Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 310 AM EST Fri Jan 17 2025 Valid 12Z Fri Jan 17 2025 - 12Z Mon Jan 20 2025 ...Great Lakes... Days 2-3... By Friday night, a deepening area of low pressure will escort a strong Arctic front south that delivers a frigid air-mass that is the coldest and most dangerous of the season this weekend and into next week (see Key Messages linked below). The Arctic front will turn on the LES machine over Lake Superior by Friday night while periods of snow develop along the northern and western coasts of Michigan's Mitten by Saturday morning. Following an initial WAA thump of snow Saturday afternoon, LES bands look to form over parts of central and western New York Saturday night that likely persist through the remainder of the weekend. Through 12Z Monday, WPC probabilities show moderate-to-high chances (50-70%) for snowfall >4" and low chances (~10%) for snowfall >8" in the Tug Hill Plateau. The Michigan U.P. sports moderate-to-high chance probabilities (50-70%) for >8" of snow in the Porcupine Mountains and in parts of the eastern U.P.. Expect these probabilities and snowfall totals to rise as LES bands increase in coverage an intensity throughout the region as winds accelerate and the heart of the bitterly cold temperatures aloft race over the Great Lakes next week. ...Rockies and High Plains... Days 1-2... The combination of both upper level diffluence ahead of a deep- layer trough and the arrival of a bitterly cold Arctic air-mass that is destined to be the coldest air-mass of the season to infiltrate the Lower 48 will bring about Snow squalls and mountain snow to parts of these regions through Saturday. This exceptional cold front (a "blue norther") out ahead of the upper trough will push south this morning while at the same time, 500mb PVA ahead of an approaching upper trough occurs over the northern Rockies/High Plains. Snow showers will breakout across Montana this morning and make its way south throughout the day into Wyoming, then into northern Colorado by Friday afternoon. Southern Montana and into Wyoming, in particular, sport notably higher snow squall parameters given the stronger surface-based heating that will cause steeper low-level lapse rates. Snow squalls are also possible today along the cold front as it tracks east across the Dakotas Snow squalls can cause rapid reductions in visibility due to a combination of heavy snow rates and whipping wind gusts, as well as rapid accumulations on roads as temperatures plummet well below freezing. Motorists in these areas should keep a close eye on potential snow squalls as it can cause dangerous travel conditions in a matter of seconds. As the front plunges south Friday evening, the snow potential shifts south into Colorado with periods of snow beginning near the start of the evening rush hour in the Denver/Boulder metro area. While the snow squall potential may not be as high by this time, the easterly upslope-enhancement of snowfall rates will make for occasional periods of heavy snow Friday evening. Snowfall rates will weaken as the front works its way south of the Palmer Divide Friday night and down the spine of the Sangre De Cristo early Saturday morning with some snow along the Front Range lingering into the day on Saturday as a dome of ~1050mb high pressure builds in from the Canadian Prairies. WPC probabilities are keying in on the Front Range as the focus for the heaviest snowfall with moderate-to-high chances (50-70+%) for snowfall >4" from the Laramie Range on south to parts of the Sangre De Cristo. The I-25 corridor from Fort Collins on south to the Colorado Springs area Friday have similar probabilities for >4" of snowfall with even low chances (10-30%) for greater than 6" between Friday evening and Saturday morning in spots. Some of the higher peaks (>10,000ft) have moderate chances (40-60%) for snowfall >8" through Saturday morning. The WSSI is currently depicting Minor Impacts for just about all of these referenced areas, suggesting the potential for winter weather conditions that would require enhanced caution while driving in these affected areas. The one notable exception is the Denver metro area as Moderate Impacts are now depicted, indicating there are likely to be disruptions ot daily life with potential closures and hazardous driving conditions. ...Mid Atlantic and Northeast... Day 3... ...Confidence is increasing in a disruptive winter storm from the Mid-Atlantic to the Northeast with locally heavy snowfall possible along the I-95 corridor... Model diagnostics: Over the past 24-36 hours, guidance has come into better agreement on a deepening 500mb trough over the Ohio Valley and higher height rises in the Atlantic south of Bermuda. AI-based guidance (ECMWF-AIFS and GFS GraphCast) have been showing this increasing potential over this time span while ensembles (ECMWF-EPS, GEFS, GEPS) have all gradually trended snowier as well. While this is the case, deterministic guidance still shows the lingering uncertainties with the 500mb trough's amplitude and position 12Z Sunday. The 00Z GFS is slightly deeper vs the 00Z ECMWF. The 00Z CMC is similar to the ECMWF 500mb disturbance's strength, but with less confluence over the Northeast, allowing for a more storm track on the northern side of the track distribution. The UKMET also has a similar look to the Canadian, albeit with a colder boundary layer setup given stronger confluence over the Northeast. Overall, the AI-based guidance and the trends in the ensembles provide an increased confidence in a synoptic- scale setup that is boosting confidence in a winter storm from the Mid-Atlantic to the Northeast, but more specific details on storm track still need to be ironed out over the next 24-36 hours. Synoptic & mesoscale analysis: The approaching upper-level trough Saturday night will amplify over the Ohio Valley and place a strengthening 250-500mb layer averaged jet streak over the Mid- Atlantic by Sunday morning. NAEFS shows 200mb and 500mb winds over the Southeast U.S. are above the 90th climatological percentile and foster healthy upper level divergence aloft on Sunday. At lower levels, a cold front Saturday night will stall out along the Mid-Atlantic coast and into southern Virginia. The front, residing beneath the synoptically-favorable jet stream setup, will spawn an area of low pressure Sunday morning in the Carolinas and gradually strengthen as it heads for the Mid-Atlantic coast Sunday afternoon. The heaviest banding of snow is likely to reside north of 850mb frontogenesis, which will become more and more pronounced as the 850mb low forms somewhere along the Jersey Coast and tracks northeast towards Long Island and the MA Capes Sunday evening. The storm is a progressive one, but given the sufficient synoptic and mesoscale processes, the deformation zone of heavy snow would be more than capable of generating ~1"/hr snowfall rates, especially for areas from the Delaware Valley on north and east through the Tri-State area and into southern New England where they are more likely to have better mesoscale forcing as the 850mb low deepens Sunday night. Timing & snowfall/impact probabilities: As mentioned before, this is a quick hitting system that is all but gone by midday Monday as it races toward the Canadian Maritimes by Monday afternoon. Snow is forecast to be at its heaviest Sunday morning and afternoon for the DC/Baltimore metros, the Philly/NYC metros Sunday afternoon and evening, then across southern New England Sunday night into early Monday morning. Latest WPC probabilities show moderate-to- high chances (50-70%) for snowfall totals >4" from the Delaware Valley on north through the Lower Hudson Valley and across southern New England. Low-to-moderate chances (30-50%) for >4" extend as far north and east as Downeast Maine. WPC probabilities even show some spotty low chance probabilities (10-20%) for snowfall totals >8" in parts of southern New England. The DC/Baltimore metro areas sport low chance probabilities (10-30%) for >4" of snowfall at the moment, while the Philly and NYC metros are in the moderate range (40-60%) for >4" of snow. While exact amounts remain unclear, the WSSI-P Minor probabilities are >60% for the full length of I-95 from DC to Boston, indicating the heightened potential for hazardous travel conditions Sunday into Monday. WPC has initiated Key Messages for this event and can be viewed at the bottom of this discussion. 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