FOUS11 KWBC 200824 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 324 AM EST Mon Jan 20 2025 Valid 12Z Mon Jan 20 2025 - 12Z Thu Jan 23 2025 ...Great Lakes... Days 1-3... Broad cyclonic flow courtesy of a massive upper low over eastern Canada will direct a series of shortwave troughs over the Great Lakes and eastern U.S. the first half of the week. An exceptionally strong shortwave trough on Tuesday (500mb heights as low as the 1st climatological percentile 12Z Tuesday) will deliver 1000-850mb temps that are also below the 1st climatological percentile. While the Great Lakes are gradually icing over with each passing day, the exceptionally colder air-mass traversing the Great Lakes paired with the favorable upper-level synoptic-scale ascent will be more than enough to support persistent and potent lake effect snow (LES) bands. In fact, lake-induced instability of >500 J/kg would support exceptional LES bands, particularly downstream of Lakes Erie and Ontario where single-banded LES streamers are most likely. LES bands should finally weaken by Wednesday morning as high pressure moves over the Mid-Atlantic and a weak Alberta Clipper approaches from the northwest. This will weaken the pressure gradient and shift winds our of the southwest. Some light snow is possible in the Great Lakes region on Wednesday as a result of the Clipper, but snowfall rates will pale in comparison to the LES bands that will be around through Tuesday afternoon. WPC probabilities for Monday and Tuesday depict moderate chances (40-60%) for snowfall >8" along the western-most portions of Michigan's Mitten and along the northern coasts of the eastern Michigan U.P., The heaviest LES bands take shape Monday night and into Tuesday downstream of Lakes Erie and Ontario. By Wednesday morning, WPC probabilities show high chances (>70%) for snowfall >18" in some of the South Towns near Buffalo and in the Tug Hill Plateau. The event total snowfall in the Tug Hill is likely to range between of 2-4 feet through Wednesday. ...Texas, Gulf Coast, and Southeast... Days 1-3... ...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. 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. Guidance is keying in on the snow/sleet/freezing rain transition from south-central TX on east to along and south of the I-10 corridor. The mechanisms for this vary, however, as in south- central TX, it has more to do with the lack of moisture above 700mb and an exceptional warm nose between 850-700mb, and from the Upper Texas Coast to the the FL Panhandle, there is no shortage of moisture but the warm nose is stronger. Some very cold and dry air to the north will likely cause a sharp precip gradient as well, but to the south of the precip gradient, a swath of heavy snow driven by strong WAA and 850mb FGEN will result in eastern TX/LA/MS/AL witnessing up to 1"/hr snowfall rates just north of the mixed transition zone, which is rare for the central Gulf Coast region. Guidance has continued to suggest more suppression of the QPF axis over the last 12 hours, and thus WSO and WSSI-P probabilities have decreased in parts of northern GA and the central Carolinas Tuesday night. That said, from the coastal Carolinas on south to northern Florida, a combination of snow and ice will make for treacherous travel conditions Tuesday night and likely into Wednesday morning stubbornly remaining below freezing. WPC probabilities indicate a moderate chances chance (40-60%) for at least 4" of snow from far eastern TX through south-central LA and into southern MS. Locally, 4-6" of snow is possible in south- central Louisiana with locations such as Lake Charles and Baton Rouge seeing such totals. These areas referenced above also ave the better chances of witnessing up to 1"/hr snowfall rates. The combination of the heavy snow, very cold temperatures, and the rarity of this event could result in major impacts and considerable disruptions to daily life. Elsewhere, WPC probabilities for 2+ inches of snow are as high as 40-60% from the northern Houston suburbs to as far east as just north of Mobile Bay. Farther east, recent trends in guidance has reduced snowfall probabilities in parts of GA and the central Carolinas with WPC probabilities showing low chances (10-30%) for over 2" of snow. Additionally, southeast of the heaviest snow, especially from the Florida Panhandle east to the northern Peninsula and parts of southern GA, freezing rain is expected with a 40-60% chance of producing at least 0.1" of ice. Most icing totals in South-central Texas should generally remain below 0.1", but these totals are still highly unusual for these parts of the Lone Star State and are likely to result in hazardous travel conditions Monday night and Tuesday morning. This event has prompted the issuance of collaborated Key Messages (KeyMessage_1) linked below. Mullinax/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 $$