Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 21 2023 20:06:57 FOUS11 KWBC 212006 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 303 PM EST Tue Feb 21 2023 Valid 00Z Wed Feb 22 2023 - 00Z Sat Feb 25 2023 ....Long duration, major coast-to-coast winter storm this week... ....Western United States through the Northern Plains, Upper Midwest, into the Northeast... Days 1-3... A complex evolution of the mid-level pattern, featuring an amplifying trough across the West with subsequent shortwave shedding into the Northern Plains will result in lowering snow levels and strong synoptic/mesoscale ascent. With well above normal moisture, a major winter storm producing far-ranging and long-lasting impacts is expected. The root of this event is an amplifying 700-500mb trough, digging across the Intermountain West tonight into Wednesday, reaching upwards of 4 standard deviations below the climo mean according to the NAEFS ensemble tables. The core of this upper low will pivot into the northern Great Basin Wednesday morning before slowly retreating back to the north and into Canada Thursday morning. However, the retreat of the core of this gyre will not at all indicate a weakening of this impressive event. Instead, dual amplified shortwaves, one upstream across the Pacific Northwest, and a second one downstream into the Northern Plains will drive even more intense ascent and continue this major winter storm and its wide ranging impacts. Beneath the primary upper trough, an arctic cold front will dig southward such that while snow levels in the antecedent WAA will be as high as 4000 ft, they will drop precipitously to less than 500 ft as far south as the central CA Valley and central Great Basin, and then even lower before leveling off at generally 0-1000 ft across the entire West. With steep lapse rates beneath the upper low, periodic PVA and height falls around the upstream shortwave, and increasing upper diffluence as the jet streak digging along the Pacific coast attempts to couple with the downstream subtropical jet streak, synoptic ascent will be quite robust through the period, with just a gradual shift southward as the front and associated fgen maximize omega. Within this regime, moisture will be impressive noted by IVT reaching +2 sigma ahead of the front, and sinking slowly southward with time such that the heaviest snow will follow suit - from the Northwest and Northern Rockies, Sierra and Central/Southern Rockies through Wednesday evening, and then finally the southern CA terrain and southern Great Basin Wednesday night into Thursday. WPC probabilities for more than 6 inches across the West are quite expansive from the Olympics and Cascades through the coastal ranges of northern CA, into the Sierra, and across much of the Rockies from the Northern Rockies through the Mogollon Rim. 3-day snowfall will likely exceed 2-3 ft in much of the higher terrain. With snow levels crashing through the period, light accumulations are expected even down into the lowlands and valleys, including the central CA valley, with significant mountain pass impacts also likely across much of the West. As impressive as the snowfall event will be across the West, potentially even more impressive and impactful will be the blizzard that is expected to develop from the High Plains through the Upper Midwest, especially Wednesday and Thursday. Before this blizzard begins, a leading wave of snowfall will likely surge northeastward ahead of the shedding vorticity lobe and associated with strong WAA along a northeastward advancing cold front. Fgen along this boundary is robust, collocated with a narrow corridor of +2 standard deviations PW anomalies. This will result in a translating band of heavy snow with snowfall rates potentially 1-2"/hr, but the overall progressive nature and somewhat more modest DGZ depth being forecast where moisture is best (a disconnect between the available moisture and deepest DGZ) should at least limit total snowfall from this first wave. Still, several inches of snow are expected and the latest WPC probabilities for exceeding 6 inches exceed 50% through Day 1. By late tonight but more so on Wednesday, the major winter storm will take shape and spread tremendous snowfall, both in coverage, rates, and amounts, across the High Plains and Upper Midwest. The primary driver of this winter storm, which will likely reach blizzard criteria in some areas, is intensely coupled upper jet streaks overlapped with robust height falls along the negatively tilted shortwave to drive surface low development. The amplification of these features will support intense downstream meridional moisture transport from the Gulf of Mexico with PWs surging to +2-3 standard deviations above the climo mean. This moisture will wrap within the WCB into a robust TROWAL, supporting strong elevated instability, while the associated WAA helps to deepen the DGZ which will almost certainly exceed 100 mb as reflected by SREF probabilities reaching 90%, with some regional soundings indicating up to 300mb of DGZ depth, exceptionally deep for heavy banded snowfall. The intense ascent into this DGZ fueled by the isentropic ascent, fgen, and increasing deformation especially late D2 into D3 could yield snowfall rates in excess of 2"/hr with high SLR in the cold column limited only by the likelihood of dendritic fracturing. Additionally, the setup seems to support that this most intense band will shift in paradigm from a translating band to a more pivoting band into D3, which would temporally extend the duration of this heavy snowfall. Confidence continues to increase in an exceptional swath of heavy snow stretching from the Black Hills of SD through southern/central Minnesota and central Wisconsin. Probabilities of exceeding 12 inches are above 50 percent across southern SD and exceed 80 percent from southern/central MN into central WI. Where the heaviest snow occurs, potentially near the Twin Cities, MN, more than 20 inches is likely, and it is possible a historic top-3 all-time snowfall event is about to unfold for Minnesota. Just south of the heavy snow areas across the Upper Midwest and Great Lakes is the growing 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 beginning Wednesday morning then continuing into Wednesday night. From central IA through southern WI, northern IL and particularly southern MI, between 0.1-0.2" of ice accretion is expected with the greatest probabilities of reaching or exceeding 0.25" of ice over southern-south central MI. Further 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. 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 reaching 50-60 percent. Further north, heavy snow is expected, from the northern NY, much of VT/NH into Maine where probabilities of 6" or greater are high (80 percent+) with localized totals exceeding 8-12" possible for the favored terrain areas, particularly the High Peaks of the Adirondacks and northern White/Green Mountains where 12" probs are above 60 percent. ....Western U.S. Friday-Weekend... The anomalously deep and cold mid/upper level low will slowly wobble southward along the West Coast later this week into this weekend. As the system approaches the California coast, it will begin to interact and direct a plume of deeper atmospheric moisture directly across central/southern California. This will provide a favorable setup for heavy precipitation across the Sierra but also southward along the coastal ranges of southern California. The deep/cold mid level low will allow for snow levels to fall quickly to as low as 1500-2000 ft and this should provide heavy snow for the higher terrain areas. Given the anomalous high moisture and lower snow levels, probabilities for exceeding 12 inches are high for the Sierra but also the higher peaks across Southern California. Weiss/Taylor ....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 this week. This will be extremely disruptive to travel, infrastructure, livestock, and recreation. -The greatest impacts will be over portions of the High Plains into the Upper Midwest. The combination of heavy snow rates of 1-2"/hr and wind gusts of 40-50 mph will create blizzard/whiteout conditions due to falling and blowing snow, nearly impossible travel, power outages, and scattered tree damage. -Across the Pacific Coast and Intermountain West, heavy snow rates and strong winds, which in some places will gust above 60 mph, will result in near-blizzard conditions producing dangerous travel and scattered power outages. In addition, impactful snow is possible into less typical foothills and valleys. -A wintry mix of heavy snow, sleet, and freezing rain will impact the Great Lakes and Northeast from late Wednesday into Friday. Treacherous travel conditions are likely, with scattered power outages possible. -Record lows and dangerous wind chills are likely over much of the West into the Plains through the week. $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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