Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 239 AM EST Wed Nov 11 2020 Valid 12Z Wed Nov 11 2020 - 12Z Sat Nov 14 2020 ...Western U.S.... Day 1... A modest shortwave dropping southeastward from WA/OR will move across the Northern Rockies today producing height falls as far south as Utah/Colorado by this evening. This feature will be accompanied by modest upper diffluence as a weak jet streak downstream of the main longwave trough axis pivots east, producing brief omega surges and mid-level RH increase across the region. Broad but persistent forcing for ascent should provide an extended period of light to moderate snow across parts of the Northern Rockies, into the ranges of NW WY, and south into Utah and parts of CO. In general, WPC probabilities are moderate for 4 inches across the area. The exception is across the ranges of NW WY, including the Tetons and Wind Rivers, where 700mb flow of a W/NW direction south of the vort max could produce enhancement through upslope and some weak frontogenesis. WPC probabilities in these ranges are high for 4 inches, with local amounts to 12 inches possible in the highest terrain. Days 2-3... A significant precipitation event is becoming more likely late Thursday and Friday. Two distinct shortwaves, one on Thursday night and a more pronounced impulse Friday evening will advect onto the WA/OR coast, embedded in increasingly confluent and zonal mid-level flow. The first shortwave will be accompanied by a modest Pacific jet streak with the LFQ producing a divergence maxima which will move across WA and race quickly towards the northern Rockies. A brief surge 700-500mb RH and ascent will produce a period of snow spreading eastward from the Olympics, into the Cascades, and towards the Northern Rockies, but with deamplification of the shortwave rapidly occurring, the heavy snow should be confined to the mountains of WA state where WPC probabilities are >50% for 4 inches, generally above 3000 ft. A more pronounced impulse rapidly follows this first one, and is accompanied by robust moist advection on the nose of a 150kt Pacific jet streak. This appears to be a setup for a moderate AR with IVT progged to exceed 700 kg/m/s into the coasts of WA and OR. Robust ascent within the anticyclonic LFQ of this jet should lift nearly orthogonally onto the PacNW coast, with intense moisture and mass confluence driving PWs as high as +1.5 standard deviations above the climo mean into the Great Basin. A strong low pressure is likely to be pushed onshore as well, with a leading warm front enhancing WAA, but also raising snow levels Friday night. Strong forcing and anomalous moisture suggest heavy snow is likely from the Olympics and Cascades, eastward through the Northern Rockies, Sawtooth and Salmon River Ranges, and into NW WY. WPC probabilities on D3 are high for 12 inches in the WA and OR Cascades, as well as ID ranges NE of the Snake River Valley. In these ranges, ascent will be enhanced via upslope, and more than 2 ft is possible, locally 3 ft in the highest peaks. With snow levels rising to 3000-5000 ft with the warm front, the valleys should see little to no accumulation, but the lower ranges feature additionally high probabilities for 4-6 inches, with lighter accumulations possible as far south as the Sierra Nevada as well. For Days 1-3, the probability of significant icing (0.25-inch or more) is less than 10 percent. Weiss