FOUS11 KWBC 190918
QPFHSD

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

Valid 12Z Sun Jan 19 2025 - 12Z Wed Jan 22 2025

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

Deep low pressure located over northern Ontario will maintain 
cyclonic flow with Arctic cold over the Great Lakes through at 
least Tuesday. NWly flow will be the prevailing wind direction 
through Sunday before backing out of the west Sunday night through
Monday. 72hr snow probabilities for >12" are over 50% in the 
western U.P., northern and southern western shores of the L.P. and 
over 80% for the eastern U.P. The shift to westerly flow over the 
eastern Great Lakes allows single banding to develop by Monday 
afternoon with heavy snow continuing into Tuesday night for 
Buffalo and the South Towns as well as northern sections of the Tug
Hill where Day 3 snow probabilities for >8" are over 60% (80% for 
the Tug Hill). In fact, there are high chance probabilities (>70%)
for snowfall totals >24" along the eastern-most slopes of the Tug Hill.

...South-Central Plains... Day 2...

The incoming longwave trough that will produce wintry weather 
along the Gulf Coast (see section below) will cause snow banding 
east from the southern CO/northern NM Rockies in a similar manner 
to the banding that occurred today east of the Rockies in the High
Plains on Monday. Weak 850mb FGEN and 700mb WAA aloft will be the
primary sources for this period of snow. Day 2 snow probabilities 
for >2" are 30-50% over southwestern KS extending south into 
northwestern OK.


...Central Appalachians, Mid-Atlantic and Northeast... Days 1-2...

...Significant winter storm to produce a swath of heavy snow from
the Central Appalachians through the northern Mid-Atlantic and New
England today through Monday morning...

Guidance has generally come into a better consensus on where the 
heaviest swatch of snowfall is expected the Lehigh valley and 
Poconos on north and east through the Lower Hudson Valley, into the
Berkshires, Worcester Hills, and White Mountains. The overall 
synoptic and mesoscale setup and progression also remains in good shape.

The Arctic cold front will push off/along the East Coast tonight 
while stalling over the Mid-Atlantic while a strengthening area of
low pressure tracks over eastern North Carolina. Upper-level 
divergence increases over the Mid-Atlantic coast through Sunday 
morning as a robust 250mb jet surpasses 150kt and its diffluent 
left-exit region aids in rapid surface/coastal development. As 
850-700mb WAA increases over the Mid-Atlantic, it will clash with 
the approaching Arctic air-mass to the north, resulting in a band 
of heavy snow that initially starts out over the Allegheny 
Highlands early Sunday morning, then spreads over the Laurels in PA
across northern VA and MD through midday. Eastern PA through 
southern NY will witness some of the best 850-700mb FGEN through 
the afternoon, allowing snowfall rates to increase through this 
swath. The 00Z HREF mean hourly snowfall hovers around 1"/hr over 
the northern half of NJ by 00Z Monday with 1"/hr rates in a rather 
wide swath tracking north through the night until it clears eastern
Maine at 12Z. The storm races north through New Brunswick early 
Monday morning with lingering periods of snow throughout parts of 
Maine tapering off by midday Monday.

WPC snow probabilities for >6" snow are 50-70% from central WV 
terrain and the Laurel Highlands on north and east through eastern
PA, northern NJ, southern NY, and into much of central New 
England. Eastern Maine has the best odds for >8" amounts with
50-70% chances for >8" through Monday morning. Moderate chance
probabilities (40-60%) for >4" of snowfall are present from the
western DC/Baltimore suburbs, into the Philly and NYC metros, and
along the southern New England coast. Hazardous driving conditions
are anticipated in these areas on Sunday and into Monday morning. 
With bitterly cold temperatures spilling in for the upcoming work-
week, snow and instances of re-freezing/black ice are likely to 
stick around for much of the week in wake of this winter storm. 


...Texas, Gulf Coast, and Southeast...

...Significant winter storm growing in confidence across the 
Southern U.S. this week..

Days 2-3... As a positively tilted trough over the southwestern 
U.S. that promotes strong upper level ascent over Texas and the 
Gulf of Mexico Monday night, low pressure forming along a 
strengthening surface front off the Lower TX coast. Low level 
easterly winds Monday evening will advect Gulf moisture into 
southern Texas that will result in an icy wintry mix over the 
region. Meanwhile, as the upper trough approaches Tuesday morning, 
the an anomalous IVT (>500 kg/m/s, or above the 90th climatological
percentile per NAEFS) will overrun a strong 850mb FGEN area from 
southeast TX to the central Gulf Coast. Moderate to locally heavy 
snow banding is expected from the northern Houston metro suburbs on
east across southern LA into southern MS on Tuesday, with snow 
moving into southern AL, the western FL Panhandle, and central GA 
by Tuesday evening. The same swath of ice then moves over northern 
FL and southern GA Tuesday night. Snow will increase in intensity 
along the GA/Carolina coast Tuesday night as the divergent right-
entrance region of a roaring 200kt 250mb jet streak aligns 
favorably with moist southerly flow. Snow will be heavy but 
progressive with snow ending along the NC Outer Banks by midday Wednesday.

Days 2-3 snow probabilities for >4" are 30-50% from the eastern
Houston suburbs to south-central MS. WPC probabilities for >4" of
snowfall rise to 40-60% from central SC on east to eastern NC.
There are ensemble members that bring the potential for >4" of
snow into the Norfolk/VA Beach area, but this remains low 
confidence given the wide range of potential scenarios. In terms of
ice accumulations, WPC probabilities show low-to-moderate chances 
(30-50%) for >0.1" of ice from the FL Panhandle and suburbs west of
Jacksonville on north through southern GA and far southern SC. 
Much of south Texas sports moderate-to-high chances (50-70%) for 
>0.01" of ice accumulation with low chances (<20%) for ice amounts 
>0.1". While exact amounts are unclear, there is no denying the 
greater impact potential given the subfreezing surfaces throughout 
the South. The WSSI shows Major Impact potential (considerable 
disruptions and dangerous to impossible travel) in southern LA, 
with a wide swath of Moderate Impacts from the northern Houston 
suburbs to southern Alabama. Details with p-ptype are better done 
with higher-res models which offer output in the Days 1/2 
timeframe, so expect greater detail to these wintry accumulations 
at that time.

Mullinax/Jackson

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

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

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

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