Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Nov 20 2023 18:58:47 FOUS11 KWBC 201858 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 157 PM EST Mon Nov 20 2023 Valid 00Z Tue Nov 21 2023 - 00Z Fri Nov 24 2023 ....Northeast... Days 1-3... Complex mid-level evolution through Thursday will result in a Miller-B type low pressure bringing wintry precipitation to the interior Northeast. A closed low over the MS VLY tonight will weaken and open into a wave over the Great Lakes by Tuesday night, while a a secondary impulse rotates through that residual positively tilted trough to become the dominant feature near the New England coast Wednesday aftn. Aloft, modestly coupled jet streaks will surge northeast downstream of the primary longwave trough axis, driving additional ascent through diffluence, and where the best mid-level height falls and upper level diffluence overlap, surface low pressures are expected to track across the area. The first of these lows, really the primary low, will likely lift from Illinois through Ontario while slowly weakening, with the secondary low development occurring along the baroclinic gradient associated with a leading warm front near DE and then moving eastward towards Cape Cod. The exact track of both of these lows will have significant consequences to p-type and intensity during this event. As the first low shifts northeast, pronounced WAA will drive increasing isentropic ascent along the 290K-300K surfaces, with an impressive axis of 700-600mb fgen demarcating the edge of this precipitation shield. Initially, cold Canadian high pressure will be in place, so this precipitation will likely begin as a band of heavy snow, especially from the Poconos and Catskills northeast through central and northern New England. During this time, snowfall rates could reach 1"/hr as shown by the WPC prototype snowband tool, and reflected by the risk for CSI evident in model cross-sections Tuesday night into Wednesday. However, the wedge of high pressure is likely to retreat quickly in response to strong mid-level divergence, and a change from snow to mix to rain is likely across all of the Northeast and New England except far northern New England. Still, a few inches of snow is likely in many areas, especially in the higher terrain, with some impactful snow, albeit brief, probable even at lower elevations before the changeover. WPC probabilities for more than 4 inches of snow are 40-70% in the Adirondacks, southern Greens, and across much of northern NH/ME, with a greater than 80% chance for the White Mountains of NH. This snow will likely have very low SLR due to the relatively warm (and warming) column, so any significant accumulations could result in impacts due to the weight of this snow which is reflected by WSSI-P of more than 30% due to Snow Load in this area. During this time as well, freezing rain could accrete significantly, especially in the higher terrain from the Allegheny Mountains through the Laurel Highlands D1, and then the Adirondacks and Greens D2 as surface temps remain colder, but any valley "trapping" gets scoured out by the strong low-level WAA. WPC probabilities for mote than 0.1 inches of ice are 20-40% D1, and 10-30% on D2. The caveat to this evolution will be as the second low pressure develops south of New England, the resultant ageostrophic flow from the coupled jets aloft and the deepening secondary low pressure may allow the cold air to retreat back to the southeast, keeping parts of NH and ME as all-snow. There is considerable uncertainty into this evolution, and even the typically colder NAM has precipitation changing over to rain almost to Caribou, ME. Additional snowfall is likely D3, primarily across northern/central Maine, where WPC probabilities for an additional 4+ inches of snow reach 20-50%. ....Northern Rockies to Central Rockies... Days 2-3... A shortwave dropping southward out of British Columbia will rapidly amplify into a closed low near the Northern Great Basin Thursday night, with the accompanying height falls and downstream divergence producing impressive synoptic ascent across the Northern and Central Rockies D3. This amplifying trough will have a two-fold response to provide additional forcing: an intensifying zonal jet streak over the Northern Plains placing increasing RRQ diffluence aloft, and pushing a cold front southward with post-frontal upslope flow into the terrain. As moisture increases (NAEFS ensemble tables suggest a ribbon of PW around +1 sigma immediately ahead of the front), the orientation of the mid-level flow becoming more perpendicular to the front into the Central Rockies will manifest as intensifying isentropic upglide, resulting in a broader footprint of snow with embedded heavier snowfall rates, especially the latter half of D3 across southern MT and into WY. Most of the snowfall across the Northern Rockies should be heaviest in the terrain where upslope flow is pronounced, and above 3000 ft. Snow levels will fall rapidly as the front sinks southward, but much of the intense ascent will also be shifting SE at this time, making the heavy snow across this region more transient. WPC probabilities are modest by D2.5, generally 30-40% for more than 4 inches near Glacier N.P. and into the Absarokas. By the latter half of D3 as moisture and forcing both become more impressive, the snowfall footprint and intensity should increase, especially as the DGZ depth increases to more than 100mb within a region of modest mid-level fgen. WPC probabilities for more than 4 inches exceed 50% for parts of the Northern Rockies southward through Yellowstone N.P. and into the Big Horns, with pronounced downstream shadowing also likely. Locally 8-12" or more is possible in some of the higher terrain. On D3 as well, light snow accumulating to more than 2 inches may extend as far east as the Black Hills of SD, with additional heavy snow possible beyond this forecast period. 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