Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Mon Mar 31 2025 Valid 00Z Tue Apr 01 2025 - 00Z Fri Apr 04 2025 ...The West... Days 1-3... Elongated Pacific jet coming into NorCal/northern Sierra this afternoon will transition towards a more amplified pattern over the next day or two as ridging builds into the Gulf of Alaska, favoring downstream troughing into the Western CONUS. Heavy snow is likely for the northern/central Sierra with major to locally extreme impacts. From the Great Basin eastward, a wide area of snowfall is forecast for the Interior West with modest snows for the northern/central Rockies. The day 1 period features the system that already moved onshore with heavy snow into the Sierra decreasing a bit by tomorrow morning behind the trough axis, but the pattern will support waves of vorticity moving into/through the West Coast as the jet digs farther south. Snow will quickly expand to the central Rockies tonight in the core of the moisture plume that reaches near the 90th percentile into the San Juans. Snow levels between 5000-8000ft to start (this afternoon at 00Z) will continue to fall behind the cold front to around 4000ft or so (Sierra into the Great Basin) but below 3500ft over the Cascades. By day 2, the trough axis will broaden out as the downstream northern Plains system develops, favoring lighter snowfall for much of the Interior West. The exception may be around SW Montana into the Bighorns near the backside of the developing upper low (again, tied to the northern Plains system). Snow will also expand southward as the jet dips into SoCal and central AZ/NM with some snow to the Mogollon Rim. By day 3, the upper ridge over the Pacific will start to drift to the West Coast, helping to lessen QPF/snowfall for the Cascades to the Sierra. Inland, northern stream height falls are forecast to move southward out of western Canada into the northern Rockies which will favor some modest snowfall over NW MT aided by upslope enhancement. To the south, continued SW flow across the Four Corners will yield some light to modest snow for the San Juans to the northern Sangre de Cristos northward to the Sawatch Range. Total snowfall over the three-day period will likely be highest over the northern/central Sierra and over parts of the northern Rockies in MT. For the Sierra, WPC probabilities for at least 18 inches of snow are high (>70%) above 5000-6000ft or so (from north to south) Inland, WPC probabilities for at least 12 inches of snow are above 50% above about 5000-6000ft (MT), 7000ft (Tetons), 8000-9000ft (Wasatch), and 10,000ft (CO). Fracasso ...Northern Plains, Upper Midwest, & Northeast... Days 2-3... The upper trough responsible for the barrage of mountain snow in the West will position the 250mb jet streak's divergent left-exit region over the Great Plains. Modest 500mb PVA and strengthening WAA at low levels will give rise to lee cyclogenesis east of the Rockies on Tuesday. Throughout the day Tuesday, a strengthening LLJ, (NAEFS shows 850mb winds exceeding the 97.5 climatological percentile 18Z Tuesday from north Texas to eastern Nebraska), will deliver both rich moisture and increased levels of WAA aloft. The upper trough to the west will also direct an IVT of 300-500 kg/m/s into the Upper Midwest, which is approaching the 97.5 climatological percentile by late afternoon. This causes strong 300K isentropic ascent and 850-700mb WAA resulting in FGEN at those mandatory height levels. There also appears to be sufficiently cold enough air present, thanks to a nearby dome of Canadian high pressure, for precipitation to transition over from rain to snow from the Black Hills on east across South Dakota and as far east as central Minnesota Tuesday afternoon. Given the early-April sun angle, snowfall will struggle to accumulate during the day on Tuesday unless rates are >1"/hr. These rates appear most likely to occur Tuesday night once the deformation zone on the northern and western flanks of the 850mb low consolidates, allowing for treacherous travel conditions to rapidly unfold into the overnight and into Wednesday. Even as we are now inside of 48 hours from the peak of the event, there are still some ongoing difference with respect to some details. The GFS/CMC suite remains on the northern envelope of track solutions, while the ECMWF/EC-AIFS are on the southern flank. When accounting for ensemble guidance, which takes into account snowfall between Tuesday afternoon and Wednesday night, most guidance favors an area of heavy snowfall extending from northeast South Dakota and southeast North Dakota across much of northern Minnesota. The latest ECMWF EFI has shifted a strong signal (values of 0.8-0.9) for a potentially disruptive winter storm across much of northern Minnesota, but still sports modest 0.7-0.8 values in eastern North Dakota. The Northern Plains and Upper Midwest will contend winter hazards such as heavy snow, blowing snow, and snow load on tree branches and power lines starting late Tuesday and continuing through Wednesday. Current WPC snowfall probabilities (through 00Z Thursday) for >8" are moderate-to-high chance (50-70%) across northeast SD, southeast ND, and northern Minnesota. High probabilities (70-90%) for >8" exist across the MN Arrowhead. There are also low probabilities (10-30%) for snowfall totals >12" in this region as well, with the highest and up to 50% in northeast South Dakota and the MN Arrowhead. In terms of ice, freezing rain is likely on the eastern edge of the WAA across northern Wisconsin, northern Michigan, and even as far east as far northeast Pennsylvania and western New York. Most ice accumulations will be <0.1", with the lone exception being northern Michigan where there are low-to- moderate chances (20-50%) for ice accumulations >0.1" on Wednesday. Residents and those traveling to/from these regions should continue to monitor the forecast as additional changes in the types of impacts, the severity of those impacts, and where/when these impacts occur can still fluctuate. Farther east, the aforementioned Canadian air-mass will be departing off the Atlantic Canadian Maritime Wednesday night, but boundary layer wet-bulb temperatures remain sufficiently cold enough for some wintry precipitation in northern New England Wednesday night and early Thursday morning. The WAA is quite strong and the air-mass in marginally cold, so ice accumulations will be limited to the northern Appalachian ranges (Adirondack, Green, White) and across northern Maine. WPC probabilities show low-to- moderate chances (10-40%) for ice accumulations >0.1" through Thursday morning. By midday Thursday, rain will be the primary precipitation type throughout most of northern New England (far northern Maine the lone exception). *Key Messages have been issued for this winter storm, and the link to view them is posted below. Snell/Mullinax ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png