Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 334 PM EST Wed Jan 8 2025 Valid 00Z Thu Jan 09 2025 - 00Z Sun Jan 12 2025 ...Great Lakes and Northeast... Day 1... Broad upper low churning near Newfoundland and southeast Canada will continue to help stream Arctic air over the Great Lakes through early Thursday. Mostly nuisance lake-effect will continue to develop in the wake of a shortwave trough currently over the lakes. Expect the greatest coverage of lake-effect snows to form off Lake Ontario, which is closest to the aforementioned upper low and will therefore have the greatest access to moisture. WPC probabilities for 2 inches of snow are over 80 percent for the Day 1/Thursday period from Oswego, NY south-southeast into the western Finger Lakes. The probabilities for 4 inches of snow drop to between 10 and 30 percent, reinforcing the idea that this snow will be mostly nuisance and will not be a classic single well-defined lake- effect band...but rather multiple bands with occasional convective snow bursts. Surface high pressure building over the lakes through the day will cause the lake-effect to diminish from west to east, so Lake Ontario's bands will die off last...hence the forecast for greater amounts of snow as compared to the upper Lakes. The multi- bands ongoing off Lake Superior and especially Lake Michigan should wane considerably this evening as the upper level shortwave quickly shifts away from the lakes. ...Northern Rockies and Northern Plains... Days 1 and 3... A northern stream shortwave will dive south down the northern Rockies and Plains tonight through Thursday. It will be riding the LER of a 110 kt jet, but with cold air well in place, it will be quite starved of moisture. Thus, for most valley locations, expect up to 4 inches of snow from eastern Montana south through Colorado. The Little Belt, Black Hills, and Bighorn Mountains of Wyoming and Montana remain areas where upslope will enhance snowfall rates, resulting in local totals up to 10 inches through Thursday. WPC probabilities are for 10-40% chance of 2 inches of snow for most of the Plains and valleys. On Day 3/Friday night into Saturday, a potent shortwave entering the Pacific Northwest will spread high elevation snow to the Cascades and Northern Rockies as it pushes eastward and snow levels are expected to start around 4500ft and fall to around 3000ft by the end of the forecast period. Snowfall amounts could exceed a foot into the Sawtooth and Bitterroot Ranges where WPC probabilities for at least 12 inches are between 30 to 50%. Up to 8 inches are probable for the Yellowstone and Tetons, the Bighorns, and Medicine Bow Ranges through Saturday. ...Southern Plains and Southeast... Days 1-3... ...Major winter storm is forecast to span from west Texas and southeast Oklahoma beginning on Thursday 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 aforementioned upper level low will support a very strong upper level jet that stretches from West Texas east through the Carolinas. Surface cyclogenesis will occur Thursday afternoon along the Texas Coast in the RER of that jet, which will bed southward, thus maximizing the upper level divergence over Louisiana. A plume of Gulf moisture will inject into the low 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 east as far as the Mid-Atlantic Saturday morning. The tight thermal gradient on the north and west side of the low will promote a similarly tight gradient in snowfall amounts and in the areas where the cold air at the surface is shallow, an area of freezing rain/sleet will develop from the Big Bend of Texas over northern Louisiana and into central Mississippi, Alabama, and Georgia, with snow to the north of that from north- central Texas east through the Tennessee Valley and into the Virginias. 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. Potential 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 (most likely by the 12z cycle today). This is particularly a concern for north-central TX and OK on D1 where overall guidance had recently trended down in amounts. For the DFW metro region, upper-end amounts have increased as some ensemble members (particularly the ECENS) pick up on this banding potential which allows for mixed precip to quickly change to heavy snow Thursday night. Given the mesoscale nature of these snowbands, conditions will likely drastically change over the course of tens of miles, with the heaviest snow north-northwest and lesser amounts to the south. 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, especially across southern Arkansas, where up to a quarter inch of accumulation is possible. This amount of ice will likely cause power outages, downed branches, and very treacherous travel conditions. Meanwhile a few miles to the north towards the Little Rock metro, up to 8 inches of snow are expected. WPC probabilities show a 40 to 70 percent chance of at least 4 inches of snow from southeast Oklahoma to the central Appalachians of West Virginia. Spanning farther east a WAA surge of snow is likely across the Mid-South Friday morning extending toward the Southeast by the afternoon hours. The southern track of this low pressure system supports snow potential reaching pretty far south compared to climatology, with ECMWF EFI values of 0.7-0.95 and an extreme shift of tail of +8 stretching from northern MS to central GA. Additionally, the deep cold air mass in place reinforced by a fresh snowpack to the north will allow for low-level cold air to hold on while warm air surges north in the mid-levels. This means that areas along and just south of the I-20 corridor that begin as snow will likely change over to sleet and freezing rain for a potentially extended period of time. WPC probabilities for moderate to major impacts extend from the DFW Metroplex area north and east to the Ozarks, through Memphis and northern MS before also spreading east to northern GA and the southern Appalachians. For ice probabilities, there is a 50 to 80 percent chance of at least a tenth of an inch of ice around the Arklatex region, with a stripe of low (10-30%) > 0.1" ice probabilities stretch from northern MS, central AL, central/northern GA and into the Tidewater of Virginia. Expect for these chances to continue to increase with time. The updated set of Key Messages follow. Wegman/Snell ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png