could result in downed=20
limbs and power lines leading to widespread long-lasting power=20
outages and impossible travel.=20

Finally, between the heavy snow and heavy icing, the forecast=20
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=20
cold layer which will allow refreezing of droplets. At the same=20
time, cold/dry wet-bulb advection on E/NE winds will maintain this=20
cold layer effectively to prevent a more rapid p-type transition.=20
Heavy accumulations of sleet are difficult to achieve, but in this=20
setup it is possible that up to 1" of sleet will accumulate causing
tremendous driving impacts. The greatest risk for the significant=20
sleet accumulations D1 extend west to east from northern KY through
southern VA.

This system has prompted the issuance of key messages which are=20
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=20
morning, a push of arctic air will dig into the Great Lakes and=20
Northeast, fueled initially by a potent shortwave dropping nearly=20
due south from Ontario Tuesday morning. 850mb temps are progged to=20
crash to -15C to -20C, steepening lapse rates considerably, and=20
resulting in a favorable setup for both lake effect snow (LES) and=20
upslope snow into the upwind terrain of the Northeast.=20

The greatest potential for upslope snow will occur D2 /Tuesday/ as
the potent shortwave dives south pulling a cold front with it.=20
This will mark the leading edge of the coldest air, and cause=20
impressive NW flow into a moistening column. The temperatures=20
during this time will be very cold, so the DGZ will be quite low,=20
resulting in effective upslope into the snow growth region. Light=20
and fluffy snow will efficient accumulations, so despite a short=20
duration, WPC probabilities for 4+ inches of snow are moderate=20
(50-70%), highest in the NW Adirondacks and N Greens.

Although some heavy LES may occur downstream of Lake Michigan=20
early D2, the greater coverage and intensity of LES is likely D3=20
behind this front. GLERL lake-surface temperatures are still=20
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=20
LES appears less intense as the last round, but WPC probabilities=20
D3 feature a low risk (10-30%) for 4+ inches of snow in the favored
N to NW snow belts.


...Western/Southwestern U.S....=20
Days 1-3...

Pacific shortwave will enter Oregon later this evening with=20
another surge in moisture to the OR Cascades into the northern=20
Great Basin as it weakens and slips southeastward into Nevada=20
overnight. High initial snow levels around 6000-7000ft over the OR=20
Cascades only drop to around 5000-6000ft under the trough axis,=20
restricting significant snow to the highest OR Cascades well above=20
pass level. Snow levels will be closer to 5000ft over the Great=20
Basin and 4000ft over the northern Rockies, where several inches of
snow are likely D1 over northern NV, southeastern ID, western=20
WY/southwestern MT and then into the CO Rockies. WPC probabilities=20
for at least 4" of snow are >50% in these mountainous regions.

Into D2, height falls (and trailing height falls) will rotate=20
through the Southwest and sharpen, eventually closing off over the=20
lower CO River Valley late Tue. With high pressure nosing down out=20
of the northern Plains, upslope snow will expand across the CO=20
Rockies into the Front Range via an easterly low level flow. The=20
upper low will likely move into northwestern Mexico by the end of=20
the period, helping to drag the snowfall southward as well through=20
the Mogollon Rim, Sangre de Cristos, Sacramento Mountains, etc. For
the D2-3 period, WPC probabilities for at least 4 inches of snow=20
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=20
 Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=
stKeyMessage_1.png__;!!DZ3fjg!67GhcKkPl9AZ3epqfeLAFe0ovocqOzfPfqEzx7LPDRI3R=
DQu7DY3j0IDZS9ngKDbIELE4DB2m8wVKKhH9BxYB-HV3-o$=20

$$

=3D =3D =3D
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