Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 419 AM EDT Thu Mar 21 2024 Valid 12Z Thu Mar 21 2024 - 12Z Sun Mar 24 2024 ...Northern Plains through the Great Lakes... Days 1-2... A fast moving 500mb disturbance exiting the northern Rockies is partly responsible for the development of low pressure in eastern Wyoming later this afternoon. Farther north, a narrow and NW-SE oriented 700mb FGEN zone stretching from southern Alberta to eastern Montana. As the upper level disturbance tracks east, a surge in 700mb WAA and 500mb PVA will coincide with an adequate plume of 700-300mb moisture aloft. Combined with upslope easterly flow (thanks in part to high pressure anchored over south- central Canada) and a boundary layer just enough below freezing, and the end result is now from the northern Bitterroots and Lewis Range to as far east as central South Dakota. While the former are largely due to upslope flow, the developing swath of heavy snow is driven by the aforementioned 700mb FGEN and the region being located beneath the divergent right- entrance region of a 250mb jet streak positioned over the Great Lakes. Meanwhile, the surface low Thursday evening will deepen with a strengthening LLJ that will direct a plume of rich low level moisture around the northern flank of the low and into the sufficiently cold air-mass in the Upper Midwest. The expectation is for a narrow band of heavy snowfall to ensue Thursday afternoon and evening over southeast North Dakota and northeast South Dakota that will track east into central Minnesota Thursday night. By early Friday morning, the band of heavy snow will race east across the Mississippi River and into central Wisconsin. Guidance continues to show coherent 700mb FGEN banding and synoptically- forced ascent thanks to the 250mb jet streak over Wisconsin that will persist over Michigan during the daytime hours. While those in the Upper Mississippi Valley will have received their snowfall at night, the bulk of the snow in eastern Wisconsin and Michigan will come during the daytime hours. While hourly snowfall rates topping 1"/hr are possible, most accumulations will be confined to grassy surfaces, sidewalks, and side streets given the strong late March sun angle in play. Snowfall will also likely compact quickly, making it a pasty snow in these areas on Friday. Snow will come to an end by Friday evening as the storm races into northeast Ohio. WPC PWPF sports high chances (70%) for >4" of snowfall in southeast South Dakota, east-central Wisconsin, and central Michigan. WPC PWPF does show low chances (10-30%) for snowfall totals >8" in localized portions of east-central Wisconsin and central Michigan. The WSSI-P does depict a swath of moderate-to- high chances (50-70%) for Minor Impacts throughout most of these affected areas (particularly in areas with >4" snowfall totals), suggesting the potential for a few inconveniences to daily life that are primarily due to possible snow covered roads and reduced visibilities. ...New England... Days 1-3 Periods of heavy snow will linger across portions of the northern Appalachians and northern Maine today as a powerful sub 990mb low over Downeast Maine tracks into southern Quebec later today. Moderate impacts, largely due to a combination of heavy snow and blowing snow, will stick around this morning but conditions will gradually improve this afternoon before finally concluding Thursday evening. While high pressure briefly builds in Thursday night and sticks around for most of Friday, the storm system responsible for periods of moderate-to-heavy snow in the Upper Midwest and Great Lakes will begin to produce snow over northern New York Friday evening. Similar to the Upper Midwest and Great Lakes, the seemingly endless swath of 700mb FGEN and upper level divergent flow courtesy of a 250mb jet streak's right-entrance region aloft will be responsible for the initial round of snow that occurs from the Adirondacks Friday night to the Northeast Kingdom and both the Green and White Mountains through early Saturday morning. However, farther south, a potent 500mb trough in the Southeast will direct low pressure in the eastern Carolinas north up the Eastern Seaboard. This upper trough will also stream anomalous moisture with Gulf of Mexico origins up the East Coast. As snow moves into northern Maine during the day on Saturday, the 250mb jet streak over southeastern Canada will strengthen at the same time as another jet streak along the Mid-Atlantic coast also intensifies. This will support a brief "kissing jet" setup along the New England coast that will produce heavy precipitation rates, perhaps as far inland as central and northern Maine Saturday afternoon and early Saturday evening. The storm will quickly race northeast Saturday evening and all snow is likely to conclude after midnight. Latest WPC PWPF shows high chances (>70%) for snowfall totals >8" Friday evening through Saturday evening in parts of the Adirondacks, Green and White Mountains, and through central Maine. Given the dearth of moisture and exceptional synoptic scale forcing aloft, there are even moderate-to-high chance probabilities (50-70%) for >12" from Vermont's Northeast Kingdom on north and east into north-central Maine. The WSSI-P currently shows moderate- to-high chances (50-70%) for Moderate Impacts beginning Friday evening and lasting into Saturday night from southern Vermont and most of interior New Hampshire to much of inland Maine. ...Pacific Mountain Ranges... Days 2-3... A large, barreling upper low off the West Coast on Friday will begin directing a healthy fetch of 850-700mb moisture at the Pacific Northwest and northern California. NAEFS is indicating an IVT of 300-400 kg/m/s (>90th climatological percentile) will steadily work its way into California on Friday. This IVT will be the primary driver in producing heavy snowfall in the Salmon/Siskiyou Mountains of northern California, as well as along the spine of the Sierra Nevada. These mountain ranges also reside beneath the diffluent left-exit region of 180kt 250mb jet streak located off the Southern California coast Friday night. NAEFS is showing 200mb winds Friday afternoon and into Saturday that are above the observed 200mb wind speeds within the CFSR climatology database for this time of year. This shows that along with the usual strong upslope ascent, that impressive synoptically-forced ascent will be present as well. Snow levels will initially start out between 4,000-5,000ft Friday night but drop to as low as 3,000ft by Saturday as the upper trough moves into the Northwest. However, snow totals that likely surpass 12" will be at elevations >5,000ft Saturday and through Sunday morning. Farther north, while the moisture transport is not as prevalent, the Pacific Northwest lies directly ahead of the approaching upper low, which will still support prolonged synoptically-forced ascent aloft and lowering snow levels. Snow levels will be a near carbon-copy of the scenario farther south in the California Ranges, but the Cascades Range and Olympics >5,000ft will have the best odds of seeing cumulative snowfall totals top 12 inches for this event. As the upper low moves ashore and over the Northwest on Saturday, the axis of Pacific moisture will be directed farther inland as well, leading to periods of snow for elevations >6,000ft in the Great Basin, the Blue Mountains of eastern Oregon, the Boise/Sawtooth of Idaho, the Tetons and Absaroka, and event as far inland as the Wasatch and Colorado Rockies by Saturday night. WPC PWPF has identified the Sierra Nevada as the only mountain range sporting high chances (>70%) for snowfall totals >12". Once the snowfall threshold dips to 6", however, just about every aforementioned mountain range above sports moderate-to-high chances (50-80%) for >6" of snowfall l;ate Friday night and through Saturday night as well. ...Northern Rockies & Northern High Plains... Days 2-3... Even in wake of the storm system that tracked into the Upper Midwest Thursday night, the large dome of high pressure over the Canadian Prairies and lower pressure in the West continues to support a prolonged low-level easterly flow regime into the western Montana Rockies, as well as far south as the Absaroka and Big Horns of northern Wyoming. Another 700mb FGEN band will strengthen throughout the day Friday and into Friday night as 700mb WAA increase over the northern High Plains. In addition, much of Montana lies beneath the divergent right-entrance region of a 100kt 250mb jet streak. This is a recipe for periods of snow on Friday, however snowfall rates will not come down heavily enough during the day on friday to do much more than Minor Impacts. Snow will be able to accumulate better Friday night once the sun sets. By Saturday morning, the approaching upper low positioned over the Pacific Northwest will spawn a new surface low somewhere over eastern Idaho Saturday morning that then tracks into northern Wyoming by Saturday evening. This new surface low grows to become rather anomalous, eventually dropping below the 1st climatological percentile according to NAEFS over central Wyoming Saturday evening. To the north of this surface low, easterly 850mb winds will increase in strength from central Montana to the Dakotas to the point where 850mb zonal wind anomalies also dip below the 1st climatological percentile. These 850mb winds will also direct a surge of 850mb moisture flux that originates from a LLJ that originates as far south as the Lower Rio Grande Valley. With a strengthening low pressure system emerging into the northern High Plains Saturday night and cold Canadian High Pressure anchored to the north, all the ingredients are coming together for a potential significant winter storm from the northern Rockies to the Northern Plains this weekend. This far out, however, there remains uncertainty in storm track, duration, precipitation type, and of course, snowfall totals. Between 12Z Friday - 12Z Saturday, WPC PWPF shows the Lewis Range as having low chances (10-30%) for >6" of snow with elevations >6,000ft most favored. By 12Z Saturday - 12Z Sunday, the low-to- moderate chance swath (30-50%) for >6" of snowfall grows substantially to include most of the northern Rockies in elevations >6,000ft, through central and eastern Montana, and into western North Dakota. Of course, the storm system will continue to produce heavy snow and blustery winds into Sunday, so those potentially in the path of this impending winter storm will want to check in with their local WFO and our WPC medium range suite of products of the latest regarding this winter storm for Sunday and into next Monday. Mullinax ***Key Messages for Northern Plains Winter Storm March 23 - March 26*** -- Winter storm becoming likely: Confidence continues to increase that a large storm system will produce a swath of heavy snow, freezing rain, and sleet across portions of the Northern Plains and Upper Midwest into early next week. -- Widespread heavy snow: Heavy snow is expected to overspread eastern Montana late Saturday, then expand into the Northern Plains and Upper Midwest by Sunday night, producing significant accumulations. Snow should exit the area during Tuesday. -- Significant impacts due to snow, ice, and wind: A combination of heavy snow and gusty winds, as well as freezing rain south of the heavy snow, will likely produce areas of hazardous travel and impacts to infrastructure. -- Forecast changes anticipated: Uncertainty remains with the timing and location of the storm track, which will affect where the most significant impacts will occur. Keep up to date with the latest forecast as this storm evolves.