Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 317 PM EST Tue Nov 09 2021 Valid 00Z Wed Nov 10 2021 - 00Z Sat Nov 13 2021 ...Pacific Northwest and California through the Central Rockies... Days 1-3... Unsettled weather remains in store for the Pacific Northwest through the forecast period. Current shortwave trough and surface front moving through the area this afternoon will lift northeast later tonight giving way to flatter upper level flow on Wednesday. A brief lull in the heavier precipitation is then expected before another surge of Pacific moisture lifts into the area Thursday into Friday. Driven by shortwave energy lifting into B.C. and a coupled jet streak providing large scale forcing for ascent, the nose of atmospheric moisture aims at the favored western slopes of the OR/WA terrain. Snow levels in the day 1 period will be quite low, between 2-3 kft before rising well above 7-8 kft Thursday afternoon into Friday ahead of the next system. This is expected to keep the best snow accumulation potential to the highest peaks. The greatest probabilities for heavy snow are in the D1 period, primarily in the 00Z-12Z Wednesday timeframe. The 6 inch probabilities are high for the WA Cascades, falling to slight/moderate for D2/3. The upper level shortwave energy and longwave trough reaches the central/northern Rockies mid-week. Sufficient moisture and lift in place moving over the favored terrain areas will squeeze out snow accumulations for the CO Rockies as well as the Uintas, NW WY ranges, and portions of the northern Rockies. The WPC probabilities for 4 inches are slight to moderate during the Day 1 period then drop off considerably in the Day 2/3 periods in the wake of the system and expected upper ridging building in. ...Northern Plains and Upper Midwest... Days 2-3... A Pacific shortwave moving quickly across the Central Rockies will lift into the Central Plains Thursday while a more robust closed mid-level low will drop southeast out of Alberta. These features will interact into a highly amplified trough across the central CONUS. Aloft, jet level energy will surge from the Pacific into the Northern Plains while only slightly weakening, and will drop southeast upstream of the amplifying trough while a secondary jet streak strengthens downstream and arcs poleward on Thursday. Together, these features will drive surface cyclogenesis in the lee of the WY Rockies, with this low then deepening through Thursday while lifting towards Lake Superior and then getting captured to become vertically stacked in southern Canada Friday. A couple periods of snowfall potential exist - initially with the warm air advection push as the warm front lifts across central/northern Minnesota. Fairly impressive isentropic lift within the marginally supportive thermal profiles will bring a quick burst of snow potential to far reaches of northern Minnesota early Thursday morning. After a period of dry slotting, the wrap around TROWAL precip will surge south/southeast across the eastern Dakotas and western/northwest Minnesota later Thursday into Friday. This TROWAL is likely to edge eastward through Thursday night/Friday such that the most robust forcing will remain within the warmer portion of the storm, but enough residual theta-e advection atop modest mid-level deformation should produce snowfall on the back side of the system, angling slowly southeastward into Friday. SLR will be on the increase as cold air advection rapidly cools the profile and the best forcing aligns in the favored DGZ. For modest amounts of QPF, this could be enough to produce several inches of snow. Compared to the previous forecast cycle, probabilities for 4 inches have trended higher and expanded across the eastern Dakotas and northwest Minnesota, exceeding 50 percent with local maximum above 80 percent close to the Canadian border. For Days 1-3, the probability of significant icing is less than 10 percent. Taylor