Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 330 AM EST Sat Mar 11 2023 Valid 12Z Sat Mar 11 2023 - 12Z Tue Mar 14 2023 ...The West... Days 1-3... The ongoing atmospheric river event will begin to wane today as the deepest moisture ahead of the surface cold front moves inland. Periods of heavy snowfall, with rates of 1-3"/hr will continue along the Sierra, as snow levels drop below 7000ft by early Sunday. Across the higher elevations, additional accumulations of 1-3ft are likely today into early tomorrow. Refer to the Key Messages below for additional information regarding the expected impacts across this region. Across the southern Wasatch into the Colorado and northern New Mexico Rockies, the moisture plume and upslope flow on the nose of a 100kt jet will yield modest to heavy higher elevation snow on Saturday, where WPC probabilities for at least 8-12in are >50% above about 9000ft. On Sunday, a nearly stationary upper low off British Columbia and continued westerly flow to its south into California 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 Monday, the upper low off British Columbia will transition to an open wave and dig south -- backing the flow along the Northwest 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 into Monday morning. The upper trough is forecast to move inland, pushing another well-defined front through the Pacific Northwest during the day on Monday before reaching the northern Rockies Tuesday morning. This may produce another period of heavy snow across the region. Snow levels are forecast to drop in the wake of the front, however waning moisture is expected to limit the potential for any heavy snow accumulations across the lower elevations. ...Northern Plains to the Upper Midwest and Great Lakes... Days 1-2... A mid-level shortwave will continue to deepen as it moves east of the northern Rockies into the High Plains this morning. These strong height falls combined with low-level warm air advection and sloped frontogenesis will contribute to a swath of moderate to heavy snow (1-2"/hr) progressing steadily east across North Dakota into the Red River Valley this morning. Surface low pressure will slip east-southeastward and weaken, but the gradient to the north will maintain strong and gusty winds that will create blizzard conditions and contribute to hazardous travel today across the region. Bands of snow will create local min/max areas of snowfall, but WPC shows high probabilities for additional accumulations of 6in from northeastern North Dakota to northern Wisconsin. The highest probabilities are centered along the North Shore, where easterly flow ahead of the low is expected to help support lake-enhanced totals of a foot or more. Please see our other Key Messages for more information. The system is expected to weaken as it moves into the Great Lakes on Sunday. Snowfall will decrease over the Great Lakes, but continued light snow is forecast 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. ...Northeast... Day 3... A Miller-B type transition of the Great Lakes system will feature a coastal low developing near the Carolina coast on Monday and moving quickly north Monday night and Tuesday morning. Beginning Monday night and continuing into Tuesday, a 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 along the coast, however significant snowfall is likely to develop over the higher elevations of northeastern Pennsylvania, eastern Upstate New York and central New England. WPC guidance shows high probabilities for snow accumulations exceeding 8in over portions of the Catskills, Adirondacks, Berkshires, and southern Green Mountains by Tuesday morning. For days 1-3, the probability of icing greater than 0.1" is less than 10 percent. Pereira/Fracasso ...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 continue to generate periods of heavy snow across portions of the Northern Plains and Upper Midwest this weekend. --Heavy snow combined with strong gusty winds will produce blizzard conditions across portions of northeast Montana, North Dakota, and northwest Minnesota. Scattered power outages are possible. --Snowfall totals greater than 6†are forecast from northern Montana to the U.P. of Michigan. Totals greater than 12†are most likely in northeast North Dakota and in the Minnesota Arrowhead. --Snow-covered roads and reduced visibility will make travel very difficult across impacted areas.