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/LatestKeyMessage_2.png