Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 334 AM EST Sat Jan 18 2025 Valid 12Z Sat Jan 18 2025 - 12Z Tue Jan 21 2025 ...Great Lakes... Days 1-3... Strong low pressure tracking through southeast Canada will escort a strong Arctic front through the Great Lakes today that will revitalize the LES machine throughout the region today. Strong NWrly flow with exceptional CAA will trigger LES bands from the U.P. of Michigan on south along the western-most portions of Michigan's Mitten. Farther east, a slug of moisture aloft will run parallel to the Arctic front as it races east today with the Chautauqua Ridge and the Tug Hill the more favored locations for locally heavy snow beginning this morning and concluding by Saturday evening. WPC probabilities through 12Z Sunday show moderate-to-high chances (50-70%) for snowfall totals >4" in the Tug Hill while there are low-to-moderate chances (30-50%) for >4" snowfall amounts from northeast Ohio to along the Chautauqua Ridge. Portions of the eastern Michigan U.P. and the Porcupines also sport moderate chances (40-60%) for snowfall amount >4". As winds accelerate over the Great Lakes on Sunday and inverted troughing ensue over Lake Superior, LES bands will increase in both coverage and intensity in the usual snow belts of the Michigan U.P. and the northern most portions of Michigan's Mitten. Including Sunday and Monday, WPC probabilities depicted high chances (>70%) for snowfall > than 8" along the eastern most areas of Michigan's U.P. and just north of the Traverse City area. Single-banded LES look to emerge off of Lakes Erie and Ontario Monday evening and generate heavy snowfall rates of 1-2"/hr through the day on Tuesday. ...Colorado Rockies and High Plains... Day 1... Mountain snow will continue over the eastern slopes and Front Range of the Colorado and New Mexico Rockies, including the adjacent high plains along the I-25 corridor today. The shortwave trough tracking south through the Intermountain West this morning will generate synoptic-scale lift over Colorado and northern New Mexico which will also have low-level, post-frontal upslope NErly flow with a 1050mb dome of high pressure moving over Montana on Saturday. Snowfall rates around 1"/hr are possible along the Raton Mesa early Saturday and will work their way south along the spine of the Sangre De Cristo Saturday afternoon. Saturday's snow probs for >4" are high (60-80%) over the Front Range from Pikes Peak on south to the Raton Mesa, while areas along I-25 have low-to-moderate chances (20-40%) for >4" of snowfall. ...Central Appalachians, Mid-Atlantic and Northeast... Days 1-3... ...Significant winter storm to produce a swath of heavy snow from the Central Appalachians and northern Mid-Atlantic to the Northeast Sunday and Monday... As model guidance continues to come into better agreement on the overall synoptic-scale setup, it is becoming clearer that a significant winter storm is likely to produce heavy snow snow from as far south and west as western Kentucky to as far north as northern Maine. As the Arctic front approaches Saturday night, the front will be draped over the Mid-Atlantic while at the same time, strong upper-level divergence ensues aloft courtesy of a ~150kt 250mb jet streak's diffluent left-exit region is in place. As 850-700mb WAA increases over the Mid-Atlantic, it will clash with the approaching Arctic air-mass to the north, resulting in a band of heavy snow that initially starts out over the Laurel Highlands and Allegheny Highlands early Sunday morning, then spreads over northern MD and southern PA later on that morning. As the 850-700mb FGEN increases throughout the day, snowfall rates will increase across eastern PA, northern NJ, and the Lower Hudson Valley through Sunday evening. 00Z HREF does suggest there are low-to-moderate chances (20-50%) for 1"/hr snowfall rates during the day on Sunday. By 00Z Monday, the 850mb low will track over Long Island and head for the New England coast with the axis of heavy snow residing along the western flank of the 850mb circulation. Heavy snow will unfold across portions of coastal New England Sunday night where snowfall rates >1"/hr are possible. The storm races northeast past Nova Scotia by Monday morning with lingering periods of snow throughout parts of Maine gradually tapering off by midday Monday. Latest WPC guidance depicts a swatch of moderate-to-high chances (50-70%) for snowfall >4" from central WV on north and east through central PA. From the Poconos on north and east through the Lower Hudson Valley, the Berkshires, and into both the Worcester Hills and White Mountains, there are moderate-to-high odds (50-70%) for snowfall >6", as well as moderate chances (4-60%) for snowfall >8". Note that WPC probabilities for >12" are generally <10%, indicating most areas are unlikely to receive a foot of snowfall, although localized amounts in the Allegheny Highlands and parts of interior New England cannot fully be ruled out. Latest WSSI shows a healthy swath of Moderate Impacts from northern MD through the Lower Susquehanna Valley, the Delaware Valley, the Tri-State area, and into portions of southern New England. Hazardous driving conditions are anticipated in these areas on Sunday. With bitterly cold temperatures spilling in for the upcoming work-week, snow and instances of re-freezing/black ice are likely to stick around for much of the week in wake of this winter storm. ...Southern Plains & Gulf Coast... ...Disruptive winter storm growing in confidence across the Southern U.S. next week... Day 3... For a winter storm to unfold from as south as the San Antonio area on east to the central Gulf Coast, the first ingredient is for an exceptionally cold air-mass to be locked in place. With the arrival of the Arctic air-mass this weekend, bone-chilling temperatures will be anchored in place by a dome of high pressure that NAEFS shows is above the 97.5 climatological percentile as of 12Z Monday over the Lower MS Valley. By 00Z Tuesday, 850mb temperatures remain below the 2.5 climatological percentile and subfreezing 850mb temps protruding as far south as the Gulf Coast. Meanwhile, a positively tilted 500mb trough tracking through the Southwest will work in tandem with the subtropical jet to increase upper-level divergence over Texas Monday afternoon. In addition, SWrly 850mb flow will deliver low-level moisture northward while at the same time, the 850mb WAA over South Texas will overrun the Arctic air-mass in place over Texas. This will result in the beginning of an icy wintry mix over south-central Texas and eventually spread east towards the Upper Texas coast and Lower Mississippi Valley by early Tuesday morning. WPC probabilities show moderate chances (40-60%) for >0.01" of ice accumulations in south-central Texas through 12Z Tuesday, which would result in slick and dangerous road conditions for the Tuesday morning commute. Farther north, WPC probabilities show low-chance odds (10-30%) for snowfall >2" in parts of eastern Texas and western Louisiana. It is worth noting that the scope of this discussion goes through 12Z Tuesday, so expect snowfall probabilities to increase over the next 24 hours. Please be sure to visit WPC's Medium Range discussion and our Key Messages for more details on the potential impacts throughout the South later this week. 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