Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 337 AM EST Thu Jan 9 2025 Valid 12Z Thu Jan 09 2025 - 12Z Sun Jan 12 2025 ...Great Lakes and Northeast... Day 1... Cyclonic flow on the backside of a large upper low in Atlantic Canada will support another ~day of mainly mountain-based snow over the northern Green and White Mountains and northern Maine. A few inches are possible in the higher elevations, with WPC probabilities for at least another 4 inches of snow around 50%. ...Northwest to the Northern Rockies/Plains... Days 2-3... Fairly sharp mid-level shortwave and sfc front will enter the PacNW with mostly higher elevation snow but lowering snow levels (from ~4500 to ~3000ft) as the front moves past the Cascades. The shortwave will move southeastward beneath a weakening upper jet, but combination of height falls and upslope enhancement will maximize snow over the mountains, from the Bitterroots to the Tetons and western MT D2 then into the Big/Little Belts into the Bighorns and eventually the Black Hills D3 as the stronger height falls move through. WPC probabilities for at least 12 inches of snow are are >50% above ~5000-6000ft or so (west to east) where two-day totals could exceed 18-24 inches above 6000-7000ft (40-70% chance). Additionally, by D3, Canadian system will move into the northern Plains/Upper Midwest with a swath of generally light snow via WAA and FGEN on the northern side of an area of low pressure. Amounts should generally be on the lighter side of a couple/few inches, but there are low-end probabilities (10-20%) of at least 4 inches of snow over eastern ND. ...Southern Plains, Southeast, southern Mid-Atlantic... Days 1-2... ...Major winter storm is forecast to span from west Texas and southeast Oklahoma starting today before crossing through much of the Mid-South and into portions of the Mid-Atlantic by the end of the week... A potent positively-tilted longwave trough containing plentiful upper level energy from an upper level low will support a very strong upper level jet that stretches from West Texas east through the Carolinas. Surface cyclogenesis will initiate this afternoon along the Texas Coast in the RER of that jet, also helping to draw in Gulf moisture with maximum PWATs along the Texas Gulf Coast rising to around 1.75 inches. While that level of moisture doesn't move too far inland, a large fraction of it will, providing ample moisture for the developing surface low. The associated low level jet will advect much of that moisture up the Lower Mississippi Valley. Meanwhile, an Arctic air mass will be in place due to a retreating high pressure system over the Central Plains and Mid-South, resulting in a strong temperature gradient which will both increase the forcing as well as lift that moisture and effectively wring it out, resulting in widespread wintry precipitation starting in TX/OK and spreading eastward to the Mid-Atlantic by late Friday/early Saturday as multiple jet streaks ride northeastward. The tight thermal gradient on the north and west side of the low will promote a similarly tight gradient in snowfall amounts. With a broad area of WAA off the deck, and in the areas where the cold air at the surface is shallow but stubborn, an area of freezing rain/sleet will develop from the Big Bend of Texas eastward over northern Louisiana/southern AR into central Mississippi, Alabama, and Georgia, then northeastward through the Carolinas. To the north, where the colder air is deeper through the column, snow will be the dominant or sole ptype across Northwest TX, central/northern AR, eastward through the Mid-South/Tennessee Valley and into the central/southern Appalachians to the VA Beaches. Ptypes will be transitioning from frozen to liquid in some southern areas where the cold air is shallower and overtaken by WAA at all levels. The models still show a fair amount of spread in the strength of the warm nose and time residence of sub-freezing air, resulting in lower confidence in these transition zones. Trended a bit warmer overall but will still have a few model cycles to hopefully gain more certainty. For many this will be a very impactful snowfall event and the first winter storm of the season for areas such as Dallas-Fort Worth north and east through the Ozarks and into the Memphis and Nashville metro areas. Additional forecasting challenges include banding potential on the northern and northwest side of the low increased by strong mid-level FGEN and isentropic ascent through the DGZ. This may lead to mesoscale banding that will become more notable once inside the full suite of CAMs, but even the 00Z runs still showed disagreement. This is particularly a concern for north-central TX and OK. From Texas through the Lower Mississippi Valley, forecast snow amounts have come down nominally along the ptype gradient, but have consequently increased for ice accumulations -- again, dependent on the strength of any warm air and how long colder 2m temps can hang on. WPC probabilities for at least 4 inches of snow are >50% near/north of D-FW across southeastern OK eastward across northern AR, the Mid-South, into Middle TN in a mostly continuous fashion. Orographic lift will also help the central/southern Appalachians see several inches of snow. Over AL/GA, initial WAA-driven snow should lead to a changeover to sleet/freezing rain and then rain for more southern areas, with the highest probabilities for at least 2 inches of snow closer to the TN border but non-zero probs to I-20/I-85. More uncertainty in snow amounts lies east of the Appalachians across southeastern VA where the guidance has wavered on the amount of QPF. Low pressure should strengthen as it leaves the coast, perhaps allowing some enhancement over SE VA early Saturday. For ice probabilities, there is a >40% chance of at least a tenth of an inch of ice around the Arklatex region through southern AR. In addition, areas from northern GA through parts of Upstate/Midlands SC into eastern NC show >30% probabilities of at least a tenth of an inch of ice. Fracasso ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png