Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 310 PM EST Wed Feb 22 2023 Valid 00Z Thu Feb 23 2023 - 00Z Sun Feb 26 2023 ...Long duration, major coast-to-coast winter storm continues... ...Unusually cold and strong winter storm will bring heavy precipitation to California... ...Four Corners through the Northern Plains, Upper Midwest, into the Northeast... Days 1-3... The major winter storm will impact areas from the Four Corners region to the Northeast over the next 48 hours, with widespread heavy snow producing major to extreme impacts and potential for significant ice accumulations. The core of this major winter storm will unfold over the next 24 hours, with a tremendous increase in coverage, rates, and amounts of snowfall over the High Plains and Upper Midwest. Currently deep troughing is across the Four Corners, which combined with a coupled upper jet streak, is providing the broad, large scale forcing for ascent across the region. This is leading to downstream low pressure development over the Plains, which will continue to deepen as it tracks to the northeast. Really impressive moisture is being advected northward and wrapped into this system, characterized by PW values reaching +2 to +3 standard deviations above normal. This moisture will wrap within the warm conveyor belt into a robust TROWAL, supporting elevated instability while the warm air advection deepens the DGZ. This combination will support really intense snow rates, possibly up to 2"/hr, as shown by the WPC Snowband Probability Tracker. Confidence is pretty high for an exceptional swath of heavy snow stretching from the eastern SD through southern/central Minnesota and central Wisconsin. WPC probabilities for more than 6 inches are above 80% from western MN through central WI, as well as the U.P. of Michigan, and northern L.P. of Michigan. 10-15" is possible where the best banding occurs, which is most likely across the Coteau des Prairies and Buffalo Ridge, eastward through the Twin Cities, and then extending across northern WI, with LES enhancement leading to locally higher snowfall along the Bayfields Peninsula. Farther to the east, and just south of the heavy snow areas across the Upper Midwest and Great Lakes is the signal for an impactful and significant ice storm. A potent warm nose with 850mb temps > 0C will lift well northward, and as a piece of the elongated Canadian high pressure arcs eastward towards northern Quebec and Ontario, an overrunning/wedge scenario appears to be unfolding resulting in a mixed precipitation event of snow, sleet, and freezing rain through tonight. From central IA through southern WI, northern IL and particularly southern MI, a band of heavy freezing rain is likely, and guidance has become a bit more aggressive with ice and also features a more southern push in the heaviest axis, likely due to the previously noted deep cold layer below the warm nose to support heavy sleet in some areas. However, elevated instability and tremendous moisture advection resulting in heavy rain rates should limit accretion efficiency despite a continued push of dry-bulb temps on low-level northeast winds. Still, freezing rain exceeding 0.25" and potentially reaching 0.5" is most likely in a stripe from eastern IA through norther IL and especially southern lower Michigan, with Chicago, Grand Rapids, and Detroit all possibly seeing a significant ice storm. Heavy sleet exceeding 1" is likely just north of the heaviest freezing rain. Even farther downstream, on the leading edge of the warm advection push across the interior Northeast and New England, a rapid transition from heavy, wet snow to mixed precip is expected this evening into tonight. The best signals for impactful ice accumulations looks to be across western/interior New York, northern PA and into parts of New England where probabilities of 0.1" or greater reach 50-80%, highest near Buffalo, NY. Farther north, heavy snow is expected, with WPC probabilities featuring a high risk for more than 8 inches in the Adirondacks, northern Greens, and Whites, with slightly lower probabilities extending across much of Maine. Lighter snow and ice accumulations are likely across much of southern New England. See additional details in the Key Message bullets below. ...Western U.S.... As the main shortwave energy currently over the Four Corners region lifts toward the Plains later today and tonight, a secondary piece of the longwave troughing over the Western U.S. will begin to sag southward, gradually closing off at 500 mb as slides down the California coast Friday into Saturday, eventually becoming cut off from the main northern stream flow. The height anomalies with this system approach -3 to -4 standard deviations below normal while a strengthening 300 mb jet (150 kts) will help provide the large scale forcing for ascent downstream of the low center. In the lower levels, favorable onshore flow directed nearly orthogonal to the terrain, especially across Southern California will direct a plume of higher moisture characterized by precipitable water values greater than 0.75". However, the colder air mass brought southward will lead to unusually low snow levels, down to 1000-15000 ft at times, will bring heavy snowfall to much of the terrain areas but also lower/valley locations. For the Day 1 period (through 00Z Friday), the greatest snowfall is expected across the coastal ranges of central/southern Oregon, down into the northwest CA coastal ranges where upwards of 8-12" or more is possible. The latest WPC probabilities show high signals for at least 8" from northwest CA to southwest OR coastal ranges with the highest peaks likely to see a foot or more of snow. By the end of day 1 period, the heavy snow should reach the northern Sierra ranges where upwards of 12-20" is likely as depicted by the high probabilities in the latest WPC PWPF. By Friday night into Saturday, the closed low becomes more intense driving the higher moisture plume directly into the southern Sierra and Transverse/Peninsular ranges, but the WAA should drive snow levels back towards 3000 ft. Still, impressive moisture being acted upon by strong ascent will result in widespread heavy snowfall, and it is likely that multiple feet of snow will occur in the terrain, with current forecasts suggesting upwards of 5-6 feet possible in some areas. Heavy snow will likely spillover into Nevada as well where WPC probabilities for more than 8 inches are 50-80%. This event will become extremely impactful for parts of southern CA, and pWSSI already indicates a high chance for extreme impacts due to snow amount and snow load for the San Gabriels. See additional details in the Key Message bullets below. Weiss/Taylor ...Key Messages for Major Coast-to-Coast Winter Storm... -The major winter storm will continue to impact areas from the Plains to the Northeast through Thursday night. Expect widespread impacts to travel, infrastructure, livestock, and recreation. -Heavy snow will impact areas from the Northern Plains to the Upper Midwest through Thursday morning. Plan on heavy snow rates of 1-2"/hr and wind gusts of 40-50 mph, which will create blizzard/whiteout conditions, nearly impossible travel, power outages, and scattered tree damage. -A stripe of freezing rain is also expected across parts of the Upper Midwest and Great Lakes. Some localized ice accumulations greater than 0.25" are possible, especially from eastern Iowa through southern Wisconsin, northern Illinois, and southern Michigan. This will lead to treacherous travel conditions and scattered power outages in the region of heaviest freezing rain. -Across the interior Northeast and New England, a quick burst of heavy snow is expected followed by a period of mixed precipitation including sleet and freezing rain. Heavy snow and ice accumulations up to 0.25" are likely to create difficult travel conditions and scattered power outages. ...Key Messages for California Winter Storm... -An atmospheric river will bring in a surge of moisture through the end of this week as cold temperatures remain over the region. -Heavy rain and mountain snow are likely Thursday into Friday with significant impacts to travel and infrastructure. Lower snow levels may bring significant snowfall to areas where snow is infrequent. -Windy conditions through the state may lead to blizzard conditions in some mountainous areas and mountain passes. Blowing dust is possible in desert areas. -Temperatures will be well below normal over much of the region, with below freezing readings possible in some coastal areas in NorCal. -Strong rip currents and high surf will be hazardous to swimmers and beach goers.