Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 217 PM EST Sun Dec 1 2024 Valid 00Z Mon Dec 02 2024 - 00Z Thu Dec 05 2024 ..Great Lakes... Days 1-3... *** Heavy Lake-Effect Snow continues through Tuesday *** Amplified longwave trough spinning across the eastern CONUS and centered near James Bay will slowly shift eastward Monday, with a final shortwave digging through the trough and across the Mid- Atlantic on Tuesday. This evolution will maintain lake effect snow (LES) across the region, but with bands gradually but inexorably shifting more to the S/SW rather than the W/NW which has plagued the Lakes the past few days. While this will continue CAA across the still very-warm lakes leading to bands of intense LES, the shifting direction will bring a reprieve to some of the hardest hit areas, while also resulting in generally less intense snow rates with less than ideal fetch direction. The exception to this will likely be along the Chautauqua Ridge SE of Lake Erie which has been crushed with snow the past few days, and late D1 into D2 will experience a resurgence as a nearly ideal upstream moisture connection to Lake Huron occurs. The other region that will see more intense LES, especially early D2 /Monday night into Tuesday/ will be south of Lake Michigan as the northerly fetch drives a robust and single band of LES into northern IN. These two areas will likely receive the heaviest additional snow, reflected by WPC probabilities exceeding 70% for 6+ inches both D1 (east of Lake Erie) and into D2 in far SW MI and northern IN (and continue east of Lake Erie). Locally 1-2 additional feet are possible. Downstream of the other lakes, including the U.P., western L.P., and down into the Finger Lakes region of NY, ***WPC probs...*** before shortwave ridging builds in across the region bringing an end, at least briefly, to most LES as WAA develops. Some light LES is likely early D3 during the wind shift, especially near Buffalo, NY and Watertown, NY, but only modest additional LES accumulations are expected. The ongoing lake-effect snow is the subject of the Key Message linked below. ...Upper Midwest/Upper Great Lakes... Day 3... A potent shortwave digging out of northern Saskatchewan will race southeast within the amplified trough plaguing much of the eastern CONUS. As this shortwave dives towards the Northern Plains, it will begin to amplify, pushing 500-700mb heights to as low as -2 sigma according to NAEFS, while concurrently enhancing downstream divergence as it interacts with modest shortwave ridging to the east. During this evolution, an impressive upper jet streak /up to 160kts/ will dig southward as well, with the favorable LFQ driving enhanced deep layer lift to produce cyclogenesis near Lake Superior Wednesday morning. This low will then deepen as it tracks progressively to the east, with the accompanying WAA driving a swath of precipitation from the Upper Midwest through the eastern Great Lakes. The warm and moist advection downstream of this low will surge PWs to nearly +1 sigma, and the column will be sufficiently cold that most of the precip will occur as snow. There remains some uncertainty amongst the various global models as to how far south the heavy snow will spread, but with impressive WAA (winds up to 50 kts at 800mb) deepening the isothermal layer beneath the DGZ, and favorable fgen driving ascent, periods of moderate to heavy snow are becoming more likely from northern MN through Upstate NY on Wednesday. WPC probabilities for 4+ inches are generally around 30% from the Arrowhead of MN through far western Upstate NY, with locally higher probabilities reaching 70% across the eastern U.P. and in some of the favored lake-enhanced areas near Traverse City, MI and downwind of Lake Ontario. The probability of significant ice across the CONUS is less than 10 percent. Weiss ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png