Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 334 PM EST Sun Nov 10 2024 Valid 00Z Mon Nov 11 2024 - 00Z Thu Nov 14 2024 ...Western U.S... Days 1-3... Back-to-back sharp troughs bring higher elevation snow to the NW quadrant of the CONUS through midweek. The first trough will continue to dig south from a parent low over the Gulf of Alaska before crossing the PacNW coast (with the base of the trough over northern CA) early Monday before ejecting east over the northern Rockies through Tuesday. The second trough reaches the PacNW coast later Wednesday. Although the first trough will be of modest amplitude and progressive, height falls, divergence and dual jet streaks (one poleward arcing downstream of the trough axis, another more zonally oriented upstream) will lead to widespread deep layer ascent from the Pacific Northwest eastward through the Northern Rockies. At the same time, this trough will drive a cold front eastward, surging onshore the WA/OR/northern CA coasts Monday morning before racing into the Northern High Plains on Tuesday. This synoptic lift will act upon an increasingly moist column noted by rapidly rising 1000-500mb RH as warm/moist advection occurs within the aforementioned jet streaks, driving IVT to 250-500 kg/m/s. This will support widespread moderate to heavy precipitation, and as snow levels drop from 6000 to 7000 ft tonight to around 4000 ft Monday night, snow will become widespread in the terrain of the Cascades down to the central Sierra Nevada and then the Northern Rockies by Monday evening. Day 1 WPC snow probabilities for >6" are 50-80% for the higher Cascades and 30-60% for the northern Sierra Nevada. Low pressure lingering near Haida Gwaii (ahead of the parent low exiting the Gulf of Alaska) maintains onshore flow over the PacNW coast through Tuesday when the moisture surge ahead of the next wave arrives at the coast. So light to moderate precip continues over the Cascades to the Northern Rockies, prolonging and connecting the main accumulating snow events. Day 2 WPC snow probabilities for >6" are categorical above 4500ft in the Cascades and 40-70% in the Blue Mtns, Bitterroots, Clearwater, Tetons, and ranges just NW of Yellowstone (such as the Gallatin and Ranges). The next wave will be accompanied by more impressive moist advection with ECENS IVT probabilities reaching above 90% for 500 kg/m/s, with several members exceeding 750 kg/m/s. The accompanying WAA will surge snow levels back to 5000-6000 ft over the Cascades Tuesday night. This will support more widespread heavy/wet snow, with snow load impact concerns. Day 3 WPC probabilities for >6" are categorical for the WA Cascades and 40-70% for the highest OR/CA Cascades higher ranges of northeast WA/far northern ID such as the Selkirk Mtns. Over the three day span, a few feet are likely above about 6000 ft in the Cascades and several feet for the highest volcanos. The probability of significant ice across the CONUS is less than 10 percent. Jackson