Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 414 PM EST Wed Jan 10 2024 Valid 00Z Thu Jan 11 2024 - 00Z Sun Jan 14 2024 ...Great Lakes and Northeast... Day 1... Shortwave over the Corn Belt this evening will shear eastward D1 as an area of low pressure zips into the eastern Great Lakes. CAM guidance indicates a narrow band of snow over northeastern IA with potential for several inches of snow. WPC probabilities of at least 4 inches are low (10-20%) but some more bullish models show over 6 inches. To the north, another shortwave with a slowing area of low pressure/trough axis over the Arrowhead will support some localized modest totals northeast of Duluth, where WPC probabilities for at least 4 inches are above 50%. East of Lake Ontario, continuing lake effect into the Tug Hill will support several more inches of snow on Thursday with additional accumulation from the approaching shortwave. ...Central Plains/Corn Belt/Upper Midwest/Great Lakes to the interior Northeast... Days 2-3... Deep troughing over the Southwest Thu evening will swing into the Mid-South and take on a negative tilt as a double-barreled jet becomes more buckled and S-shaped across the Great Lakes. This will drive strong upper level divergence and rapid cyclogenesis at the surface with a deepening area of low pressure lifting from the Ozarks into the Midwest and into eastern/southeastern Lower Michigan (strikingly similar to the exiting system in strength and track). Snow will increase in coverage and intensity across the Corn Belt into the Upper Midwest/western Great Lakes on Friday and slowly wind down into Saturday with lingering lake-effect snow over the U.P. into northern Lower MI. Additional northern stream shortwave over the central Plains will help bring light snow across the top of the northern extent of the developing precipitation shield early Friday across the Corn Belt, as Gulf moisture streams northward ahead of the cold front on strong 60kt 850mb flow across the OH Valley. PW anomalies around +1 (values ~0.50") will nose into the Corn Belt and near +2 sigma around 0.75" near the rain/snow line. Initial WAA into colder air will likely support some front-end snow across parts of the Midwest before a changeover to rain in areas southeast of the track of the low. Models over the past day or so have trended a bit farther northwest with the system and have adjusted the heavy snow axis and rain/snow line northwestward as well. This yields significant snowfall on the northwest side of the low as a deformation band forms and slowly pivots across the area, coincident with an intense TROWAL as the WCB wraps into the low. High-end potential is in excess of 20 inches per the NBM probabilistic guidance, driven by strong UVV into a sufficiently deep DGZ. CAM guidance will offer more details on placement and amounts once they fully come into view tomorrow, but for now WPC probabilities of at least 12 inches of snow for this storm are moderate or higher (>40%) from eastern IA across southern and eastern WI/northern IL across much of central/northern Lower Michigan. Within this area, there are also moderate (40-70%) probabilities of at least 18 inches over eastern WI into portions of northeastern Lower Michigan. As the low lifts into Canada, wrap around northerly to NNW flow will support lake-effect snow in the U.P. snow belts into northwestern Lower Michigan. On the eastern side, cold air mass in place will slowly be eroded as broad southerly flow once again erodes sub-freezing temperatures. The uneven erosion in the column will likely lead to a period of snow to freezing rain to rain in most areas outside northern Maine as a triple point low moves across the area. Before the changeover, several inches of snow will likely accumulate over interior portions of the Northeast, where WPC probabilities of at least 4 inches of snow are at least 50%, and highest over the Adirondacks into the Green and White Mountains. ...The West... Days 1-3... A busy and wintry pattern looks to continue through the end of the week and linger into the upcoming weekend. The atmospheric culprits responsible are a series of shortwave troughs; the first tracking through the northern Rockies and northern Great Basin today, a second and more elongated disturbance along the British Columbia coast tonight and into Thursday, and a more compact Pacific storm system approaching the Pacific Northwest coast Friday night into Saturday. As the first storm system races east through the Intermountain West tonight, a strong cold front and surge in IVT (>90th climatological percentile according to ECMWF in portions of central/southern California and the Southwest overnight) will accompany it to generate heavy snowfall rates from the Sierra Nevada and Wasatch to the central and southern Rockies. Latest WPC PWPF sport high chances (>70%) for additional snowfall totals >6" in the Wasatch of central Utah and Gila Mountains of eastern Arizona. Similar high chances (>70%) are present for >8" amounts in the Sierra Nevada, the Elk Mountains of west-central Colorado, and the San Juans and Sangre De Cristo of the southern Rockies. The Sierra Nevada and San Juans have moderate odds (50-60%) for snowfall totals through Thursday afternoon. In terms of impacts, the Probabilistic WSSI (WSSI-P) show high chances (>70%) for Moderate Impacts from just south of the I-80 pass in the Sierra Nevada down the spine of the range itself, as well as in the higher ranges of western Nevada and the Gila Mountains of eastern Arizona. The second shortwave trough over British Columbia will happen to have the most frigid and bitterly cold air-mass of the winter season to date at its disposal as a strong Arctic slowly advances south through northern Washington, the northern Rockies, and Northern High Plains Thursday afternoon and into the interior Northwest on Friday. Northwesterly flow off the coast of British Columbia will strengthen as the longwave upper low over Alberta and an expansive ridge cresting as far north as the Arctic Circle help funnel Pacific moisture into the Northwest and up the Columbia River Gorge on Thursday. With a strong dome of Arctic high pressure (possibly approaching record high pressure observations for the time of year in southwest Canada), a plume of moisture running into the boundary, and the added help of strong topographic ascent along ranges oriented orthogonally to the mean flow, heavy snow is likely to unfold in the mountain ranges of the Northwest. Latest WPC PWPF shows high chances (>70%) for >12" of snowfall in the Cascades, the Blue Mountains, and portions of the Bitterroots and Boise Mountains. In fact, this stream of moisture will become advected as far inland as the Wasatch throughout the day on Friday and lingering into Friday night. WPC PWPF shows high chances (>70%) for >12" of snow in the Wasatch on Friday. In addition to the heavy snow, strong wind gusts could result in blizzard conditions across some portions of the interior Northwest. WSSI-P shows as much as a 20-40% chance for Minor Impacts due to Blowing Snow in south central Idaho Thursday night and into Friday afternoon. A vorticity maximum shearing off from a long wave trough over the northeast Pacific will develop an area of low pressure that tracks towards Oregon coast Saturday morning. This storm system will tap into a conveyor belt of moisture that has origins into the subtropics. The combination of anomalous moisture and a strong 850mb jet will lead to IVT values topping the 97.5 climatological percentile being directed at southern Oregon and norther California. This is a recipe for heavy snowfall in the Oregon Cascades and northern ranges of California. Farther north, there remains uncertainty in where the tight norther gradient in QPF occurs. If a more northern storm track can occur, it could mean significant snow for parts of the northern Willamette Valley. If the track is suppressed to the south, lesser impacts as far north as the Portland and Tacoma metros would occur Friday night into Saturday. Confidence is highest in heavy snowfall accumulations along the warm front from southern Oregon and northern Nevada to the Wasatch as they also are coincidentally at the nose of the strong IVT plume emanating out from the northeast Pacific. The WSSI-P shows moderate chances (40-60%) for Moderate Impacts in the Oregon Cascades, the Trinity/Salmon of northern California, and the Wasatch. WPC PWPF sports moderate-to-high (60-80%) for >12" of snowfall on Day 3 for these ranges as well. Similar probabilities exist in northern Nevada where there is higher confidence in totals topping 8". In summary, many of the mountain ranges throughout the West can expect rounds of heavy and impactful snow to end the work-week and open the upcoming weekend. Fracasso/Mullinax ***Key Messages for Jan 8-13 Major Winter Storm*** -Snow and Blowing Snow Spread South across the Sierra Significant snowfall accumulations of 1-2 feet are likely (70-90%) in the central Sierra through tonight. Strong winds of 40-50 MPH during the heaviest snow will produce difficult travel. Snow levels will lower from about 3000-4000 feet to 2000-3000 feet by early Thursday. -Snow Squalls over the Great Basin, Wasatch, Four Corners Snow squalls will accompany a cold front through early Thursday. Intense bursts of snow and gusty winds can produce rapid changes in visibility, occasional whiteouts, and possible flash freezes on roadways posing significant hazards to motorists. -Next Great Plains Winter Storm Ramps Up Thursday The disturbance will emerge into the Plains Thursday and strengthen into another potent low pressure system that will track across the Mid-Mississippi Valley through Friday. This major storm system is forecast to produce a swath of heavy snow from the Central Plains through the Great Lakes. The probability of snowfall exceeding 4 inches is above 70% (likely) from eastern Nebraska through lower Michigan. Considerable blowing and drifting snow will result in dangerous travel. -Much Colder Air Mass to Follow A very cold air mass will begin to infiltrate the northern Rockies and Northwest U.S. in the wake of this disturbance over the next few days. Temperatures are forecast to be 20 deg to more than 40 deg Fahrenheit below normal from northern Washington and Idaho and Montana to Kansas by Friday.