Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 358 PM EST Thu Dec 17 2020 Valid 00Z Fri Dec 18 2020 - 00Z Mon Dec 21 2020 ...Pacific Northwest to the Northern Rockies.... ...A couple of episodes of moderate snow on days 1-2 should be followed by a heavier event on Day 3 across the mountains of the Pacific Northwest to the Northern Rockies... Day 1... An upper level trough approaches from the northeast Pacific tonight into the Pacific northwest on Fri. Out ahead of the trough, the low-mid level flow backs to the southwest. In turn, this leads to moisture advection across the region, with a region of 0.50 precipitable water values streaming into the WA Olympics and then the Cascades. This leads to high elevations snows which become locally heavy, which may accumulate to 12-18 inches in the northern WA Cascades. Day 2... The trough moves inland into the northern Rockies Fri night, with snow then picking up in the ranges of northern ID and northwestern MT. A persistent upper jet maxima cross the WA cascades, with embedded upper divergence maxima within the jet producing long lasting ascent. periods of snow, with additional heavy accumulations possible across the northern Cascades. Day 3... As a strong upper level jet maxima drifts south from British Columbia towards WA State, moisture advection rapidly increases relative humidity values coming into th the WA Olympics and Cascades. The strong synoptic lift combined with orographic enhanced areas in windward terrain leads to high confidence in a long last lasting heavy precipitation event, with heavy valley rains and mountain snows. Snow levels are forecast to be relatively high at the onset, confining the threat for heavy snowfall accumulations largely to the higher terrain in the northern Cascades and Olympics. Where it remains cold enough for snow, multiple feet of snow are possible, as 3-4 inches of liquid equivalent QPF is forecast in the northern WA cascades. This in turn is supported by sustained low-mid level integrated vapor transport,a s the 12z ECMWF forecasts the 850 mb jet maxima to increase to near 50 kt, along with 1 inch precipitable water values onshore into western WA, then OR as the plume drifts south Sun. Snow levels drop Sun night as a cold front crosses the region. The deep westerly flow extends into the northern Rockies, with heavy snow in the mountains of northern ID and northwest MT, with an additional 1-2 feet expected in the mountains. UT/CO Day 1... An upper level trough is forecast to depart NV and cross UT tonight and western CO Fri. Out ahead of the trough, low-mid level warm/moisture advection increases mean layer relative humidity. Upper level divergence maxima spark lift, producing snow, with aid of terrain in windward areas. Once the trough crosses UT late tonight, ascent declines and then drying aloft occurs, decreasing snow amounts and coverage. In western CO, this sequence of events occurs on Fri. The system has produced several inches of snow in portions of the mountains of CA/NV, and several inches of snow is expected with the terrain of UT and western CO, too. For Days 1-3, the probability of significant icing (0.25-inch or greater) is less than 10 percent. Petersen