Subj : HVYSNOW: Probabilistic He To : All From : Mike Powell Date : Thu Mar 21 2024 08:18:00 FOUS11 KWBC 210819 QPFHSD 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. $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .