Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 315 PM EST Sun Feb 16 2025 Valid 12Z Sun Feb 16 2025 - 12Z Wed Feb 19 2025 ...Great Lakes/Northeast... Days 1-2... A dynamic winter storm is unfolding today with heavy snow across northern Maine, while a protruding warm nose of >0C air at low levels causes precipitation to fall in the form of sleet and/or freezing rain from central Maine and the northern Appalachians to the Maine coast. WPC's Snowband Probability Tracker shows the HREF guidance suite has members that are producing as much as 1-2"/hr snowfall rates across northern Maine through this evening. Heavy snow is expected in parts of NY's North Country this evening, and on the western flank of the storm as it tracks into Maine tonight. WPC probabilities show high chances (>70%) for snowfall >8" in the North Woods, Central Aroostook, and St. John Valley, with the latter of the three having high chances (>70%) for over a foot of snow. A couple additional inches of snow are possible in the Adirondacks and the rest of VT as the storm pulls away. Meanwhile, soundings will be more supportive of freezing rain and sleet farther south with another tenth to a quarter inch of ice expected from southern and central NH/VT on east through the southern tier of Maine. Strong wind gusts topping 50 mph could add further stress to trees and power lines with heavy/wet snow or ice, keeping the potential for additional power outages and tree damage as a potential impact through Monday morning. On the backside of the winter storm, much colder air races across the Great Lakes and into the central Appalachians, giving rise to lake effect snow bands and upslope snow in these regions. For the Potomac Highlands of eastern WV on north to the Laurel Highlands, upslope flow will generate locally heavy snow totals with WPC probabilities sporting moderate-to-high chances (50-70%) for >6" in these mountain ranges. Snow will taper off in the central Appalachians by midday Monday, while the lake effect snow machine looks to keep producing bands of heavy snow through Tuesday as an upper low heads for the eastern Canadian Maritime and an elongated 500mb trough axis moves in overhead. Low level flow backs from NW to WNW and favor the typical lake belts, but most notably over the eastern U.P. of Michigan and southeast of Lake Ontario. WPC probabilities for at least 8" of snow over the next few days are high (>70%) and are moderate-to-high (50-70%) for 18 inches of snow downwind of Lake Ontario. ...Pacific Northwest/Rockies/Northern High Plains... Days 1-3... An active Pacific jet will direct a storm system and its associated IVT to the south of the storm into the Northwestern U.S. through Monday, followed by another system by Tuesday into Wednesday. Snow levels will dip just below pass level and oscillate there through Monday night. 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. For the first system, Sun-Mon (though into Tue over WY), the highest snow totals will be over the Cascades eastward into the central Idaho ranges, western MT/WY and into northern CO/southern WY. Several feet of snow are likely in the higher terrain with only a brief break between systems. IVT values exceeding the 90th percentile will move through the region today, but then drop after tonight as the jet flattens out. Nevertheless, the nearly continuous terrain- enhanced snow will accumulate well over 1-2 ft with WPC probs > 70% in the mountains. Snow will be fairly heavy over western-central MT on the upslope side as well, where WPC probs for at least 8" of snow are >50% around Billings up to Great Falls. The next system will enter the Pacific Northwest starting Tuesday Night with some periods of snow for the Cascades. Snow will then extend into the Northern Rockies throughout the day on Wednesday. WPC probabilities shows moderate-to-high chances (50-70%) for >6" of snowfall for the Cascades, Blue, and Sawtooth mountain ranges. ...Central Plains/Mid-MS Valley/Mid-South... Days 1-3... 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 moderate snow from the Black Hills on south and eats through NE and into the Lower Missouri River Valley tonight and into Monday. This band eventually weakens to where mainly light snow is expected in these areas through Monday afternoon. By Monday night, 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. By early Tuesday morning, the aforementioned jet streak will co-locate its divergent left-exit region with the divergent right-entrance region of a strengthening jet streak over the eastern U.S.. As southerly and moist 850mb winds increase over Texas and low-level WAA results in 850mb FGEN from the OK/TX Panhandles to as far east as the Ozarks and Ouachita Mountains, periods of heavy snow will envelope these regions with deteriorating travel conditions throughout the day. There are still some different scenarios on guidance as to where and how long the heaviest bands of snow take shape. WPC probabilities are keying in on southeast KS, southern MO, and as far east as the Mississippi River bordering southeast MO and western KY as most likely to witness heavy snow where there are high chances (>70%) for snowfall amounts topping 6". In fact, there are low-to-moderate chances (30-50%) in southern MO for >12" of snowfall. Farther south across northern OK, northern AR, and northwest TN, a disruptive blend of snow and ice would make for treacherous travel conditions Tuesday and continuing into Tuesday night. As the storm tracks east into the Mid-South on Wednesday, heavy snow will blanket areas from the OH-MS River Confluence on east into the Ohio Valley. WPC probabilities do depict moderate chances (40-60%) for snowfall totals >4" in these areas. For ice accumulations, the Ouachita Mountains sport low-to-moderate chances (30-50%) for ice accumulations >0.1" through Wednesday morning. WPC's WSSI-P shows low-to-moderate chances (30-50%) for Major Impacts (dangerous or impossible travel conditions; widespread closures and disruptions) for northeast OK, southeast KS, southern MO, and far northern AR. As high as 60% chances exist for Moderate Impacts (hazardous driving conditions; some closures and disruptions) stretch as far west as Wichita and the I-35 corridor on east to northern TN and western KY. ...Central Appalachians/Mid-Atlantic... Day 3... As the upper-level trough driving the Mid-South storm system tracks east on Wednesday, modest PVA aloft combined with 850-700mb WAA will direct the shield of heavy snow in the Mid-South towards the Appalachians and Mid-Atlantic. What guidance largely agrees upon is the southern stream storm system tracking ENE towards the Southeast coast Wednesday afternoon and steadily strengthening beneath the divergent right-entrance region of a strengthening 140kt 250mb jet streak to the north. There is an anomalous IVT topping 500 kg/m/s over the Southeast that will help direct plenty of moisture into the southern Mid-Atlantic. As 850-700mb FGEN occurs over southern VA, bands of heavy snow should take shape and lead to hazardous travel conditions from southwest VA and the central Appalachians of WV and western VA to the VA Tidewater Wednesday afternoon. Probabilistic guidance is suggesting an area from eastern KY and the WV/western VA Appalachians to the DelMarVa Peninsula as having the best odds of seeing heavy snow. WPC probabilities show a growing area of moderate-to-high chance probabilities (50-70%) for snowfall >4" in southwest VA through Wednesday evening, with heavy snow spreading across southern VA and the DelMarVa Peninsula by Wednesday evening. With the storm also deepening as it reaches the coast Wednesday night, blowing snow and heavy snowfall rates of 1-2"/hr would cause near whiteout conditions, especially for the DelMarVa Peninsula. There is also the concern for significant icing as WPC probabilities show moderate-to-high chances (50-70%) for >0.01" of ice over central NC and north-central SC. These areas would be most at risk for treacherous driving conditions on Wednesday. However, 500mb skewness plots continue to show some subtle difference in the depth/strength of the large upper low over the Midwest and how it interacts with the lead shortwave trough in the Southeast. Farther southeast, while not as pronounced there are also lingering discrepancies in the depth/speed of the lead trough in the Southeast as well. While most ensemble guidance has started to come into focus with southern VA and the DelMarVa Peninsula being most favored for heavy snow, a deeper/more phased upper- level pattern would allow for heavy snow to creep northward up I-95 and get closer to the major metro centers of Washington D.C. and Baltimore. There are also some solutions that show the lead shortwave could be flatter as well, and make northern NC and far southeast VA the focus for heavy snow. Residents in the Mid- Atlantic should keep a close eye on the forecast as any subtle adjustment in speed/timing/phasing of these atmospheric variables could mean 50-100 mile shifts in where the heavy snowfall axis sets up. Mullinax ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png