Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Feb 19 2023 09:42:28 FOUS11 KWBC 190942 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 441 AM EST Sun Feb 19 2023 Valid 12Z Sun Feb 19 2023 - 12Z Wed Feb 22 2023 ....Long-duration, widespread, major winter storm expands from the Northwest Monday night to much of the West and northern tier states through the middle of next week... ....Pacific Northwest, Northern/Central Rockies, Northern High Plains... Days 1-3... A broad and elongated mid-level trough centered near the MS VLY will amplify through Monday in response to periodic shortwaves and associated vorticity impulses from Canada rotating through it. However, a more amplified trough will begin to take shape Tuesday morning as a strong impulse digs south out of British Columbia, manifesting as a closed 500mb low over the Pacific Northwest by the end of the forecast period. This evolution will drive rapid height falls, with NAEFS ensemble tables suggesting 700mb to 500mb height anomalies reaching -4 standard deviations over the much of the Intermountain West by D3. This amplifying trough will help drive an arctic cold front southward out of Canada D2-3, with impressive 925-700mb fgen developing along and behind this front. This fgen will likely be enhanced by the favorable ageostrophic circulation that will develop aloft as a Pacific jet streak dives down the coast and couples with a downstream jet streak over the Great Lakes. This enhanced ascent, combined with increasing upslope flow behind the front, and the synoptic UVVs created by height falls/PVA will support widespread heavy precipitation, spreading from the Pacific Northwest through the Great Basin, Central and Northern Rockies, and into the Northern High Plains by D3. With moisture progged to reach +1 to +3 standard deviations thanks to confluent mid-level flow overlapped with the Pacific jet streak to intensify moist advection, the stage is set for a major winter storm with heavy snow across much of the region. Snow levels D1-D2 will likely climb to 3000-4000 ft on the warm advection downstream of the digging trough, but will then quickly crash during D3, reaching the surface for states bordering Canada and east of the Continental Divide, and falling to as low as 500-1000 ft into the Great Basin. WPC probabilities for more than 6 inches on D1 are confined to the higher terrain from the WA Cascades through the northern Rockies, Bitterroots, and into the Absarokas, Big Horns, and other NW WY ranges where they reach 50-90%. On D2, many of these same areas remain with elevated WPC probabilities for more than 6 inches, but the coverage begins to expand both due to better ascent/more moisture, but also due to lowering snow levels. 2-day snowfall across the higher terrain of the WA Cascades and Northern Rockies could exceed 5 feet, with widespread 2-3 feet across these ranges likely. During D3, the heaviest snowfall expands across nearly all of the Intermountain West as the upper trough moves overhead, and mid-level fgen begins to increase as the front sinks southward. The guidance has become a bit faster with this southern push, and WPC probabilities for more than 6 inches exceed 50% for nearly all the terrain above 3000 ft from the Olympics southward along the Cascades and into the northern CA ranges, as well as into the Sierra. Additionally, high probabilities encompass much of the terrain from the northern Rockies through the Great Basin and central Rockies. Locally 2+ feet of snow is likely above 3000 ft, and with snow levels falling to around 500 ft for the northwest, light accumulations should spread into the valleys as well. Additionally on D3, increasing WAA as the warm front lifts northward into the Plains will result in a band of heavier snow driven by WAA/fgen into SD/MN. The guidance has become much more impressive with this event overnight, and despite some concern about the placement of the greatest moisture, WPC probabilities for more than 6 inches reach 40-70% centered near the Coteau and Buffalo Ridge. The long duration, widespread, major winter storm which spread across much of the Western U.S. and the north-central Plains to the central/northern Great Lakes is described in the key messages below. ....North Dakota to the Northern Great Lakes... Days 2-3... A potent vorticity lobe sheared into confluent mid-level flow upstream of broad trough axis centered over the MS Valley will drop southeast from Alberta/Saskatchewan and into the Upper Midwest Monday into Tuesday. The guidance has backed off slightly on the intensity of this feature from previous runs, but it is still likely to result in a clipper type low pressure system moving progressively across the region, aided by the periphery LFQ of a powerful subtropical jet streak centered over IL Tuesday morning. Modest moist advection from the S/SW will manifest as mixing ratios of 2-3 g/kg within brief but impressive 280-285K isentropic upglide, providing sufficient moisture and mesoscale ascent to produce heavy snow along and ahead of this wave. A subtly deepening DGZ along and behind the wave will help increase the potential for larger aggregates supporting higher SLRs, but overall this looks like more modest event. The exception may be where some lake enhancement occurs both across the Arrowhead of MN ahead of the low, and then along the southern shores of Lake Superior as CAA occurs in its wake. WPC probabilities for more than 4 inches of snow are above 70% along the Arrowhead/Iron Ranges and the western U.P. where the lake enhancement is expected to be most substantial. Otherwise, snowfall across MN and the northern Great Lakes should be just 1-3 inches. Significant icing is not forecast across the CONUS Days 1-3. Weiss ....Key Messages for Major Coast-to-Coast Winter Storm... --A major winter storm will spread a large swath of heavy snow from the West Coast to the Upper Midwest Monday through Thursday. --The combination of heavy snow and gusty winds will likely result in widespread hazardous travel and cause impacts to infrastructure. --While subtle shifts in storm track are possible, confidence is high that this winter storm will be extremely disruptive to travel, livestock, and recreation in affected areas. --Very cold temperatures are likely 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. --Additionally, a heavy wintry mix of snow, sleet, and freezing rain may impact the Great Lakes and Northeast later in the week. $$ = = = 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) .