Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 253 PM EST Tue Feb 18 2025 Valid 00Z Wed Feb 19 2025 - 00Z Sat Feb 22 2025 ...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. See the latest Key Messages linked at the end of the discussion. Current radar/sat composite is pretty revealing this afternoon with the development of a significant swath of precip developing in-of the Central Plains into the adjacent Lower to Mid-Mississippi Valley. Shortwave trough ejecting out of the Central Rockies will continue downstream with general mid-level ascent pattern maximized within the corridor extending from southeast KS to points east within Southern MO/Northern AR with a heavy snow footprint forecast to align within the above corridor. WPC prob snowbands at the 12z update was still pretty aggressive for a several hr time frame of 1-2"/hr rates embedded within the primary precip shield moving across the Ozarks until about the three-river confluence at the MO/TN/AR border. Heavy snows over the span of several hours will produce total accumulations generally >8" across that same expanse from KS through northern AR and southern MO through Wednesday morning. High probs (50-80%) for >4" exists just for the 00-12z Wednesday period across southern MO, northern AR, and far southwestern KY with lower probs (25-50%) extending further east through the Ohio Valley for the remainder of the period. Further south into Central AR down through northern LA, dominant ptype will continue to trend more favorable for IP/ZR as a shallow warm layer is progged due to the meridional push of Gulf air advecting over top the shallow cold air mass being pushed south on the southern flank of the Arctic high drilling south over the course of today and tomorrow. Thankfully the ascent pattern is strongest further north, so ice accums are not anticipated to be severe, but still relatively impactful with probs of ice >0.1" holding between 10-30% across the AR/LA border over the course of the forecast period. Ice probs decrease downstream for D2, but still maintain some low probs for between 0.01-0.1" ice accums across northern MS/AL/GA for mainly the 06-14z time frame on Wednesday before precip moves out of the area the second half of the D2 period. By early Wednesday morning, increased jet dynamics over the Southeast and Mid-Atlantic will aid in generating a surface low along the Central Gulf coast, propagating east-northeast over the course of Wednesday with a surface low located off the NC coast by early Thursday morning. The primary disturbance to the west will still motion across the Ohio Valley into the mountains of the central and southern Mid-Atlantic leading to moderate snow accumulations on D2 across eastern KY/southern WV/southwestern VA leading to probs around 30-50% for at least 4" of snowfall in those locales during the 12z Wednesday to 12z Thursday period falling in-between both D1 and D2. Further east into the coastal southern Mid-Atlantic, our aforementioned surface cyclone over the Gulf into the Western Atlantic will lead to the greatest QPF footprint for the D2 period with heavy synoptic based precipitation over places like the Tidewater of VA down into northeastern NC, mainly in the form of snow. Moderate precip focus will align within the NC Piedmont across into southeastern NC with a mixed ptype regime likely to cause some formidable ice accums in the form of sleet (IP) and freezing rain (ZR). The area across the VA Tidewater will be the epicenter for the most significant snowfall forecast with totals between 6-12" expected for areas like Norfolk up through the U13 corridor between VA Beach/Chincoteague, well-documented with the latest >4" probs a solid 80+% within that entire zone and >8" probs up into the 40-60% range. Modest probs for >4" also extend south into northeastern NC with a decent area of 40-70% probs existing from Elizabeth City and points northwest. Freezing rain prospects exist across a good portion of eastern NC and the northern Piedmont of SC with the heaviest totals likely to fall over areas along and east of I-95. WPC probabilities between 50-80% are forecast for >0.1" ice accretion across portions of eastern NC with low probs (10-30%) for the northern most portion of the SC Piedmont. 10-30% probs also exist for as much as 0.25" of ice accretion over eastern NC as well, although the deterrent for the threat is the heaviest precip falling within marginally cold enough air at the surface (30-32F) with periods of heavier precip falling during the day time which historically curbs ice accretion rates, especially at these latitudes. Kleebauer ...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 unwind, 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 heavy lake effect snow, but then those height falls will bring in a general/light snow to the region on Thursday and Friday. WPC probabilities for at least 8 inches of snow only exist on Day 1 (Wednesday) parts of the Michigan U.P. and off southern Lake Ontario. For central NY between ROC- FZY- SYR, the strong single band will steal the show and continue to dump snow at 1-3"/hr rates with significant accumulation in a narrow swath -- perhaps more than 12 inches of additional snow (10-20% probs). By days 2 and 3, snowfall will be light with most areas seeing less than 4 inches of snow. The greatest potential for additional snowfall over 4 inches (20-50%) is forecast off Lake Michigan into western L.P. and far northern Indiana on Thursday, positioned along the eastern and northern periphery of the upper low. ...Pacific Northwest through the Central Rockies... Days 1-3... Initially for tonight, additional height falls out of Canada will drop through central MT with some light snow to the central ID ranges, southwestern MT, and the western WY ranges where WPC probabilities for an additional 6 inches of snow are low (<40%) to the north but high (>70%) over southern WY and the high peaks of the northern CO Rockies. By early Wednesday, a potent but progressive 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 the end of D2 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. The next system begins to enter the Northwest by the end of D3 with lighter snow entering the northern Cascades. ...Southeastern Mass... Day 2-3... The exiting system from the Southeast/southern Mid-Atlantic will track northeastward, and may clip southeastern MA/Cape Cod on Thursday 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 and pump up the downstream ridge. Recent 12z guidance has nudged northward, which is common in the short range with developing low pressure systems off the coast as CAMs incorporate latent heat release and stronger WAA in relation to convection in the warm sector of the system. However, the best chances for snowfall over 4 inches remains over Nantucket (70-80%) as well as both Martha's Vineyard and Cape Cod (40-60%), with low probs (10-20%) clipping far eastern Long Island, southern CT and the remainder of southeast MA. Fracasso/Snell ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png