Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Tue Apr 18 2023 Valid 12Z Tue Apr 18 2023 - 12Z Fri Apr 21 2023 ..Upper Midwest... Days 1-2... Warm air advection late tonight into Wednesday morning will bring a round of precipitation to parts of Minnesota, central/northern Wisconsin, and into the U.P. of Michigan. A pocket of residual cold air will be in place such that light icing will be possible before changing over to rain. The latest WPC ice probabilities are above 50-60 percent for at least 0.01" of ice and reach near 20 percent for 0.1" across northwest WI and parts of northeast MN. Day 3... Mid to upper level troughing is expected to form a closed low over North Dakota by Thursday into Thursday night as a sharp shortwave trough tracks through the central Rockies. A coupled upper jet streak positioned over the central U.S. will then induce deepening cyclogenesis over the Midwest with the low track through Wisconsin. The increasing baroclinicity will drive a frontogenetical band of precipitation northwest of the low center across portions of central to northern/northeast Minnesota and as colder air pours into the region, a changeover to wet, heavy snow is increasingly likely Thursday evening into Thursday night. Probabilities for a swath of moderate to heavy snow have increased, the latest WPC snow probs show near 20 percent around Duluth to near 40 to 50 percent closer to MN/Canadian border for 6 inches. Trends continue toward a wetter and potentially colder solution and later updates may show potential for localized higher amounts. ...Pacific Northwest through Northern California to the Northern Rockies and onto the Northern Plains... Days 1-3... Upper level low off the British Columbia coast will continue to shed lobes of vorticity into the Pacific Northwest over the next several days. The combination of strong forcing, modest amounts of moisture, and colder air spilling eastward will help bring decreasing snow levels and locally heavy snowfall to the WA Olympics and the OR/WA Cascades and southward into the far northwest CA ranges. As the energy moves eastward, height falls and orographic flow will squeeze out locally heavy snowfall for much of central/northern Idaho and western Montana ranges and also much of western Wyoming and northern Utah. WPC probabilities for 6" or more top out at 60 to near 70 percent for these locations, greatest for the WA Olympics and Cascades. Then late Tuesday into Wednesday, the shortwave energy rounding the base of the main upper trough takes on a negative tilt as it swings through Wyoming and southeast Montana. Guidance over the last couple of days has trended stronger and a bit further south/southwest with this feature such that the deepening low pressure is expected over western North Dakota. A favored left exit region coupled with the deepening low and orographic upslope flow will bring locally heavy snowfall across much of north-central to eastern Montana and northwest North Dakota. Probabilities for 6" or more have increased for the higher terrain areas of north-central and northeast MT (locally 60-80 percent) with a broader area of 40-60 percent probabilities for 6" across northeast MT and northwest ND. The WY Bighorn Mountains also stand out for high probabilities for 6"+ through the day 1-2 period. Another piece of shortwave energy then slides across the region Wednesday into Wednesday night, continuing the unsettled and snowy regime for the region. Additional light to moderate snowfall is expected from the Central Rockies northward into the Northern Rockies where probabilities for 6" or more remain moderate (40-50 percent). Taylor