Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 418 PM EST Thu Jan 04 2024 Valid 00Z Fri Jan 05 2024 - 00Z Mon Jan 08 2024 ...Southern Rockies into the Southern High Plains... Days 1-3... A strong closed low continues to evolve across the Four Corners this afternoon, and is forecast to eject eastward into the Rockies/Southern Plains tonight as a cyclonically curved 110 kt jet streak rounds the base of the closed low over the Plains. As pronounced height falls overspread the area, lee-side low pressure will propagate eastward toward the Red River, ushering in increasing snowfall coverage initially focused along and east of the Sangre De Cristos at the beginning of the forecast period tonight associated with increasing low-level WAA and an emerging deformation axis on the back side of the system. Deterministic guidance continues to highlight a steady influx of moisture drawn into this system to overlap with the impressive dynamical forcing, which includes a healthy supply of 850-700 hPa moisture flux with source regions originating from both the Gulf and Eastern Pacific. The last three cycles of the WPC Super Ensemble have increasingly emphasized a focused area of Southeast CO into the OK/TX Panhandle for 24 hour snowfall amounts exceeding 4 inches (40-50%) through tomorrow evening within a regime of northeasterly upslope flow. Meanwhile, a lower (5-10%) but appreciable signal for snowfall exceeding 4 inches is also noted over south-central Kansas within a potential frontogenetic snow band tomorrow morning, although confidence is low with this feature. Yet another bout of snowfall is expected over the Sangre De Cristos by D2 associated with the next shortwave and 110 kt jet streak diving into the Southern Plains on the western periphery of a large mean-trough. Compared with the previous forecast cycle, WPC probabilities remain in the 10-30% range over portions of the Sangre De Cristo with this weekend system. The more impactful round of wintry weather is anticipated to arrive later in the D2-D3 time frame as the next amplifying upper-trough and closed low arrives from the Pacific Northwest. Strong height falls will support plummeting snow levels and widespread mountain snowfall across the Southern Rockies, with 70-80% WSE probabilities of 24 hour snowfall exceeding 4 inches by Day 3. ..Pacific Northwest/Great Basin/Sierra/Four Corners... Days 1-3... Zonal flow out of the northeastern Pacific will transition toward a much more amplified ridge/trough pattern over the Gulf of Alaska/western US, respectively. This will drive a digging shortwave within a broader longwave trough through the interior West by the end of this forecast period behind a strong cold front that will bring in much colder temperatures and lower snow levels. Lead system tonight/Friday will continue through the Pac NW as the weak mid-level shortwave dives southeastward. Snow levels around 4000 this evening with the greater snowfall rates will lower to around 3000ft as snow tapers off. WPC probabilities of at least 6 inches in the Cascades through Friday evening are moderate or higher (>40%) above about 4500ft. Much of Friday will see a break in the snowfall ahead of the much more robust second system. Sharpening and digging mid-level shortwave and stream of vorticity will push into the Pac NW by late Saturday morning as an increasing upper jet (>130kts) noses into NorCal. Strong surface cold front will move ashore early Saturday and continue eastward/southeastward through the Great Basin and into the Four Corners region by early Sunday. Overall moisture surge with the system will be somewhat lacking (PW anomalies close to normal), but will be balanced by strong dynamics via the upper jet/height falls and upslope enhancement into the terrain, especially over the WA/OR Cascades. Snow levels will fall behind the front as precipitation continues through Sunday, from around 1500ft in northern areas to 3000ft across the Sierra/Great Basin, reaching most valley floors in the region. Measurable snow is likely down to the foothills of the Sierra and Cascades with significant impacts to most passes in the West. Track of the cold core will be toward NorCal and into the Sierra/Great Basin where 700mb temperature anomalies will likely fall to near -2 sigma (nearing -15C). Expanse of snowfall will be throughout much of the West to the Rockies over the weekend, even well into the Four Corners as the front tracks to Mexico. WPC probabilities of at least 4 inches of snow are moderate (40-70%) to elevations as low as 1500ft on day 2 in the Northwest and as low as 3000ft into northern CA into the Great Basin into day 3. Much higher totals are expected for the higher mountain areas in excess of a foot. At pass levels, 4000-5000ft or so, WPC probabilities for at least 8 inches of snow are at least moderate (>40%) especially over the Cascades. In the Sierra, WPC probabilities for at least 8 inches of snow are around and higher than 50% at/above 4000ft or so. Significant snow is also likely into Utah, Mogollon Rim, and into the San Juans as the deep upper trough and surface cold front move through the region on Sunday. ...Northern MN into the U.P. of Michigan... Day 2... As the upper low over the southern Rockies exits onto the Plains tonight into early Friday, northern portion of the embedded vort max will split off and move NNE into the Corn Belt by Friday evening as the arcing northern stream jet lifts through the Great Lakes. Broad upper divergence will promote lift across the Upper Midwest, with sufficient column moisture and sub-freezing temperatures to favor light snow across much of the region into Saturday. In addition, an incoming shortwave from southern Canada will help maintain PVA as a surface boundary moves through the northern Plains and a weak area of low pressure forms over northern MN. PW values will rise over the next day or so and WAA will help maintain the relatively moist environment (+1 to +2 sigma anomalies) but thermal structure of the atmosphere should preclude even moderate snow through the period aside from near the front with some enhanced low-level convergence. This will be offset by a drawn-out duration of light snow, with total accumulations of a few inches on day 2. Northeast flow off Lake Superior may enhance the accumulations along the North Shore in the Arrowhead. There, WPC probabilities for at least 4 inches of snow are above 50%. Lower probabilities exist across much of the rest of northern MN and into some portions of the U.P. of Michigan. ...Southern Appalachians through New England... Days 2-3... As an upper low originating over the Southwest lifts to the northeast and fills while mid-level energy moves through the base of a broadening trough to its south, guidance continues to depict a typical Miller Type-A storm, with low pressure developing over the northern Gulf of Mexico by late Friday. As the system tracks along the central Gulf Coast into the Southeast, this will channel moisture northward into the southern to central Appalachians and Piedmont region. An in-situ cold/dry air wedge will support a wintry mix, with guidance still showing a good signal for impactful ice accumulations across portions of western North Carolina and along much of Virginia's I-81 corridor. WPC PWPF probabilities for ice accumulations for 0.10 inch or more have increased a little with the latest run, with the 50 percent or greater probabilities expanding along the region. The surface low will begin to steadily deepen as it moves along the Mid-Atlantic Coast Saturday. Models show a pivoting deformation band beginning to develop well northwest of the low, with moderate to heavy snow developing by late Saturday over the interior northern Mid-Atlantic. By late Saturday, WPC PWPF shows moderate to high probabilities for accumulation of 4 inches centered over the eastern WV Panhandle, western Maryland, and south-central Pennsylvania. As an amplified upstream shortwave begins to assume a negative-tilt while moving through the Mid-Atlantic into the Northeast, the surface low is forecast to deepen into a strong cyclone moving along the Long Island-southern New England coasts on Sunday. While notable differences in the details remain, the overall trend in the models has been wetter across the Northeast, with heavy snow appearing more likely from northeastern Pennsylvania and interior northern New Jersey through interior southern New England to coastal Maine. WPC PWPF shows moderate to high probabilities for snow accumulations of 8 inches or more across this region. As is typically the case, there remains a good deal of uncertainty in the rain-snow transition that is likely to linger not far from the coast and along the I-95 Corridor from New York to Boston. Asherman/Fracasso/Pereira ***Key Messages for Jan 6-7 East Coast Winter Storm*** --Heavy Snow Likely in the Northeast-- For the interior Mid-Atlantic and New England, there is increasing confidence in heavy snow from Saturday Afternoon into Sunday. The greatest uncertainty in the rain-snow transition is from southeast Pennsylvania and northern New Jersey, into southern New England. People in those areas should still be prepared for the possibility of snow, and changes to the forecast. North of those areas, confidence in heavy snow is higher. --Wet Snow and Wind in Southern New England-- The combination of heavy, wet snow and gusty winds in Connecticut, Rhode Island, and Massachusetts may lead to some power outages and tree damage. --Minor Coastal Flooding Possible in the Northeast-- Gusty onshore winds may lead to minor flooding along the Mid-Atlantic and southern New England coasts, particularly for the Sunday morning high tide cycle. --Ice Impacts to Travel in Southern Appalachians-- In the Appalachian region of western North Carolina, western Virginia, and eastern West Virginia, accumulations of ice in excess of 0.1 inches, due to freezing rain, are likely with locally higher accumulations possible. This icing, along with some areas of sleet, may produce hazardous travel conditions on Friday Night and Saturday.