Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 245 PM EST Mon Dec 2 2024 Valid 00Z Tue Dec 03 2024 - 00Z Fri Dec 06 2024 ..Great Lakes... Day 1... Reinforcing shortwave trough axis over the Ohio Valley this afternoon will continue to round the longwave trough axis over the Northeast by tracking near the VA/NC line tonight before moving offshore. Flow will continue to be NNWly over the Lakes tonight before shifting more westerly on Tuesday ahead of the next approaching shortwave. Michigan Lake Effect... NNWly 850mb flow up to 20 kts over Lakes Superior and Michigan will continue to support locally heavy snow over the eastern U.P. and along the western shore of the L.P. into far northern IN. Day 1 PWPF for >4" is 40-70% around Grand Traverse Bay and far SW MI to the IN border. Eastern Lake Effect... NWly flow will continue to produce heavy LES over the Chautauqua Ridge from fetch over Lake Huron and then over Lake Erie. Day 1 PWPF for at least 6" is 60-80% over that portion of extreme western NY with probabilities dropping off toward Erie PA. Banding off Lake Ontario is also expected to continue for the Syracuse area into tonight with an additional 4" possible. Day 2... High pressure currently centered over ND will shift down the Mid- Mississippi Valley Tuesday with a more amplified ridge extending north over the Great Lakes. Behind this ridge axis, flow over Lakes Michigan and Superior quickly shifts to the SW which ends the LES bands from overnight. However, as this new fetch saturates, warm air advection induced snow develops across the U.P. Tuesday night, particularly as a surface low ahead of the next wave approaches Lake Superior from the NW late early D2 and pivots east Wednesday. The air remains cold enough for SWly flow driven lake enhanced snow into the far eastern U.P. Day 2 PWPF for >6" is high (>70%) for the far eastern U.P. and medium (40-60%) for the Keweenaw Peninsula. The ridge axis does not shift over the eastern Great Lakes until early Wednesday morning, so LES will continue and essentially shift back toward a single- band appearance on SWly flow, though the warm air advection makes for a wetter snow than recent days. Day 2 PWPF for >4" is 20-50% around the Chautauqua Ridge with low probs for the south towns of Buffalo. Higher probs (50-70%) are found near the Tug Hill Plateau. Day 3... The low over Ontario further develops and sends a strong cold front over the Great Lakes Wednesday into Thursday. 850mb temps quickly drop into the DGZ behind the front, so LES resumes with a vengeance on NWly flow through the day over Michigan and that night/Thursday morning for the eastern Great Lakes. Snow-liquid ratios would normally rise to around 20:1 or higher, but very strong 850mb winds of 40-55 kts could fracture dendrites and lead to areas of blowing snow. Day 2-3 PWPF >6" is >80% for NWly snow belts across the northern U.P. and the northern L.P. Marginal thermals for snow and warm air advection limit eastern Great Lakes snow Wednesday, but quickly shifts snowy overnight. Day 3 PWPF for >6" is around 60-80% off Lake Erie and prolonged moderate snow over the Tug Hill makes for >90% probs there. Additionally, snow squalls are likely along and just behind a strong cold front crossing the Upper Great Lakes on Wednesday and into the L.P. of Michigan by Wednesday night as 3-6 km AGL lapse rates rise to around 7 C/km underneath the arctic airmass.This could lead to intense multi- band convective squalls enhanced by flow off Lake Superior and Lake Michigan Wednesday evening. Potential also exists for snow squalls across the northern Mid- Atlantic and Northeast by Thursday, but timing and mode is more uncertain. Either way, gusty winds with maximum values exceeding 40 mph are likely to be widespread and persistent between the northern Plains and central Apps/Northeast, which could lead to lowering visibility where snow is falling and where fresh snow cover exists. LES looks to continue over the eastern Great Lakes on the NWly flow into Friday. ...New England... Day 3... The upper low tracks across northern Lake Huron Wednesday night with strong warm air advection and increasing precip into an initially cold airmass over New England allowing moderate to locally heavy snow over the terrain of the Greens/Whites/and much of Maine. The low crosses ME on Thursday when there is a greater snow threat for interior New England. Day 3 PWPF for >6" 30-60% for the higher terrain of VT/NH, with a corridor of 60-80% extending from the central highlands to northern portions of ME. The low crosses ME on Thursday when there is a greater snow threat for interior New England. ...Southern Appalachians... Day 1... A clipper-type trough currently over the Ohio Valley shifts to eastern TN by this evening. Upslope flow enhances precip which is cold enough for snow and SLRs around 17-18:1 to produce 30-60% probs for >4" snow tonight for mainly the western slopes of the Great Smokey Mtns. Enough lift directly beneath the shortwave could also lead to minor snowfall accumulations (<1") Tuesday morning across central NC/SC. The probability of significant ice across the CONUS is less than 10 percent. Snell/Jackson