Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 437 PM EST Sun Feb 19 2023 Valid 00Z Mon Feb 20 2023 - 00Z Thu Feb 23 2023 ...Long-duration, widespread, major winter storm expands from the Northwest Monday night through much of the West and northern tier states through next week... ...North Dakota through the Northern Great Lakes and Interior Northeast... Days 1-2... A potent vorticity lobe on the leading edge of the strong NWly jet coming over BC will continue to shift southeast tonight from Alberta, across ND before turning easterly over MN and northern WI Monday and over New England Tuesday. This clipper is progressive, but given the enhancement from the left exit of the jet, low level fgen oriented in the direction of motion, and ample DGZ depth and saturation warrants a risk for 6" over portions of northern MN, particularly on the North Shore of MN where Lake Superior enhancement followed by wrap around banding should maximize snowfall. Day 1 snow probs are 10 to 30% for 6 or more inches on the central North Shore. The combo of warm air advection snow followed by lake enhancement brings high Day 1.5 probabilities for 4 or more inches over the Porcupine Mtns of the western U.P. with moderate probabilities. The low tracks up the St. Lawrence Valley with warm air advection over New England and low terrain-based probabilities for 4 or more inches on Day 2. ...West Coast to the Rockies... Days 1-3... A strong shortwave trough is carving into the strong northeastern Pacific ridge as it approaches the Gulf of Alaska this afternoon and will be the main weather feature for the CONUS through next week after reaching the PacNW Monday night. This evolution will drive rapid height falls, with NAEFS ensemble tables suggesting 700mb to 500mb height anomalies reaching -4 standard deviations over the much of the Intermountain West by Tuesday night. This amplifying trough will drive an arctic cold front southward out of Canada Monday night, with impressive 925-700mb fgen developing along and behind this front. This fgen will be enhanced by the favorable ageostrophic circulation that will develop aloft as a Pacific jet streak dives down the West Coast through Tuesday night and couples with a downstream jet streak over the Central Plains to the Great Lakes. This enhanced ascent, combined with increasing upslope flow behind the front, and the synoptic UVVs created by height falls/PVA will support widespread heavy precipitation, spreading from the Pacific Northwest through the Great Basin (with potential for snow squalls along the front across the Inter-mountain West), the full extent of the Rockies, and onto the north-central Plains Tuesday night spreading to the Midwest Wednesday. With moisture progged to reach +1 to +3 standard deviations thanks to confluent mid-level flow overlapped with the Pacific jet streak (that is sourced from Hawaii) to intensify moist advection, the stage is set for a major winter storm with heavy snow across much of the region. Snow levels will climb to 4000-6000ft tonight on the warm advection downstream of the digging trough, but will then quickly crash with the upper trough and behind the cold front Monday night through Tuesday night, reaching the surface/sea level for all areas outside of coastal and Central Valley of CA. Ahead of the main Monday night trough is a vort lobe moving southeast from BC that crosses central MT tonight into Monday with marginal thermals on the central MT Plains until the cold front moves in Monday night. WPC Day 1 snow probabilities for more than 12 inches are high for the WA Cascades and the northern Rockies of northern ID/western MT and northwest WY with moderate probabilities for 4 or more inches over the central MT foothills and generally not the plains. Day 2 snow probs for 12 or more inches are expanded farther south and higher than Day 1 for all but the Big Horns of WY with high probabilities over the WA/OR Cascades and Northern Rockies across ID, western MT and the Absarokas/Tetons/to the northern Wasatch. Probs for 4 or more inches spread over the Coastal Ranges of WA/OR and into northeast NV. 2-day snowfall across the higher terrain of the WA Cascades and Northern Rockies could exceed 5 feet, with widespread 2-3 feet across these ranges likely. During D3, the heaviest snowfall expands across nearly all of the Intermountain West as the upper trough moves overhead, and mid-level fgen begins to increase as the front sinks southward. The guidance has become a bit faster with this southern push, and WPC probabilities for more than 6 inches exceed 50% for nearly all the terrain above 3000 ft from the Olympics southward along the Cascades and into the northern CA ranges, as well as into the Sierra. Additionally, high probabilities encompass much of the terrain from the northern Rockies through the Great Basin and central Rockies. Locally 2+ feet of snow is likely above 3000 ft, and with snow levels falling to around 500 ft for the northwest, light accumulations should spread into the valleys as well. As a result of these low snow levels, there are moderate probabilities for 2 inches in the Santa Lucia Range south of Monterey, CA. As the western trough/low reaches its peak intensity Wednesday with 522dam heights over ID widespread heavy snow occurs across the Great Basin (with the threat of a snow squall with the cold front) and southern Intermountain West. The closed low currently stalled off the northern Baja ejects east Tuesday ahead of the approaching trough, crossing northern Mexico Tuesday night which effectively adds moisture and lift to the Southwest. Day 3 probabilities for 12 or more inches are moderately high across eastern NV ranges, all UT ranges, southern WY and western CO as well low probs over the Mogollon Rim in AZ. The eastern slopes of the WY Rockies are especially prone to heavy snow with an easterly component with upslope flow continuing behind the cold front as a 1040mb high stalls over northern Alberta and lee-side cyclogenesis gets strong off southeast CO, wrapping Gulf moisture in. There are low Day 3 snow probs for 12 inches or more over southeast WY. ...Northern Plains to the Midwest... Days 2-3... The digging trough over the Northwest Monday night allows lee-side cyclogenesis to develop near the MT/WY border by Tuesday which is along the Arctic cold front coming in the wake of the clipper. this allows moderate to eventually heavy snow to develop over the Plains of MT late Monday night/Tuesday before spreading east over the Dakotas and MN Tuesday and WI Tuesday night before intensifying as it settles south over the north-central High Plains Tuesday night. Then the southern stream system emerging from northern Mexico Wednesday allows a rapid expansion of precip east over the southern/central Great Lakes Wednesday that continues into or through Tuesday, making for a nearly continuous 36 to 48 hr snowfall over portions of the north-central Plains and Midwest. Day 2.5 snow probs for 6 or more inches are moderate over northeast SD and southern MN and then on Day 3/Wednesday they are high over the north-central High Plains of WY into western SD and northwestern Neb and moderately high over the rest of SD and southern MN into central WI. Again, snow then continues into or through Thursday, so much more and impactful snow is expected as highlighted in the Key Messages below. With the southern stream spreading in Wednesday, a stripe of wintry mix is expected with Day 3 ice probabilities for a tenth inch or more from east-central IA, along the WI/IL border and over southern MI. Notable sleet accumulations are likely just north of the freezing rain stripe too. Jackson ...Key Messages for Major Coast-to-Coast Winter Storm... --A major winter storm will spread a large swath of heavy snow from the West Coast, over the Rockies northern and north-central Plains, Midwest, through New England through Thursday night. --Confidence is high that this winter storm will be extremely disruptive to travel, livestock, and recreation in affected areas. --The combination of heavy snow and strong wind gusts may result in blizzard conditions in parts of the West and Midwest with power outages and tree damage possible. --Very cold temperatures are likely from the West Coast into the Northern Plains behind this system, with record lows and dangerous wind chills possible. The potential also exists for a flash freeze in portions of the northern Rockies on Tuesday. --Farther east, heavy snow and an icy wintry mix of snow, sleet, and freezing rain will impact the Great Lakes and Northeast later in the week. Treacherous travel conditions are possible in these areas.