Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 309 PM EST Fri Mar 03 2023 Valid 00Z Sat Mar 04 2023 - 00Z Tue Mar 07 2023 ...Upper Midwest into the Great Lakes and Northeast down into the Mid-Atlantic... Day 1... A mature cyclone will move through the Midwest into the Great Lakes and weaken as its triple point low forming over the Mid-Atlantic becomes the dominant surface low by early Saturday. The Miller-B evolution will spread and area of heavy snow and mixed precipitation across portions of the eastern Great Lakes and Northeast D1 before quickly moving out to sea. Early D1, on the northwest side of the parent low, an increasingly intense deformation band will translate along the NW edge of the precipitation shield through Lower Michigan. Sloped FGEN and strong UVVs will likely produce 1-3"/hr snow rates this evening per the WPC Snowband Probability Tracker. WPC probabilities for at least 4 inches of snow are high (>70%) over much of southeastern Lower Michigan. Farther to the east, strong WAA driving the meridional moisture transport will be most intense and lead to an expanding precipitation shield from the Mid-Atlantic through Upstate New York and much of New England. Elevated warm nose (~850-700mb) will cause p-type changes from snow to wintry mix to rain across PA and parts of western/central and mid/lower Hudson Valley in NY, northeast PA, and southern New England. The best chance for significant snow accumulations continues to focus across eastern Upstate NY/The Adirondacks into central and parts of northern New England where the leading WAA snow will transition to a colder snow with time as the triple point low takes over very near Long Island and southeast MA, maintaining the colder air mass. Higher elevations with easterly exposure are favored in this event including the eastern Adirondacks and into the Green and White Mountains. WPC snowfall probabilities for at least 8 inches are high (>70%) across these areas. For snowfall of at least 4 inches, probabilities are >50% over much of interior eastern NY and toward Lake Ontario through much of western/central MA and northeastward into the southern half of Maine. Totals may exceed 12-18" in some favored locations. South of this heavy snow, a region of moderate sleet and freezing rain is expected, especially in some of the terrain from the Potomac and Laurel Highlands through the Catskills where ice accretion may exceed a tenth or two. By Saturday evening the coastal low will have pulled far enough into the Atlantic with rapid dry advection occurring in its wake to bring an end to precipitation over New England. ...Pacific Northwest through Central California and the Intermountain West... Days 1-3... Building upper ridging into Alaska will Saturday will help dig troughing into the Pacific Northwest through the next several days with multiple waves of mid-level vorticity moving generally west to east as the main axis stall just off the WA coast. At the same time, a pronounced and intensifying Pacific jet streak, progged to reach 150+kt by Sunday, will overlap with the mid-level confluence to surge moisture into much of the West this period. The overlap of height falls, PVA, and left jet exit diffluence along the upper jet will provide widespread deep layer synoptic lift to support rounds of moderate to heavy snowfall each day, at times into lower elevations. Though the heaviest snow is forecast for the mountains, snow levels through the period will generally remain low (~1000-1500 ft west of the Cascades), and 500 ft or less to the east. As the lead cold front pushes into OR/CA Saturday, snow levels will surge as high as 3000ft over CA during the period of most intense WAA. Impactful snow is likely at many of the mountain passes with at least moderate accumulations in the coastal ranges and light snow likely even in the lowlands. As the upper trough settles into the West Coast, the focus for heavier snowfall will also trend southward in time. On D1, WPC snow probabilities for at least 6 inches of snow are high (>70%) in the WA Cascades and Olympics southward into northern CA and the northern Sierra. By D2, the heavy snow focus shifts farther south through the central to southern Sierra Saturday with the heaviest snow of the period (likely over 1-2ft in the Sierra) occurs. In addition, with mid-level vorticity streaming eastward across the Interior West, WPC probabilities for at least 6 inches of snow are high over the Wasatch and central Rockies. By D3, as the best dynamics move to the Plains and moisture abates, the northern Sierra has the highest probabilities (still above 70%) for at least 6 inches of snow with lower probabilities into much of the northern California ranges. In the lowlands, and especially D1, confidence is lower in heavier amounts greater than 4 inches but probabilities are non-zero. ...Northern Plains to Upper Midwest/western Great Lakes... Days 2-3... A lobe of vorticity ejecting from the Pacific coast will be aided by the left exit region of the eastward-translating 150+kt Pacific jet streak over the Intermountain West to increase lift across the Northern Plains north of a lee-side low on Sunday. Moist advection ahead of this low will lift up the MS Valley and into the Northern Plains as PWs surge to around 0.5in (+1 sigma) just north of a warm front. At the same time, a cold front will dig out of Canada in advance of a cold high pressure centered over the Northwest Territories, with increasing fgen beneath the upper jet supporting a stripe of moderate to heavy snowfall developing from west to east. Recent guidance remains in good agreement on an axis near and north of the SD/ND border D2 that will carry eastward into northern WI and the U.P. of Michigan D3. East to northeast flow atop the surface low will enhance snowfall into the North Shore and Iron Range in northeast MN where WPC probabilities for at least 6 inches are highest (>60%). Two-day probabilities for at least 4 inches of snowfall are greater than 50% along and just south of I-94 from western ND into MN then extending due eastward into the U.P. of Michigan. Fracasso/Jackson ...Key Messages for Midwest through Northeast Winter Storm... --A significant winter storm will move east from the Great Lakes this evening, producing a swath of heavy snow over portions of northern New York and New England tonight into Saturday. Accumulating sleet and freezing rain are likely south of the heaviest snow. --For portions of southern Lower Michigan, heavy snow rates of 1-3"/hr are likely this evening. Heavy snow and winds gusting to 45 mph winds will produce dangerous travel conditions. Isolated power outages are possible. --Across northern New York and New England, heavy snow rates of 1-2"/hr and winds gusting as high as 45 mph will reduce visibility and create hazardous travel conditions. Downed trees and power outages are possible. --Freezing rain and sleet are likely for interior portions of the Mid-Atlantic states into southern New England. This will produce slippery road conditions and difficult travel in some areas.