Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 20 2023 09:59:27 FOUS11 KWBC 200959 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 458 AM EST Mon Feb 20 2023 Valid 12Z Mon Feb 20 2023 - 12Z Thu Feb 23 2023 ....Long-duration, widespread, major winter storm expands from the Northwest Monday night through much of the West and northern tier states through next week... ....Northern Great Lakes through New England... Days 1-2... Broad cyclonic flow enveloping much of the eastern 2/3 of the CONUS will become reinforced by a shortwave trough digging out of Alberta Monday morning. This trough and the associated strung-out vorticity lobe will surge eastward through the base of the trough over the Great Lakes by Tuesday morning, and then pivot northeast across New England Tuesday night into Wednesday. During this evolution, the shortwave is expected to deepen and take on a negative tilt, and guidance has become more intense with this feature since last night. Initially this shortwave will yield a weak clipper type surface low ejecting along the international border with Canada, and amplification of this wave is expected to be minimal due to modest height falls/PVA, being well displaced from the best upper jet streaks, and remaining progressive in subtly confluent flow. A lack of robust meridional moisture transport noted by weak IVT and PW anomalies around to slightly below normal, also suggest the accompanying precipitation from MN through the Great Lakes will be modest. However, a cold column and well aligned 280-285K isentropic ascent with the deepening DGZ should produce at least a brief period of moderate to heavy snow across northern MN and the northern Great Lakes on D1, with some enhancement likely into the Arrowhead of MN due to lake enhancement and upslope flow into the Iron Ranges, with an additional maxima of snowfall along the western U.P. where additional lake enhancement occurs as the low pulls away with accompanying CAA. WPC probabilities for more than 6 inches have increased to 50-70% in narrow corridors along the shores of Lake Superior, where locally up to 10 inches is possible. Otherwise, D1 snowfall should be generally less than 4 inches. On D2 /Tuesday/ the deepening shortwave taking on the negative tilt combined with subtly enhanced diffluence along the LFQ of a surging subtropical jet streak moving into the Atlantic will result in secondary low pressure development along or just south of New England. This is a relatively new development, but is supported by most global models, and it is likely that dual surface waves will move invof New England Tuesday. This secondary low will lead to better downstream moisture advection noted by NAEFS IVT exceeding 200 kg/m/s as 290K isentropic upglide intensifies with mixing ratios of 2-3 g/kg into New England. While the thermal structure of the column is progged to be marginal for wintry precip across southern/central New England, it is likely this secondary low will allow somewhat colder air to remain entrenched even as the surface high becomes displaced to the east. This has led to higher confidence in moderate accumulating snow across northern New England, with some potential for light accumulations even into southern New England and coastal Maine. This low will remain progressive, but WPC probabilities for more than 4 inches reach as high as 50% in the terrain from the Tug Hill Plateau through the Adirondacks and into the Green/White Mountains, with 10-30% chances encompassing much of northern New England. ....Western United States through the Northern Plains, Upper Midwest, into the Northeast... Days 1-3... A complex evolution of the mid-level pattern this week will yield widespread heavy snow across nearly all of the West, with an impressive snowstorm developing over the Northern Plains on Tuesday. Lowering snow levels, strong height falls, and well above normal moisture will combine to produce a major winter storm with far ranging and long lasting impacts for much of the region. The event begins to take shape today as a potent shortwave trough dives from the Gulf of Alaska and advects towards the Pacific Northwest coast embedded within confluent NW flow around a broad and expansive mid-level trough centered near the Hudson Bay. This shortwave and the accompanying vorticity lobe will surge into Washington state overnight Monday into Tuesday and then amplify into a closed low over the northern Great Basin by Wednesday morning. Around this closed feature, which is progged to have height anomalies that could reach -4 standard deviations below the climo mean according to the NAEFS ensemble tables, potent sheared vorticity will rotate, producing additional ascent through PVA, which will already be impressive in response to height falls and downstream divergence. This closed low is progged to reach peak amplitude over the Great Basin Wednesday morning, before slowly beginning to fill and lift northward towards Canada. However, as this occurs, split shortwaves will emerge and deepen, dumb-belling around the filling center, to drive more intense local ascent both into the Plains, and along the Pacific Northwest coast, at the end of the forecast period. While there continue to be, as expected, placement differences among the global guidance as to where these features will setup, the general trend is well agreed upon and confidence is high in a major event this week. As this mid-level trough evolution occurs, strong Pacific jet streaks, both a southern and northern stream, will merge across the High Plains and into the Central Plains, producing increasingly strong upper diffluence through the coupled jet structure. Additionally, these jet streaks will help to surge moisture onshore and across the western/central parts of the CONUS, noted by IVT reaching +2 standard deviations and dropping slowly southward downstream of the primary trough through the middle part of the week. Additionally this deepening and southward advancing trough will push an arctic cold front southward beginning Monday night, and although it may waver downstream at times due to waves of low pressure rippling along it providing bouts of WAA, in general this front will push south with increasingly strong 925-700mb fgen, and intensifying upslope flow into the terrain in its wake on N/NE winds. Across the west, as snow levels will be 4000-6000 ft within the most intense WAA D1, but crash rapidly to the surface by the end of D2 in the Pacific NW, and then to below 500 ft as far south as the central Great Basin and Four Corners D3. With continued ascent, including the upstream secondary shortwave that pushes a surface low near the OR coast D3, snow will likely accumulate even into the valleys and lowlands of the west, with moderate to heavy accumulations by D3 in even some of the lower terrain, including the coastal terrain around San Francisco, CA. High probabilities for more than 6 inches exist each day through the forecast period, spreading southward with time such that D1 probabilities are highest in the WA Cascades, Northern Rockies, and into the NW WY ranges, but by D3 the greatest risk for more than 6 inches extends from the OR Cascades and coastal ranges through the Sierra, San Bernadinos, and into the Four Corners including the San Juans and Mogollon Rim. Widespread 3-day snowfall of several feet is likely above 2000 ft elevation, with impactful snow spreading into the passes as far south as southern CA. As impressive as the snowfall will be in the west, the heaviest and most impactful accumulations still are suggesting across the High Plains and Northern Plains D2 Tuesday and spreading into the Upper Midwest Wednesday. This will be a long duration event with two primary waves of heavy snow. The first will occur from eastern MT through southern MN Tuesday as the aforementioned wavering front surges briefly northward as intense WAA occurs from the SW. This will drive strong omega through isentropic upglide, leading to strengthening mid-level 700-600mb fgen just beneath the DGZ, supporting efficient snow growth, while the low level WAA helps to produce a deepening isothermal layer for efficient aggregate maintenance. The signal for excessive snow rates above 1"/hr within this band is modest due to modest conditional instability above the DGZ, but a translating band of heavy snow is likely D2 and WPC probabilities for more than 6 inches are 20-40% stretching discontinuously from the High Plains of WY, the Black Hills of SD, and into the Coteau des Prairies, Buffalo Ridge, and southern MN. However, by D3 /Wednesday/ the event really begins to ramp up as an area of low pressure developing in the lee of the Rockies begins to strengthen beneath the increased upper diffluence of the coupled jet structure and the downstream secondary shortwave, and despite this low occluding and becoming vertically stacked late D3, still very impressive warm/moist advection surging into a TROWAL aloft will enhance both elevated instability and moisture, resulting in expansive heavy snow from the central Plains through the Upper Midwest. Both WSE and NBM probabilities continue to increase for this region, and despite strong winds which may fracture dendrites beneath the DGZ, a cold column with robust UVVs will likely yield fluffy SLRS which will accumulate rapidly, especially in the presence of increasingly -SEPV aloft to support convective snow rates, and a major storm with extreme impacts appears almost certain. WPC probabilities for more than 6 inches ramp up quickly and expand extensively, reaching more than 70% for northern NE, much of SD, southern MN including the Twin Cities, and into central WI. Widespread snowfall of more than 12 inches is likely in this area, with locally up to 2 feet of storm total snow possible where the bands persist with the longest temporal duration. Farther downstream, increasingly impressive meridional moisture advection within the amplifying theta-e ridge on WAA out of the Gulf of Mexico will yield an expanding precipitation shield into the Great Lakes and Northeast late D3. 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. Uncertainty into the exact thermal evolution create lowered confidence this far out into the forecast period, but it is becoming increasingly likely that impactful snow and ice will spread across the L.P. of MI, northern PA and Upstate NY, and into New England on Thursday. The guidance is quite aggressive with ice accumulations D3 for the L.P. of MI and into the Northeast, but examination of regional soundings suggests the cold layer below the warm nose is quite deep, nearing the 95th percentile from the McCray climatology, which could indicate this will be more sleet than freezing rain in some areas, or could push the freezing rain axis farther south with time. Uncertainty at this time range precludes these specifics, so will continue to mention heavy mixed precip, but WPC probabilities for more than 0.25" of freezing rain range from 20-50% from far eastern IA through southern MI, with lower probabilities including the Chicago metro area and as far east as the Poconos. WPC probabilities for more than 6 inches of snow are above 70% for northern lower Michigan and into the Adirondacks. In between, an axis of heavy sleet exceeding 1" is also possible. Weiss ....Key Messages for Major Coast-to-Coast Winter Storm... --A prolonged major winter storm will spread a large swath of heavy snow from the West Coast to the Northeast today through Friday, and confidence is high that this winter storm will be extremely disruptive to travel, infrastructure, livestock, and recreation in affected areas. --The combination of heavy snow rates and strong wind gusts may result in blizzard conditions in parts of the West and Midwest, leading to difficult travel, as well as potential power outages and tree damage. In some places winds will likely gust over 50 mph. --Farther east, heavy snow and an icy wintry mix of snow, sleet, and freezing rain will impact the Great Lakes and Northeast late Wednesday and Thursday. Treacherous travel conditions are likely, with scattered power outages possible. --Very cold temperatures are expected from the West Coast into the Northern Plains behind this system, with record lows and dangerous wind chills possible. The potential also exists for a flash freeze in portions of the northern Rockies on Tuesday. $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .