Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 342 AM EDT Wed Apr 05 2023 Valid 12Z Wed Apr 05 2023 - 12Z Sat Apr 08 2023 ...Northern Plains and Upper Midwest... Day 1... ...Major late-season blizzard to gradually conclude across the Northern Plains and Upper Midwest today... A closed low embedded within a lifting longwave trough over the central Rockies will slide northeastward and into southwestern Ontario tonight. The associated strong surface low with an estimated central pressure (mb) in the low 990s is forecast to also lift northward from southeast MN this morning to the Arrowhead of Minnesota and eventually southern Canada tonight. Modest 700 mb frontogenesis with a TROWAL wrapping around the surface cyclone will likely lead to a few more hours of snowfall rates up to 1"/hr across northeast ND and northern MN this morning. The heavy snow threat rapidly diminishes across the region after 16z as dry air surges in behind the system once it slides into Canada. Gusty winds up to 45 mph continue and may increase on the backside of the low throughout the afternoon and evening per the 00z HREF, leading to continuing blowing and drifting snow. Additional snowfall amounts through D1 are expected to be in the 3-6" range across far northeast ND and far northern MN, with WPC probabilities for an additional 6" between 40-60%. Farther east into the Upper Midwest and Great Lakes, warm air advection and light precipitation should continue early this morning from northern WI to the Upper Peninsula of MI with surface temperatures near or slightly below freezing. This should lead to additional glaze to 0.1" of ice. ...Pacific Northwest... Days 2-3... A closed mid-level low dropping out of the Gulf of Alaska will weaken through late week as it approaches the Pacific Northwest coast, but will likely shift onshore as a lobe of vorticity with modest amplitude as it takes on a negative tilt on Friday. This feature will be accompanied by weak but persistent upper level diffluence within the LFQ of an approaching and digging jet streak, resulting in increasing large scale ascent across the Northwest Wednesday night into Friday. Downstream of the aforementioned closed low, mid-level flow between 700-500mb will initially become zonal and point orthogonally into the region, but will gradually back to be more SWly on Thursday, driving enhanced warm and moist advection. This will drive a modest atmospheric river (AR) onshore, with IVT values potentially reaching up to 500 kg/m-s inland towards the Cascades D2. While this overlap of moisture and ascent will result in expanding and intensifying precipitation across the region, the accompanying WAA will help surge snow levels to above 4000-5000 ft late Thursday into Friday, such that heavy snow accumulations should generally be above the major pass levels. WPC probabilities for more than 6 inches reach between 30-60% on D2 and D3, but remain confined generally to the higher terrain above pass level throughout the Olympics, Cascades, and parts of the Shasta/Trinity region of northern CA. ...New England... Day 1... An amplified closed low characterized by 700-500mb heights dropping to -4 standard deviations according to NAEFS ensemble tables will lift across the Northern Plains and then into Ontario Thursday, coincident with an impressive coupled jet structure that will overlap the best mid-level height falls and PVA with increased diffluence to drive robust deep layer ascent across the Upper Midwest. An accompanying surface low will become vertically stacked during D1, and then occlude to a triple point over the Great Lakes, with a secondary wave of low pressure developing and then lifting northeast along a warm front into New England on Thursday. Strong meridional moisture transport out of the Gulf of Mexico will intensify driving anomalous PWs northward, noted by anomalies reaching +3 standard deviations across New England on Thursday. During this evolution, a cold Canadian high pressure will gradually retreat, but should maintain at least a modest CAD, especially across New England, keeping surface temperatures below freezing even as the strong WAA pushes a warm nose > 0C northward. This will result in a difficult p-type across northern New England and Upstate NY. Precipitation is expected to move into the region around or just after 18z today, with significant mixed-precip stretching from the St. Lawrence Valley eastward through the White Mountains of NH and much of central/northern ME through early Thursday. Robust moisture transport in a region of intense ascent will result in sufficient forcing spreading across the region north of the warm front, while the cold high gradually shifts eastward and maintains a more impressive CAD into New England. A brief burst of snow is likely across northern ME, where the warm nose between 850-700 mb remains below freezing. Areas just to the south should start out with predominantly sleet as ptype. CAD should erode relatively quickly across Upstate New York, Vermont, and coastal Maine due to strong WAA, but precip falling into the CAD will likely result in some intensification of this wedge before erosion, lengthening the duration of mixed precip. Regional soundings indicate the warm nose will be quite strong, as high as +9C, which is above the 75th% for warm nose temperatures from McCray, 2019, but is accompanied by a cold depth of 1500-2000m which is also above the 75th%, suggesting the potential for significant sleet. For this area, anticipate the slightly colder NAM solution is better, and will hedge in that direction which suggests impactful sleet and freezing rain across the area, which is also reflected by medium-high probabilities for moderate impacts due to ice in the PWSSI. WPC probabilities for more than 0.25" of ice are between 40-70% across the White Mts of NH and ME through 12z Thursday, with sleet accumulations greater than 0.5" also likely across parts of central and northern ME. WPC probabilities for greater than 4" of snow across northern ME are low, between 10-30%. Snell ...Key Messages for April 3-5 Major Winter Storm... --Blizzard Conditions in the Northern Plains Strong winds and lingering snow will continue to create dangerous driving conditions and significant drifting snow today throughout portions of the Dakotas and northern Minnesota. --Additional Snowfall Periods of moderate snow will continue across parts of eastern North Dakota and northern Minnesota this morning, leading to an additional 3-6 inches of snow. --Freezing Rain across the Upper Great Lakes Pockets of freezing rain early this morning will create icy conditions for portions of northern Wisconsin and the Upper Peninsula of Michigan. Isolated power outages and slippery travel is likely. --Life-Threatening Cold for Anyone Stranded Wind chills below zero throughout the Northern Plains will be dangerous for anyone stranded outside.