Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 308 PM EST Fri Mar 10 2023 Valid 00Z Sat Mar 11 2023 - 00Z Tue Mar 14 2023 ...The West... Days 1-3... Atmospheric river event will start to wind down on Saturday as the heaviest moisture influx ahead of the surface cold front pushes inland. Snowfall over the Sierra will be heavy this evening and into early Saturday with high snow levels above 8000ft falling below 7000ft by Sunday. 1-3 additional feet of snow is likely in the higher elevations. Refer to the Key Messages below for additional information regarding the expected impacts across this region. Across the Wasatch into the CO Rockies, the moisture plume will and upslope flow at the nose of a 100kt jet will yield modest to heavy higher elevation snow on Saturday, where WPC probabilities for at least 12 inches are >50% above about 9000ft. Farther to the north, an ex-closed low opening into a sharp shortwave trough will move across the Divide in tandem with an area of low pressure at the surface and a robust cold front. Periods of heavy snow and some strong gusty winds are expected as the front moves across the region this evening/tonight with embedded snow squalls. On D2 (Sunday), a nearly stationary upper low off British Columbia and continued westerly flow in CA will maintain an unsettled pattern over the West Coast. Heaviest snowfall will be over the Sierra where another 1-2ft are forecast for elevations above about 7000ft as snow levels waver in the 6000-7000ft range. By D3 (Monday), upper low off British Columbia will devolve into punctuated shortwaves as the mid-level flow backs a bit across much of the West Coast. While additional snowfall is likely for at least the northern Sierra (snow levels around 7000ft), the moisture focus will shift into the Pacific Northwest and the NorCal ranges. With higher heights, snow levels will remain fairly high, confining the threat for heavy accumulations to the higher elevations of the Shasta Cascade region and the northern Sierra. There remains uncertainty with any surface low development off the WA/OR coast, but the consensus favors the Olympics/Cascades for locally heavy, high elevation snow on Monday. There, WPC probabilities for at least 8 inches of snow are highest (>70%). ...Interior Mid-Atlantic/New York/western New England... Day 1... A closed low over the eastern Great Lakes will move east-southeastward past the 40/70 benchmark on Saturday. An area of low pressure crossing the Appalachians will weaken as a new low pressure center develops off the DelMarVa this evening/overnight and continues eastward. A rather wet snow amid marginal boundary layer conditions will progress eastward early Saturday with higher terrain areas of WV turning from rain to snow as enough colder air moves in behind the front. Higher elevations have the greater chance of accumulating snow, especially over the Catskills and into the Berkshires as well as the Poconos. There, WPC probabilities for snow accumulations of 4 inches or more are >50%. As the low moves east of 70W, an inverted surface trough extending into New England and western NY may contribute to some additional snows across northeastern Pennsylvania and Upstate New York before the precipitation ends later on Saturday. ...Northern Plains to Upper Midwest and Great Lakes... Days 1-3... As the D1 system moves out of the northern Rockies, heavy snow will develop quickly across portions of northeastern Montana this evening before shifting east across North Dakota and into the Red River Valley overnight into early Saturday. Mid-level shortwave will close off once again, and these strong height falls combined with potentially strong low-level warm air advection and sloped frontogenesis will contribute to a swath a moderate to heavy snow progressing steadily east across the region. Surface low pressure will slip ESE-ward and weaken, but the gradient to the north will maintain strong and gusty winds that will create blizzard conditions and contribute to hazardous travel. Bands of snow will create local min/max areas of snowfall, but WPC shows high probabilities for storm total amounts exceeding 8 inches from far northeastern Montana across northern North Dakota to the Red River, through northern MN and into the Iron Range/North Shore and Bayfield Peninsula. Please see our other Key Messages for more information. The system is expected to weaken as it moves into the Great Lakes D2. Snowfall will decrease over the Great Lakes, but continued light snow is expected around the northwest side of the circulation that may try to re-form over southern Ontario. As the upper trough/low moves through late Sunday into Monday, snow will continue over the Upper Great Lakes where an additional few inches are possible before winding down late Monday. WPC probabilities for at least 4 inches of snow on D3 are generally less than 50% except for a portion of the U.P. of Michigan. ...Northeast... Day 3... Miller-B type transition of the Great Lakes system will feature a coastal low developing near the Carolina coast and moving northeastward into D4. By the end of this forecast period (00Z/14), combination of approaching upper dynamics and increased moisture from the Atlantic will yield an expanding area of precipitation over the Mid-Atlantic into the Northeast. Temperatures will again be marginal, and higher elevations will be favored for accumulating snowfall. This will be over the Adirondacks, Catskills, Berkshires, and Poconos (into north central PA) where WPC probabilities for at least 4 inches of snow are moderate (>40%) with lower probabilities across the NY Southern Tier into the central Appalachians. For days 1-3, the probability of icing greater than 0.1" is less than 10 percent. Fracasso/Pereira ...Key Messages for Mid-March (Western U.S.) Winter Storms... --Significant Impacts Ongoing into Early Saturday: Heavy rain and snowmelt will continue widespread considerable flooding impacts below 5000 foot elevation along the central California coast, San Joaquin Valley, and southern Sierra Nevada foothills tonight into early Saturday. Heavy, wet snow will continue at higher elevations in the Sierra Nevada. --Occasional Precipitation this Weekend and Monday: Rain in low elevations and foothills, and snow at high elevations, will refocus in the northern half of California, northwest Nevada, and Oregon this weekend, with a slightly lower intensity. --Rain and Snowmelt Will Lead to Flooding: The most significant snowmelt, and overall flooding threat, is expected below 5000 foot elevation, in areas with shallow snowpack. Creeks and streams in the foothills of the Sierra Nevada will be most vulnerable to flooding from rain and snowmelt. --Heavy, Wet Snow at Higher Elevations: Higher elevations in northern California and in the Sierra Nevada will receive a heavy, wet snow. This will lead to difficult travel, and combined with an already deep snowpack, may lead to increasing impacts from the depth and weight of the snow. ...Key Messages for March 10-12 (Northern Plains/Upper Midwest) Winter Storm... --A winter storm will generate periods of heavy snow across portions of northern Montana, North Dakota, and into the Upper Midwest starting this evening and continuing into the upcoming weekend. --Heavy snow combined with strong gusty winds will produce blizzard conditions across portions of North Dakota. Scattered power outages are possible. --Snowfall totals greater than 6" are forecast from northern Montana to northern Wisconsin. Totals greater than 12" are most likely in northern North Dakota and in the Minnesota Arrowhead. --Snow-covered roads and reduced visibility will make travel difficult across impacted areas.