Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 233 PM EST Mon Jan 20 2025 Valid 00Z Tue Jan 21 2025 - 00Z Fri Jan 24 2025 ...Great Lakes... Days 1-2... A large cyclonic gyre encompassing much of the eastern 2/3 of the CONUS and draped from a massive upper low centered over eastern Canada will maintain CAA across the Great Lakes through Thursday, with periodic shortwave impulses rotating through the flow and overhead the region. The impressive upper low will maintain height anomalies well below normal into Thursday, with persistent CAA pushing W/NW flow atop the Great Lakes. The most impressive ascent will accompany a sharp shortwave which will cross the region from the west to east on Tuesday, driving enhanced ascent through height falls and strong PVA. This will also result in one final subsequent cold surge reflected by 850mb temps crashing to as low as -20C to -30C over the Lakes, coldest across the U.P. of MI. The continuous cyclonic flow aloft with embedded shortwaves will result in a favorable environment for rounds of lake effect snow (LES) in the favored W/NW snow belts downstream of all the lakes. Despite lake surface temperatures that have cooled dramatically the past few weeks (now generally 0C to +5C, with ice present in some areas according to GLERL), the extreme cold air funneling through will produce steep lapse rates, inversion depths towards 10,000 ft, and lake-induced instability of 500-1000 J/kg. This will support heavy bands of LES, with single bands off of Lakes Erie and Ontario supporting 2-3"/hr rates at times, and 1"/hr rates elsewhere. The heaviest and most widespread snow is likely D1 into D2 behind the potent shortwave and accompanying weakening front, and coverage/intensity is likely to wane D2 into D3. However, at least modest LES is progged to persist into the early part of D3 across the eastern lakes. 2-day snowfall probabilities from WPC ending 00Z/Thursday indicate a high chance (>70%) for more than 8 inches for portions of the NW L.P. of MI, as well as downstream of Lakes Erie and Ontario, especially into the Chautauqua Ridge and Tug Hill Plateau. The heaviest snow totals are likely in the Tug Hill, however, where WPC probabilities for more than 24 inches reach 30-50%, and locally 2-4 feet of snow is possible. ...Texas, Gulf Coast, and Southeast... Days 1-3... ...Significant, potentially historic, winter storm to spread across the Gulf Coast and portions of the Southeast through Wednesday... A rare, potentially historic, Gulf-Coast and Southeast Atlantic Coast winter storm begins tonight and spread across the region through Wednesday. The anomalous to extreme cold plaguing the eastern 2/3 of the CONUS will set the stage for this upcoming low pressure system. The primary mechanism driving this impressive event will be a shortwave digging across the Great Basin and then sharpening/intensifying as it pivots towards the Gulf Coast Tuesday morning. This will interact with a northern stream impulse to produce a positively tilted full-latitude trough surging eastward through Wednesday. Downstream of this amplification, a SW to NE oriented jet streak will amplify to above 150 kts, leaving its favorable diffluent RRQ atop the Gulf Coast, leading to pronounced deep layer ascent. At the same time, the aforementioned Great Basin shortwave will continue to sharpen to produce impressive height falls/PVA, overlapping with the upper diffluence and the baroclinic zone positioned across the Gulf of Mexico to spawn a wave of low pressure moving east across it. Downstream of this developing wave, moisture advection will rapidly intensify on 300K isentropic ascent and accompanying WAA, driving PWs northward into the Gulf Coast and Southeast, with PW anomalies approaching +1 sigma to fuel widespread and expanding precipitation. While there remains some uncertainty as far as where the mixed precip zone will occur, the guidance has continued to trend a bit farther south/colder, suggesting that even the immediate beaches from Houston to Panama City will experience snowfall. Additionally, with extremely cold and dry air positioned just to the north, and low-level northerly flow ageostrophically enhancing the low-level baroclinic gradient, some increased fgen is expected from eastern TX through the FL Panhandle. This will be enhanced by the response to the upper jet aloft, and the WPC prototype snowband tool indicates a high chance for 1+"/hr snow rates from near Houston to Mobile Bay despite an elevated DGZ. This could additionally cool the column causing the mixed precip zone to push even farther south. These types of snow rates in areas that do not typically receive them will create major to extreme impacts, and historic snowfall is possible across this area. WPC probabilities are moderate to high (50-70%) for more than 4 inches on D1 from near Beaumont, TX through southern LA, with locally more than 6 inches possible (30% chance) where the most intense banding occurs. Surrounding this region, there exists a broad moderate chance (30-70%) for more than 2 inches from east-central TX through the NW Florida Panhandle. Farther east into Georgia, the northern Florida Peninsula, and the Carolinas, the guidance has been flip-flopping as far as the northern extent of moisture. However, today's runs have pushed the moisture a bit farther north despite the dry/cold airmass positioned to the north, so even these areas may see rare significant snowfall accumulations, despite more modest rates overall. In this region, and generally on D2, WPC probabilities for more than 2 inches are 30-50% from southern AL and south-central GA eastward to Cape Fear and the Outer Banks of NC. Finally, across portions of south Texas, and areas from the FL Panhandle to the northern FL Peninsula, a mixed zone of snow, sleet, and freezing rain is likely. There continues to be uncertainty in the latitudinal placement of the heaviest freezing rain/icing amounts, but current WPC probabilities for at least 0.1" of ice are as high as 30% in south Texas, and 50-70% across the northern FL Peninsula, where locally more than 0.25" of ice is possible. 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