Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 353 AM EDT Mon Apr 17 2023 Valid 12Z Mon Apr 17 2023 - 12Z Thu Apr 20 2023 ..Upper Midwest and Western Great Lakes... Day 1... The significant late-season winter storm impacting portions of the Upper Midwest begins to wind down today. Early morning analysis across the region showed the strong storm system has become vertically stacked over eastern Wisconsin with radar imagery showing a continuation of steady deformation band snow affecting the western U.P. of Michigan southward through western Wisconsin, southeast Minnesota, and far northeast Iowa. With little movement expected over the next several hours, this band of moderate to heavy snow should persist through the mid to late morning hours, bringing an additional 1" to locally 4" over west-central Wisconsin (beyond 12Z today) while northeasterly flow lake enhancement off Superior (along with a cooling air mass supporting higher SLRs) will bring greater totals and potential 1"/hr rates to northwest Wisconsin and far western U.P. where the latest WPC snow probabilities are high for an additional 8" and slight for localized 12" amounts. Lighter snows (a couple inches) will also be likely downwind of Lake Michigan across lower Michigan as well as the upslope region of the central Appalachians of eastern West Virginia later today into tonight as the storm system slowly drifts eastward. ...Pacific Northwest and Northern California to the Northern Rockies... Days 1-3... Deep upper low positioned west of British Columbia this morning will move southeastward toward the Pacific Northwest today. Multiple embedded vorticity maxes will pivot around the low, bringing a few rounds of precipitation to the Pacific Northwest, Northern California and into the Northern Rockies over the next few days. The initial frontal passage early this morning will then be followed by another stronger frontal passage later this evening. This one will bring snow further south and east, reaching most of the OR/WA Cascades, into Northern California ranges, and northern Sierra Nevada as well as spilling eastward into the central Idaho ranges. For the day 1 period, the combination of the height falls, upslope flow and a cooler air mass will support several inches for the higher terrain from the Olympics, Cascades into the Klamath Mountains, Shasta-Siskyous and Trinity Ranges. Localized 8-12" totals will be possible. For Monday night through Wednesday night, a continued unsettled weather pattern will be found across the region with the upper troughing in place and additional series of embedded vort maxes rotating through. A stronger one is set to arrive Tuesday along OR/CA coast, quickly lifting northeast toward WY/MT. This wave will have sufficient forcing for ascent and a colder air mass in place to bring widespread snowfall to much of the region though amounts look to be on the lighter side and confined to the higher elevations. As the energy reaches the Plains, low pressure may deepen over North Dakota, with a deformation band of heavier snow possible across portions of eastern/northeast MT into western ND. There remains some uncertainty in the strength and placement of the shortwave energies coming out into Plains but for now, the best potential for 4" based off the WPC snow probabilities is for far northeastern Montana with probabilities of 10 to 25 percent. For Days 1-3, the probability of significant icing greater than 0.25" is less than 10 percent. Taylor