Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 404 PM EST Sun Jan 19 2025 Valid 00Z Mon Jan 20 2025 - 00Z Thu Jan 23 2025 ...Great Lakes... Days 1-3... Broad cyclonic flow encompassing much of the eastern CONUS will be reinforced by dual shortwaves - one moving east of the Great Lakes to start the period, and a second more amplified wave Tuesday night. The overall flow pattern is reflective of a true mid-winter airmass with cross-polar flow surging extremely cold air into the eastern 2/3 of the CONUS, with 850mb temps progged to fall from around -15C to as low as -30C (falling to below the 1st percentile within the CFSR climatology). This will create persistent CAA favorable for lake effect snow (LES). Notably, the water temperatures across the Great Lakes have cooled considerably thanks to a cold January, and ice is now evident in satellite imagery and as analyzed by GLERL, which may somewhat limit the intensity of LES. However, with such an extremely cold airmass in place, and surges of enhanced ascent associated with the aforementioned shortwaves, inversion depths to 10,000 ft and lake- induced instability of 500-1000 J/kg will still support periods of intense LES, especially downstream of Lakes Erie and Ontario, with 2"/hr rates or more possible. WPC probabilities D1 are high (70%+) for 4+ inches of snow across the Western U.P, and northern L.P. of MI, as well as downstream of Lakes Erie and Ontario. The heaviest LES commences D2 downstream of Lakes Erie and Ontario where WPC probabilities for 8+ inches reach above 50%, although upstream snowfall across MI may lessen in coverage and intensity. Another surge of CAA will enhance LES across the L.P. of MI once again D3, but the heaviest snow will continue into the Tug Hill Plateau where event-total snowfall of 2-4 feet is forecast. ...South-Central Plains... Day 1... A shortwave digging through the broad cyclonic flow encompassing much of the eastern 2/3 of the CONUS will dig E/SE from the Great Basin as a southern stream impulse. This will eventually interact with a northern stream shortwave to drive a full-latitude trough development, and while it remains positively tilted, it will drive downstream jet intensification, with the resulting overlap of PVA, jet level diffluence, and mid-level isentropic ascent causing an expansion of snowfall Monday afternoon through Monday night. Total ascent is modest and the best ascent does not seem to overlap with the DGZ, but an extremely cold column should still result in efficient/fluffy SLR where snowfall occurs. This will cause some modest snowfall accumulations for the TX/OK Panhandles and points east into OK/KS, where WPC probabilities are as high as 30-50% for 2 inches of snow. ...Mid-Atlantic and Northeast... Day 1... Fast moving low pressure will bring significant snow to the Mid- Atlantic and Northeast through Monday as it tracks northeast just offshore the coast. This low is being driven by an overlap of a negatively tilted shortwave trough currently emerging from the TN VLY, with its track likely to take it over the Washington, D.C. area before lifting over Cape Cod and Downeast Maine overnight. A potent 150+ kt jet streak arcing poleward will overlap with this shortwave, resulting in a favorable environment for low pressure strengthening just offshore. The resultant surface wave will deepen at a moderate pace, but lift rapidly northeast, exiting into Canada by late Monday morning. Despite the rapid pace of this system, a swath of heavy snow is likely just inland from the coast, with the heaviest snowfall accumulations likely just NW of I-95 from Washington, D.C. to Boston, MA. The primary driver of this heavy snow will be an overlap of enhanced moisture rotating around the system on the accompanying isentropic ascent, and a corridor of strengthening 700-600mb fgen. While the forecast soundings do not scream intense snowfall due to very dry air above the DGZ resulting in modest available PWs, the most intense fgen does appear to overlap efficiently with the DGZ suggesting potential for banded snow structures, within which the WPC prototype snowband tool indicates the potential for 1-2"/hr snowfall. The progressive nature of this system will limit overall accumulations, and precip may start as a rain/snow mix in many areas before cooling dynamically to support all snow, but WPC probabilities feature a high risk (>70%) for more than 4 inches of snow in a continuous swath from far eastern Upstate NY through much of southern and central New England from 00Z tonight through 18Z Monday. Locally, more than 8 inches is possible (30-50% chance) in Downeast Maine. With snowfall rates exceeding 1"/hr at times, this will result in hazardous travel in many areas. The progressive nature of this system will limit more substantial impacts, but heavy snow and hazardous travel impacting much of the northeast I-95 urban corridor has prompted the issuance of collaborative Key Messages linked below (KeyMessage_2). ...Texas, Gulf Coast, and Southeast... Days 2-3... ...Significant winter storm likely across portions of the Southern U.S. this week... A rare Gulf-Coast and Southeast Atlantic Coast winter storm is becoming more likely beginning Tuesday as an anomalously cold airmass settles across the eastern 2/3 of the CONUS and sets the stage for the upcoming low pressure system. Amplified troughing deepening across the east will spill cold air all the way to the Gulf Coast as reflected by ECMWF EFI for minimum temperatures falling below -0.9 on Tuesday, and expanding/intensifying on Wednesday. This cold air will be entrenched even before the low develops, and despite some variability in model solutions among the various ensemble camps, the colder ECMWF is favored at this time. As this cold air floods across the region, an intensifying shortwave will dig out of the Great Basin and then sharpen as it pivots eastward from West Texas through the Gulf Coast Tuesday into Wednesday. As this occurs, some interaction with a northern stream impulse will yield a positively tilted full latitude trough shifting east, with the resultant downstream jet streak intensifying and shifting poleward to place the favorable RRQ atop the Gulf Coast. The overlap of height falls and upper diffluence will help spawn a wave of low pressure across the Gulf of Mexico, and as 300K isentropic ascent maximizes to the north, precipitation will begin to overspread eastern Texas Tuesday morning, before becoming more expansive across the Gulf Coast and eventually coastal Southeast Tuesday night into Wednesday. The challenge with this event is where the snow/sleet/freezing rain transition will occur, and where the northern cutoff of precipitation will occur. Some very cold and dry air to the north will likely cause a sharp precip gradient, but south of there will be a swath of heavy snow driven by strong WAA, and places across LA/MS/AL that rarely seen snow could experience 1"/hr snow rates just north of the mixed transition zone. While there is still some uncertainty into the placement of the mixed precip axis, especially farther east into the Carolinas/Georgia/Florida, the guidance has trended a bit colder and farther south, which is impacting the recent WPC probabilities. At this time, WPC probabilities indicate a high chance (50-90%) for at least 2" of snow from far eastern TX through southern/central LA and into southern MS. Locally, 4-6" of snow is possible, especially where any banding sets up which could produce snowfall rates above 1"/hr. The combination of the heavy snow, very cold temperatures, and the simply rarity of this event could result in major impacts and considerable disruptions to daily life. Farther east, as the event translates downstream, WPC probabilities for 2+ inches of snow are as high as 30-50%, focused in the eastern Carolinas, however uncertainty here is higher due to latitudinal discrepancies in the axis of QPF from the available models. Additionally, southeast of the heaviest snow, especially from the Florida Panhandle east to the northern Peninsula and parts of coastal GA/SC, freezing rain is likely, which has a 30-50% chance of producing at least 0.1" of ice. This event has prompted the issuance of collaborated Key Messages (KeyMessage_1) linked below. 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 https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png