Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 412 PM EDT Thu Mar 30 2023 Valid 00Z Fri Mar 31 2023 - 00Z Mon Apr 03 2023 ...Intermountain West... Day 1... The latest water vapor imagery and 500 mb heights shows an impressively deep upper trough centered over the Intermountain West. The latest NAEFS shows the 500 mb height anomalies to be between 2 and 3 standard deviations below normal. This is providing sufficient vertical ascent aloft, enough anomalous moisture, and cold air to support continued periods of heavy snow from the northern Rockies through the Wasatch and central Rockies to the Mogollon Rim. The heaviest snowfall through tomorrow afternoon is favored over the Wasatch and central Rockies where the latest WPC snow probabilities for at least 6 inches are high (above 70%). The best chances for totals exceeding 12 inches are across the Wasatch east of Salt Lake City (50-70% in the latest PWPF) ...Pacific Northwest to Northern Rockies... Days 1-3... After a brief period of mid/upper level ridging over the region tonight through Friday, another shortwave trough will approach the region beginning late Friday. A rather lengthy jet streak (from the Aleutians to the western Oregon coast) will position itself for the favored left exit region lift. As the longwave troughing settles over the region through the weekend, a series of embedded shortwaves will move onshore, bringing with it a steady barrage of mid level moisture into the Pacific Northwest and northern Rockies. The first round of heavy mountain snow arrives Friday evening over in the Olympics and Cascades. Snow will pile up quickly in these ranges thanks to the mean flow oriented orthogonally to these mountain ranges. As much as 1-4 feet of snow will accumulate in these ranges through Sunday evening. The Day 3 WSSI is depicting Moderate to Major impacts in portions of these ranges. Farther east, the nose of the aforementioned jet streak will be aimed at the northern Rockies Saturday. Favorable forcing for ascent will be provided through the left exit region of the jet streak, the PVA associated with the incoming 500 mb vort maxes, and a long duration of upslope flow into some mountains. Heavy snow is expected from the Bitterroots, Sawtooth, Salmon River, and Boise mountains of Idaho, the northern Lewis Range of Montana, and the Tetons of western Wyoming and eastern Idaho. The latest WPC snow probabilities are high (70-90%) for >8" of snowfall in these aforementioned ranges this weekend. The Boise and Tetons feature the best odds for >12" of snowfall Saturday into Sunday. ...Northern Plains and Upper Midwest... Days 1-3... ...A multi-day powerful winter storm is forecast to unfold from the Dakotas and northern Nebraska to the Upper Great Lakes... The stage is set for a prolonged, disruptive winter storm across portions of the Northern Plains and Upper Midwest beginning later this evening through Saturday. This winter storm comes in 2 waves, the first being a large area of precipitation north of an advancing warm front tonight through Friday morning and then the main cyclone and its associated heavy snow threat Friday through Saturday. Robust 850 mb moisture transport will continue to nose over a slowly advancing warm front positioned over the Midwest today into tonight. An initial thump of heavy snow on the northern edge as well as a zone of mixed precipitation is expected. Snowfall on the order of several inches are expected from far northern Wisconsin and the U.P. of Michigan. South of that, a zone of wintry mix including freezing rain is likely to develop from parts of central Minnesota through central Wisconsin, where the latest WPC probabilities for ice accumulation greater than 0.1" is between 50-70% (locally near 80% across northeast Wisconsin). The other part of this system begins to ramp up considerably early Friday morning through Saturday as the main upper trough pushes out into the Plains. An area of low pressure deepens as it moves northeast from CO to Wisconsin early Saturday. A warm conveyor belt of 850-700mb moisture wraps around the 700mb low Friday afternoon and it is beneath this developing "Trough of Warm Air Aloft" (TROWAL) across portions of the Dakotas where the most intense snowfall banding and rates are expected. The TROWAL will pivot across central SD where both 1-2"/hr snowfall rates and wind gusts of 30-40 mph will track across the state and pivot northeast into eastern SD and central MN by Friday afternoon. As colder northeast winds strengthen Friday night via both the intensifying low over the Great Lakes and the dome of high pressure to the north, any precipitation that was originally a rain/wintry mix will transition to snow by Friday night and continue into Saturday. The U.P. of MI and northern WI will not only have the TROWAL to contend with, but also some lake enhancement that could drive snowfall rates above 2"/hr at times. Wind gusts from the Upper Mississippi Valley to the Upper Great Lakes could range between 35-45 mph with peak gusts above 50 mp through Saturday morning. The storm races into southeast Canada by midday Saturday, which aside from lingering lake effect snow showers, will mean the storm effectively comes to an end in the Great Lakes by Saturday afternoon. In terms of impacts, there are several factors to take into account that can be broken down by utilizing the experimental PWSSI. 1.) Snow Rate. With the intense banding and vertical velocities supported across just about all guidance, snow will be able to accumulate quickly in areas where hourly snowfall rates are above 1"/hr, even during the daytime hours (although night time will be the best chance for rapid accumulations). There are currently 70-90% probabilities for Moderate impacts via Snow Rate from central SD and northern NE to northern Michigan and is the primary driver in the PWSSI. Even a slight signal (10-30%) for Major impacts in Snow Rate are found across portions of central SD. 2.) Snow Load. With such an impressive fetch of moisture from the Gulf of Mexico and off the Great Lakes themselves, parts of central/northern WI and northern MI feature up to 70% chances for Moderate impacts from the weight of the snow on trees and power lines. There are also 30-40% chances for Major impacts as a result of Snow Load in these areas Friday night into Saturday. 3.) Blowing Snow. The wind gusts mentioned earlier will undoubtedly lead to significantly reduced visibilites on roads with blizzard conditions causing drifting snow and power outages. The pWSSI for Moderate to Major impacts due to Blowing Snow reach 20-30% across portions of central South Dakota. In areas where snow load is a problem, the wind will only compound the risk for tree damage and power outages in the upper Great Lakes. There is still lingering uncertainty in track and where the TROWAL sets up, but confidence is growing in a major winter storm that produces numerous travel delays, closures, and cancellations from the northern Plains to the upper Great Lakes Friday and into Saturday. Key Messages for this storm system are below. Mullinax/Taylor ...Key Messages for Mar 30 - Apr 1 Winter Storm... -A powerful winter storm is expected to track across the central High Plains and Great Lakes through Saturday. -A combination of heavy snow rates (1-2â€/hr) and strong wind gusts as high as 50 mph may cause blizzard conditions from portions of the Dakotas to western Minnesota. -In the Great Lakes, similar wind gusts along with a heavy and wet snow may cause extensive tree damage and power outages. -There is also the potential for treacherous ice accumulations from eastern South Dakota to central Minnesota and northern Wisconsin. -Hazardous travel conditions are expected in impacted areas that could include snow covered roads, near impossible travel from zero visibility and whiteout conditions.