Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Nov 17 2024 20:42:44 FOUS11 KWBC 172042 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 342 PM EST Sun Nov 17 2024 Valid 00Z Mon Nov 18 2024 - 00Z Thu Nov 21 2024 ....Western U.S.... Days 1-3... Active winter pattern for the Northwest at least through the next week. A leading wave over the PacNW coast this afternoon will track east over the northern Rockies through Monday before rapidly developing into a deep low over North Dakota on Tuesday. A deep low currently over the Gulf of Alaska is rounded by a powerful shortwave Monday night allowing for particularly rapid development to a surface low well off Vancouver Island on Tuesday. This low then stalls/becomes a gyre off the PacNW coast through at least Friday which will result in prolonged moisture-potent onshore flow into the PacNW starting later Tuesday. The cold front from the leading wave currently crossing the Cascades crosses the northern Rockies tonight with low levels plummeting by this evening to 2,000ft in the WA Cascades and 2,500ft in the OR Cascades. These snow levels then gradually drop an additional 500ft through Monday with moderate precip rates persisting from continued onshore flow. Snow levels fall over the northern Rockies from 5000-7000ft now to 2000-2500ft overnight. The unusually cold air-mass is in place at the same time as persistent onshore flow, courtesy of the longwave trough to the north, strikes mountain ranges such as the Cascades at an orthogonal angle to maximize the snowfall potential through Monday. Day 1 PWPF for >6" are categorical for the Cascades above about 3000ft and the northern Rockies from the Tetons to the Bitterroots above about 4000ft. Following a brief lull in minor ridging Monday night, a rapidly strengthening storm system (the 12Z GFS/ECMWF both have a sub 950mb low by 21Z Tuesday!) will direct a stronger atmospheric river at the Pacific Northwest with not only exceptional moisture advection (>750 kg/m/s IVT on NAEFS would be above the 97.5 climatological percentile Tuesday afternoon), but intense winds along the mountainous terrain. Warm air advection will raise snow levels to above 2,500ft in the WA Cascades, around 4,000ft in the OR Cascades, and 6,000ft in the northern California ranges by Wednesday morning. The concern is for a heavier/wet snow type to rapidly accumulate, while powerful wind gusts potential lead to tree damage and power outages Tuesday night and Wednesday. All told, there is no shortage of snow in the forecast for the Pacific Northwest and Northern Rockies. Snow is likely to be measured in feet along the Cascades above 3,000ft, the Salmon/Shasta/Siskiyou of northern California, and well inland through the Blue, Sawtooth, Boise, Bitterroot, Tetons, and Lewis Mountain Ranges. The latest WSSI shows Moderate to Major Impacts in the Cascades and Bitterroots with impacts including not just dangerous travel conditions (including notable Cascade passes), but very heavy/wet snow and strong winds by Tuesday night that could result in tree damage and power outages in some areas. As of now Day 3 PWPF for >6" are categorical in the Shasta/Siskiyou of CA (which is the center of the prolonged moisture plume, but where topographical lift should keep snow levels from rising too far above 4000ft) and the WA Cascades where a burst of snow is expected from the arrival of the occluded low. ....Northern Plains... Days 2/3... A powerful late November cyclone will track from the Southern Plains tonight to Minnesota Monday night. The 12Z ECMWF is not as far east as previous runs and a bit more in line with the 12Z GFS in the track north over MN. This exceptionally strong storm system tracking over Minnesota coupled with high pressure building in over the Northern Rockies will make for a very windy setup over the Northern Plains. Snow is expected to fall across much of North Dakota and winds of 30-40 mph would result in whiteout conditions for areas like central and eastern ND would experience their first round of accumulating snowfall this season. The Day 2.5 PWPF for >6" is 20-70% over much of ND west of the Red River Basin and north of I-94. The probability of significant ice across the CONUS is less than 10 percent. Jackson $$ = = = 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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .