FOUS11 KWBC 170823
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 AM EST Mon Feb 17 2025

Valid 12Z Mon Feb 17 2025 - 12Z Thu Feb 20 2025

...Great Lakes... Days 1-2...

Behind the exiting winter storm, cyclonic flow will remain over 
the Great Lakes with 805mb temps < -20C. NW to WNW flow will favor 
the typical MI belt regions but with an emphasis over the eastern 
U.P. due to persistent light to moderate snow and especially 
downwind of Lake Ontario. There, an intense single band may set up 
today into Oswego and just north of Syracuse with some wavering N-S
into Tuesday. Two-day totals could exceed 18" (WPC probs > 50%) 
with intense snowfall rates of 2-3"/hr. Strong pressure gradient in
place will keep winds elevated as well, creating blowing and 
drifting of snow with especially hazardous whiteout conditions 
within the band. Snow will gradually wind down into day 3.

...Pacific Northwest/Rockies/Northern High Plains... Days 1-2...

An active Pacific jet will maintain unsettled weather into the 
Northwestern U.S. for the next 24 hours or so over the Cascades 
into the northern Rockies. Height falls will continue moving inland
this morning with modest snows following along out of the Cascades
and into the Bitterroots/western MT this evening. East of the 
Continental Divide a very cold air mass will be in place thanks to 
high pressure over Canada that pushes southward through the Plains,
setting up upslope flow into western MT. Snow totals will be light
to modest in the higher elevations where WPC probabilities for at 
least 8 inches of snow are >70%. Over central MT, light snow due 
to upslope/easterly flow is expected from Billings to Kalispell but
WPC probabilities for an additional 4 inches are low (<40%) in the
lower elevations. As the height falls move into the Plains, snow 
will gradually diminish on Tuesday. 

Day 3...

The next system will enter the Pacific Northwest starting Tuesday 
Night with some periods of snow for the Cascades, including into 
the passes. Moisture plume will aim into Oregon but then move 
quickly inland, helping to spread snow into the Northern Rockies 
throughout the day on Wednesday (especially into the Blue Mountains
and into the central Idaho ranges). WPC probabilities for >6" of 
snowfall are high (>70%) in these areas.

...Central Plains/Mid-MS Valley/Mid-South... Days 1-2...

The anomalous IVT responsible for periods of heavy snow in the 
Rockies will accompany a period of 700mb WAA over the Central 
Plains. The combination of 700mb FGEN and an accelerating 250mb jet
aloft will support a stripe of light/moderate snow from the Black 
Hills southeastward through NE and into the Lower Missouri River 
Valley today, weakening later today. Then tonight, the stronger 
500mb shortwave trough tracking across the Central Rockies will 
gradually deepen while upper-level diffluence increases at the nose
of a 250mb jet streak rounding the base of the Central Rockies 
trough, setting up another area of snow from NW to SE. By early 
Tuesday morning, KS will be within the entrance region of the upper
jet which will help promote an area of snow across central KS 
where lower-level FGEN is maximized. Temperatures will be cold 
behind an Arctic front, which should allow for SLRs ~15:1 
especially in healthier bands. This area of snow will move 
southeastward and increase in intensity into southeastern KS and 
southern MO late Tue afternoon/evening where the 00Z CAM guidance 
shows the potential for 1-1.5"/hr rates (per WPC snowband tool). By
early Wednesday the snow may diminish in intensity into KY as the 
upper jet reorients itself and shifts focus to the Gulf coast, 
setting up the coastal part of this event farther east. WPC 
probabilities of at least 4 inches of snow are high (>70%) from 
central KS eastward into MO and extreme southern IL. Within this 
region, WPC probabilities for at least 8 inches of snow are 
moderate (40-70%), especially over southern MO.

To the south of this swath of snow, cold surface temperatures 
beneath >0C air aloft will promote an area of sleet and freezing 
rain over central OK eastward into AR. A few hundredths of an inch 
of icing is possible, especially into the Ouachita Mountains.

...Central/Southern Appalachians/Mid-Atlantic... Day 3...

The next phase of the system moving out of the Mid-MS Valley has 
been in quite a bit of flux. Question revolves around the timing 
and amount of interaction between the shortwave responsible for the
snow across KY/northern TN and the lead height falls ahead of the 
upper low over the western Great Lakes/Corn Belt. A surface wave 
should develop near the Gulf coast by early Wed and track eastward 
then east-northeastward just off the Southeast coast to become the 
new focus for QPF. Models have been wavering quite a bit on the 
amount of development and how close to the coast this may be, 
influencing how much QPF to surge into the southern Mid-Atlantic. 
Overall trend continues to be a flatter and drier system and have 
trended that way tonight. 

Nevertheless, there is confidence in the wave development and at 
least light to moderate QPF that will spread into the Southeast and
southern Mid-Atlantic from the system. The result will be a swath 
of snow over southern and southeastern VA and northern NC with an 
area of sleet and potentially significant freezing rain to the 
south over eastern NC where QPF will be plentiful. WPC 
probabilities for at least 4 inches of snow are >50% across
southern/southeastern VA. Over NC, where the cold air in place 
behind the departing system Sunday will be overrun with >0C air 
aloft, the icing potential is significant. WPC probabilities for at
least 0.25" ice are at least 30% but may rise in future updates. 

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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