Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 407 AM EDT Fri Mar 22 2024 Valid 12Z Fri Mar 22 2024 - 12Z Mon Mar 25 2024 ...Great Lakes... Day 1... Early this morning, a band of heavy snow is racing east across the Upper Mississippi River Valley and approaching the Great Lakes. This band of snow is supported by a ribbon of 850-700mb FGEN and additional upper level lift aloft thanks to the diffluent right- entrance region of a 250mb jet streak overhead. The WPC Snowband Probability Tool (SPT), utilizing the 00Z HREF, depicted a narrow band of snow tracking from northeast Iowa to along and south of I-94 in southern Wisconsin where 1-2"/hr snowfall rates are expected with even some cases of >2"/hr rates possible in some cases. HRRR area averaged soundings within this area between 09-15Z showed exceptional vertical velocities within a highly saturated DGZ that will support rapidly accumulating snowfall before sunrise, as well as significantly reduced visibilities. This band will begin to lose its punch as it crosses Lake Michigan and heads for central Michigan during the daytime hours with hourly rates topping out around 1"/hr in most cases. In cases where snowfall rates are <1"/hr during the day, most accumulations will be confined to grassy surfaces, sidewalks, and side streets given the strong late March sun angle. The band of snow will works its way through southern Michigan late afternoon and exit east of Lake Erie by Friday evening. WPC PWPF depicts moderate-to-high chances (50-70%) for >4" of additional snowfall from along I-94 near Madison, WI on east into central Michigan. There was also some localized low chances (10-30%) for >6" of additional snowfall close to the Milwaukee metro area and in central Michigan. ...New England... Days 1-2... While high pressure briefly builds in for most of Friday, the same storm system responsible for the swatch of heavy snow in the Midwest and Great Lakes looks to produce periods of snow starting late morning Friday in western New York and Upstate New York by Friday afternoon. Similar to the Great Lakes, snow is blossoming due to strong 850-700mb WAA aloft and 290K isentropic glide overhead. As the primary 500mb vorticity tracks over the Great Lakes, strong PVA and increase 850-500mb WAA will strengthen a SW- NE oriented 250mb jet streak over Ontario and Quebec that will place its divergent right-entrance region over northern New England Friday night. This will further enhance the snow shield over Upstate New York and into northern New England Friday night as heavy snow unfolds from the Tug Hill and Adirondacks to the Green and White Mountains. By Saturday morning, heavy snow will spread into western Maine and eventually into northern Maine during the day. Meanwhile, farther south, a potent 500mb trough in the Southeast will strengthen low pressure along the Mid-Atlantic coast that will track up the coast throughout the day. While this is occurring, both the jet streak over southeast Canada and another 250mb jet streak along the Southeast coast will position their right-entrance (southeast Canada jet streak) and left-exit region (Southeast coast) over the coastal Northeast. This will support a "kissing jets" setup that further enhances precipitation rates at the same time strong 850mb moisture flux is being advected into the sufficiently cold air-mass in the White Mountains and much of interior Maine. In addition to the snow, surface temperatures in parts of the Catskills, Lower Hudson Valley, Berkshires, Worcester Hills, and both southern New Hampshire and Maine are cold enough to support some ice accumulations, but ice will be more likely to accumulate on trees and other vegetation during the day since the late March sun angle will make most roads primarily wet. Heavy snow from the Green and White Mountains to central Maine, as well as ice from southern New Hampshire to south-central Maine, will continue Saturday afternoon with snow changing over in parts of southern Maine Saturday evening. Snow will finally come to an end by early Sunday morning over Downeast Maine. Latest WPC PWPF shows high chances (>70%) for snowfall totals >8" from the Tug Hill and Adirondacks through northern Vermont, northern New Hampshire, and both northern and central Maine. Northern Maine in particular could see exceptionally heavy snowfall in northern Maine with WPC PWPF showing moderate-to-high chances (50-70%) for snowfall totals >12". The WSSI-P does depict moderate chances (40-60%) for Major Impacts from the Green and White Mountains to northern Maine with Snow Amount and Snow Load the primary drivers in the WSSI-P algorithm. "Major Impacts" suggest the potential for considerable disruptions to daily life that include dangerous to impossible driving, widespread closures, and potential tree damage and power outages due to excessive snow load. ...Pacific Mountain Ranges... Days 1-3... A large, barreling upper low off the West Coast today will direct a healthy fetch of 850-700mb moisture at the Pacific Northwest and northern California. NAEFS depicts an IVT of 300-400 kg/m/s (>90th climatological percentile) that will steadily work its was into California today and into Friday night. This IVT will be the primary factor in producing heavy snowfall in the Salmon/Siskiyou Mountains of northern California, as well as along the spine of the Sierra Nevada. These mountains also reside beneath the diffluent left-exit region of a roaring 180kt 250mb jet streak located off the southern California coast Friday night. NAEFS shows 200mb wind speeds that are above the observed levels in the CFSR database for this time of year. This shows that along with the usual strong topographically-forced ascent, there is also impressive synoptically-forced ascent 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 low moves into the Pacific 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 pronounced, the Pacific Northwest lies directly ahead of the approaching upper low, which still supports prolonged synoptically-forced ascent aloft to go along with falling snow levels. Snow levels will be a near carbon-copy of the scenario farther south in the California Ranges with the Cascade Range and Olympics >5,000ft having the better odds of seeing cumulative snowfall totals through Sunday topping 12". As the upper low moves ashore and over the Northwest on Saturday, the axis of Pacific moisture will be oriented farther inland and lead to periods of heavy mountain snow for elevations >6,000ft in the Great Basin, the Blue Mountains of eastern Oregon, the Bitterroots, Tetons, Absaroka, and as far inland as the Wasatch and Colorado Rockies by Saturday night. WPC PWPF continues to identify the Sierra Nevada and the Salmon/Siskiyou/Trinity mountains as having high chances (>70%) for >12" of snowfall for the event. In fact, at elevations >8,000ft, high chances (>70%) for >18" of snowfall are depicted. Once the snowfall threshold dips to >6", the Olympics and Cascades, as well as the Absaroka, Tetons, and portions of the western Colorado Rockies sport moderate-to-high chances (50-70%) for those snowfall totals. ...Northern Rockies & High Plains, Central Rockies, Upper Midwest... Days 2-3... **Significant late season winter storm becoming increasingly likely from the Northern Rockies and Northern High Plains to as far east as the Upper Midwest this weekend In wake of the storm system that tracked through the Upper Midwest Thursday night, the large dome of high pressure over the Canadian Prairies working in tandem with lower pressure in the West supports a prolonged low-level easterly easterly flow regime into the western Montana Rockies as well as central Montana and as far south as the Absaroka and Big Horns. Another 700mb FGEN band will strengthen throughout the day today and into Friday night as 700mb WAA increases 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 today, however snowfall rates will not come down heavily enough during the day to do much more than Minor Impacts. Snow will accumulate more effectively Friday night. By Saturday morning, the approaching upper low in the Pacific Northwest will spawn a new surface low somewhere over eastern Idaho Saturday morning that will track into northern Wyoming Saturday evening. The surface low will continue its trek southeast into eastern Colorado, where NAEFS suggests it will eventually reach MSLP values that are <0.5 climatological percentile. Meanwhile, the 850mb LLJ will accelerate over the Great Plains with winds topping the 97.5 climatological percentile over the southern Great Plains around 06-12Z Sunday. The LLJ will advect anomalous 850-700mb moisture northward and wrap around the northern flank of the strengthening surface low, and give rise to a blossoming shield of heavy snow early Sunday morning from western Montana and northern Wyoming to the Dakotas. Strong 850-700mb WAA and copious amounts of Gulf of Mexico moisture will make its way into the Upper Midwest where heavy snow will then ensue over southern Minnesota. By 00Z Monday, a significant winter storm will be well underway, causing strong winds and heavy snow from the Central Rockies on east through the Black Hills, the Dakotas, and into the Upper Mississippi Valley. NAEFS by 00Z Monday shows MSLP values in parts of southwest Kansas and the TX/OK Panhandles that fall outside the observed MSLP minimum for this time of year (~980mb), showcasing the remarkable strength this storm will reach Sunday evening. By early Monday morning, the storm will track northeast towards the Missouri River with a pronounced TROWAL over the eastern Dakotas and western Minnesota being the focus for the heaviest snowfall. There remains some uncertainty in regards to the position and track of the storm, but most ensemble members are on board with this becoming an impressive late March winter storm. Latest WSSI-P shows moderate-to-high chances (50-80%) for Moderate Impacts from eastern Montana and along the ND/SD border to south-central Minnesota through early Monday morning. There are similar probabilities for Moderate Impacts that stretch as far south as the Sand Hills of northern Nebraska. There are even some low-to- moderate chances (30-50%) for Major impacts in parts of eastern South Dakota, southwest Minnesota, and throughout the Twin Cities metro area. The primary drivers in the WSSI-P algorithm are Snow Amount and Blowing Snow, the latter of which, shows >60% chances for Minor Impacts as far south as the Palmer Divide and into central Nebraska as a powerful cold front passes through these areas on Sunday. There are more details on the event in the Key Messages below, but residents in these areas are encouraged to closely monitor the latest forecasts this weekend as this storm will likely continue to cause hazardous impacts for parts of the Northern Plains and Upper Midwest into the start of next week. Mullinax ***Key Messages for Northern Plains Winter Storm*** --Significant winter storm likely Confidence continues to increase in a large storm system that will produce an area of heavy snow over the Northern Plains and into the Upper Midwest, along with some areas of mixed precipitation this weekend into early next week. --Widespread heavy snow expected Heavy snow is forecast to overspread eastern Montana late Saturday, then expand into the Northern Plains and Upper Midwest by Sunday afternoon. There is a high chance (>70%) of at least eight inches of snow from the North Dakota/South Dakota border eastward into Minnesota and northern Wisconsin. --Hazardous impacts due to snow, snow load, and wind A combination of heavy snow and gusty winds will likely produce areas of blowing and drifting snow, as well as possible blizzard conditions Sunday into Monday. Hazardous travel, along with some disruptions and closures are expected late Saturday into Monday. The combination of strong winds and heavy, wet snow on trees and power lines may result in tree damage and power outages. --Additional forecast changes anticipated Uncertainty remains with precipitation type for southern-most areas and a possible sharp northern snowfall gradient in northern-most areas. Closely monitor the latest forecasts as the storm evolved over the next 24-48 hours.