FOUS11 KWBC 180811
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
311 AM EST Tue Feb 18 2025

Valid 12Z Tue Feb 18 2025 - 12Z Fri Feb 21 2025


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

Cyclonic flow over southern Canada and the Great Lakes on the 
western side of the elongated upper trough axis will favor a 
continuation of the lake effect snow for the next couple of days. 
With 850 temperatures in the -20sC and NW to WNW flow, multi-bands 
will form off the Upper Lakes with a strong single band off Lake 
Ontario with an impressive multi-lake moisture feed. THe pattern 
will be slow to unfold, but by early Wednesday a closed low will 
begin to form over the Corn Belt which will help disrupt the 
pattern enough to diminish the lake effect snow, but then those 
height falls will bring in a general/light snow to the region on 
Thursday. WPC probabilities for at least 8 inches of snow over the 
next two days are near 50% over the eastern U.P. of Michigan. Over 
central NY between ROC-FZY-SYR, the strong single band will 
continue to dump snow at 1-3"/hr rates with significant 
accumulation in a narrow swath -- perhaps more than 12-18 inches 
(>60% probs). By day 3, snowfall will be light with most areas 
seeing less than 4 inches of snow.


...Pacific Northwest through the Central Rockies...
Days 1-3...

Height falls over the Great Basin this morning will continue 
southeastward today, giving some additional snow to the CO Rockies.
To the north, additional height falls out of Canada will drop 
through central MT with some light snow to the central ID ranges, 
southwestern MY, and the western WY ranges where WPC probabilities 
for at least 6 inches of snow are low (<40%) to the north but high 
(>70%) over western WY and the high peaks of the northern CO Rockies.

By day 2, a Pacific system will start to move into the Northwest, 
spanning the latitudes from NorCal up to the Canadian border. IVT 
will be modestly high (~90th percentile) into NorCal/southern 
OR/northern NV toward Idaho, supporting light to moderate snows for
the terrain with snow levels near 3000ft to the north and 5000ft 
to the south that will rise a bit as the core of the moisture plume
pushes through. The system will move steadily along and clear the 
coast with QPF starting early Thursday, pushing into northern 
UT/southern ID and into the Rockies. However, moisture will wane 
and snow will become lighter overall into D3 with the snow confined
mainly to the Unitas and higher CO Rockies by then. For the 3-day 
period, WPC probabilities for at least 8 inches of snow are highest
(>70%) over the higher terrain above 6000ft closer to the coast, 
7000-8000ft over the Great Basin, and 9000ft over CO/WY. 


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

A significant winter storm is underway over the Central Plains 
which will spread eastward through the Mid-South into the southern
Mid-Atlantic with potentially major impacts including heavy snow 
and significant ice. 

Snow is already underway across KS into MO this morning, driven by
lead shortwaves ahead of the Rockies trough. Through this morning,
as the stronger height falls approach the region, snow will expand
over the central and southern Plains eastward beneath the RRQ of 
the upper jet and atop the Arctic air mass driven in by the strong 
cold front overnight. With sufficient moisture influx from the 
Gulf, snow rates will increase over southern KS into southern MS 
later this morning into the afternoon with >1"/hr rates at times 
per the WPC snowband tool and HREF probs. Snow will advance 
eastward into the Mid-South later this evening and overnight but 
likely diminish somewhat in intensity. To the south, the shallow 
Arctic air mass will continue southward but southerly flow aloft 
will maintain >0C temperatures, favoring a mix of sleet and 
freezing rain over parts of OK into AR, northern LA, and western 
MS. For the D1 period, WPC probabilities of at least 4 inches of 
snow are high (>70%) from southeastern KS and northeastern OK 
eastward to western KY. Within this region, probabilities of at 
least 8 inches of snow are >60% near Joplin to Springfield, MO. 
Icing to the south should generally be light but there is a low 
change (10-30%) of at least 0.10" accretion near the Oachita 
Mountains in AR. 

By Wednesday morning, the next phase of the system will take shape
over the northern Gulf as the entrance region of the upper jet 
touches the LA/MS coast, helping to favor cyclogenesis. The 
northern part of the system will still spread snow across the Mid-
South eastward to the central Appalachians where some orographic 
lift will help wring out several inches of snow into eastern 
KY/southern WV/southwestern VA. The larger QPF-maker will be the 
forming Gulf low that is forecast to track ENE to NE across north 
FL before turning northeastward along the Gulf Stream just off the 
SC/NC coasts. With the cold air mass in place through the East, 
this sets up a southern slider snow event with an axis of heavy 
snow along the VA/NC border into the Tidewater and southern 
DelMarVa. The incoming northern stream upper low will lag behind 
just enough to not phase until farther downstream, but enough 
ingredients will be coming into place in the formative stages of 
this system to yield a modest to heavy snowfall for parts of the 
southern Mid-Atlantic, with a cutoff near a line along I-66/I-95 
into southern NJ. WPC probabilities for at least 6 inches of snow 
are highest over southeastern VA and northeastern NC, squarely 
through the Norfolk area.

Like places farther west, the >0C air aloft will rise up and over 
the cold surface temperatures over eastern NC, setting up a 
freezing rain event that may be significant along and east of I-95
between the VA border and into northeastern SC. There, WPC 
probabilities for at least 0.10" icing are >50% with some areas 
receiving more than 0.25" accretion. 


...Southeastern Mass...
Day 3...

The exiting system from the Southeast/southern Mid-Atlantic will 
track northeastward, and may clip southeastern MA/Cape Cod D3 on 
its way into Atlantic Canada. The question revolves around the 
speed of the incoming northern stream upper low and how much it can
capture the exiting system. Given the uncertainty leading into the
event in the Southeast/Mid-Atlantic, there are still a wide range 
of solutions from zero to moderate snow over part of the region. 
For now, there are low (10-40%) probabilities of at least 4 inches 
of snow over Cape Cod and the Islands. 


Fracasso


...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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