Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 405 AM EST Fri Jan 22 2021 Valid 12Z Fri Jan 22 2021 - 12Z Mon Jan 25 2021 ...Great Lakes and Northeast... Day 1... Lake effect snow is likely downwind of Lake Ontario and Lake Erie through Fri evening. A cold front crosses both lake today, with snow showers occurring in conjunction with pre-frontal convergence. As the mid level trough moves into New England tonight, flow veers towards the northwest. Pr-conditioning of the airmass crossing Lake Ontario by Lake Huron allows moistening to occur. The confluent northwest flow set up a band of lake effect snow south of Lake Ontario into the Syracuse, Rochester, and Auburn NY areas. Likewise, northwest flow across Lake Erie supports lee shore ascent and snow showers in southwest NY into northwest PA. As the low-mid level ridge builds across the upper lakes, dry air aloft reduces activity in the upper lakes later today and near lake Erie late tonight. WPC probabilities for 4" are moderate on D1 in the Tug Hill Plateau, continuing southeast of the Lake. For Lake Erie, the heaviest snow is expected D1, with moderate probabilities for 4 inches of snow. Snow showers are expected in the Green Mountains of VT and White Mountains of NH/ranges of southwest Maine as the cold front progresses across the region, followed by moist west-northwest flow that produces snow showers in windward facing terrain. A few snow squalls are possible in the period of peak diurnal instability this afternoon. ...California to the Southwest/Great Basin/WY-CO-NM Rockies... Days 1-3... A closed low over CA will move east across NV by 12z Sat, with mid level convergence near the low resulting in snow across the ranges of NV into UT. Pre-frontal convergence also extends into the ranges of UT and northwest AZ, with the GFS showing a 300 mb divergence maxima crossing northern UT, leading to snows in the Wasatch. Further south, the primary upper jet extends from AZ and NM into CO, so several inches of snow area expected where moist flow turns upslope in windward terrain of western CO. As the upper trough builds into the southwest later Saturday, the continuing upper southwest jet from AZ across NM into southern CO favors snow across the Mogollon Rim of AZ and then into the White Mountains and San Juans. Snow is expected to continue along the path of the 700 mb wave crossing northern UT and then WY, so several additional inches of snow are expected early Sat before the wave departs for the Plains states. On Day 3, the trailing low level front moves east out of AZ across southern CO and AZ. One more period of snow is expected before drying aloft occurs after the front passes. Several more inches of snow are possible in the Mogollon Rim to the San Juans before snow tapers. There remain timing differences as the NAM and ECMWF are slower to move the upper trough across the southwest and Southern Rockies, suggesting the possibility of more snow than the faster GFS and UKMET. WPC probabilities are greatest for 8 inches in the UT Wasatch/ Uintas, CO/NM San Juans, and AZ Mogollon Rim and White Mtns. Moderate snow is possible as far south as the San Gabriels and San Bernadinos of southern CA, where a low probability of 8 inches is indicated. ...Northern Plains to Upper Midwest... Days 1-3... A 700 mb wave ejects east from Wyoming and move east across the northern Plains, phasing with a northern stream wave coming down from south central Canada. As this wave moves east, it will be accompanied by moisture advection and convergence, with snow developing in advance of the wave Sat morning in parts of western SD/NE and then expanding as it moves east across MN/IA/WI on Saturday/Sat night. While this system is fast moving, it does have plentiful moisture with which to work, and upper divergence maxima in a coupled jet structure that aids ascent. Regional soundings indicate an environment favorable for high SLRs, in the mid to upper teens to 1, due to a deepening dendritic growth zone, and lift within the dendritic growth zone. Several inches of snow are expected despite modest QPF, and WPC probabilities for 4" are 40-60% from eastern SD eastward across southern MN and into western Wisconsin. Light freezing rain/freezing drizzle is possible at the end of Day 2 into Day 3 in the eastern central Plains to the mid MS Valley and adjacent midwest. The NAM model soundings indicate sub-saturated conditions where lift occurs aloft, so if lift occurs below, freezing drizzle will result, with light amounts as a result of modest lift and resultant rain intensity. ...Pacific Northwest/California... Day 3... An upper trough builds southeast from the northern Pacific and moves onshore into WA/OR/CA. The ECMWF shows a 300 mb jet of 110-130 kt off the coast, with difluent flow aloft over WA/OR/CA allowing upper divergence maxima to develop to drive ascent, in conjunction with low level frontogenesis in western WA/OR. Increasing moisture and ascent will spread precipitation from western WA/OR into northern and central CA. The heaviest precipitation is likely in the OR Cascades and Coastal Ranges. WPC probabilities for 4 inches of snow are high from the Siskiyous northward along the Cascades of OR and WA and into the Olympics. There is spread on Day 3 on the order of half an inch liquid equivalent in CA as the 00z GFS shows stronger 700 mb ascent than other solutions, with the GFS showing higher QPF than most models/ensemble members in the ranges of northern to central CA. Consequently, the GFS was given less weight here. For Days 1-3, the probability of significant icing (0.25 inch or greater) is less than 10 percent. Petersen