Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 303 AM EST Fri Dec 20 2024 Valid 12Z Fri Dec 20 2024 - 12Z Mon Dec 23 2024 ...Great Lakes... Day 1... The gradually weakening clipper system will track across the Lower Great Lakes this morning with lingering periods of snow across much of the region, although most totals will be on the light side. As the storm tracks east towards the Mid-Atlantic, northerly winds will accelerate over Lakes Superior, Michigan, and Huron and lead to a handful of lake-effect streamers containing occasional bursts of heavy snow on Friday. Latest WPC probabilities show low chances (10-20%) for snowfall totals >4" in parts of northwest Indiana, near Traverse City, MI, and along the coastal areas of Michigan's thumb. Otherwise, additional snowfall totals will generally hover around 1-3" in parts of Michigan, Indiana, and western Ohio. ...Central Appalachians to Northeast.... Days 1-2... As the remnant circulation of the weakening clipper system approaches the central Appalachians, its residual moisture will flow directly into the mountain range Friday evening. By Friday night, as low pressure off the East Coast deepens and high pressure builds in from the north, low-level NW winds will be favorably oriented into the Central Appalachians with some periods of snow as far south as the Smokeys. Upslope ascent will continue into the central Appalachians through Friday night and gradually taper off by Saturday afternoon. Snowfall totals in east-central WV will likely range between 1-4" with localized totals up to 6" possible in the tallest peaks of the WV Appalachians and the Smokeys. Farther east, an area of low pressure will organize along a strengthening frontal boundary off the East Coast this morning. This low will strengthen as low-level WAA to its north and a jet streak couplet takes shape over the Gulf Stream. Meanwhile, broad upper level divergence out ahead of the 250-500mb mean trough axis will supply sufficient ascent aloft in tandem with the residual moisture from the clipper system to support periods of snow over northern PA and western NY. Colder temperatures aloft supporting higher SLRs will also support As the low off the East Coast deepens, easterly flow is expected to strengthen and a low level convergence trough will setup over the northern Mid-Atlantic coast. Latest CAMs guidance is keying on this convergence boundary setting up over the Delaware Valley and stretching north into the Poconos. WPC probabilities do show moderate chances (40-60%) for snowfall >4" in parts of the Poconos, while there are low-to- moderate chances (30-50%) for snowfall >2" in parts of the Lehigh Valley, Delaware Valley, and into northern New Jersey. Localized amounts in the highest elevations of the Poconos have low chances (10-20%) for snowfall amounts topping 6". There is a chance for minor accumulations (coating-2") along I-95 from the Philadelphia metro on north through the NYC and Tri-State metro regions this afternoon Coastal New England sports the higher "boom" scenarios given the combination of ocean-enhanced snowfall over eastern MA Friday afternoon and the deformation axis of the coastal storm extending far enough west to produce heavy snow over Downeast Maine Friday night into early morning Saturday. Latest guidance is starting to come into better agreement with a swath of 1-3" of snowfall over eastern MA with some CAMs suggesting higher end totals (>4") within the range of possibilities. Downeast Maine could feature the heaviest snowfall for the event in coastal New England with WPC probabilities showing moderate-to-high chances (50-70%) for snowfall >4" through Saturday morning. ...Pacific Northwest... Days 2-3... A pair of Pacific storm systems will bring rounds of high elevation mountain snow to the Olympics, Cascades, Blue, and Sawtooth Mountains this weekend. Snow levels will generally be as low as 4,000ft in the Cascades, but the heavier snowfall totals (>6") will likely be confined to elevations >5,000ft in the Olympics and Cascades. In the Blue and Sawtooth Mountains, >6" snowfall totals will likely be confined to elevations >6,000ft. These heavier totals will generally be above pass level in these mountain ranges, so expect the more hazardous travel impacts to be in the more remote and complex terrain of these mountain ranges this weekend. The probabilities for significant ice accumulations are less than 10 percent. Mullinax