Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 341 PM EST Fri Nov 29 2019 Valid 00Z Sat Nov 30 2019 - 00Z Tue Dec 03 2019 ...California to Great basin and Rocky Mountains... Days 1-3... Upper low centered near Washington state will retrograde SW to off the California coast through the forecast period. South of this evolution, a prolonged and robust Pacific jet will angle onshore advecting copious moisture into the West, noted by PWAT anomalies of more than 2 standard deviations above the mean. This Pacific jet will drop only very slowly southward into early next week, so the LFQ and max diffluent region will remain over the Sierra and other northern CA ranges for multiple days. This enhanced ascent combined with nearly unidirectional 850-600mb flow will produce heavy snowfall across much of the Great Basin and into the Northern Rockies, as well as California. The heaviest snow is likely in the Sierra where prolonged moist advection, robust synoptic lift, and enhanced upslope ascent combine, especially on D2-D3. In the highest terrain of California, including the Sierra and other ranges of northern CA, WPC probabilities are high for 12 inches on both D2 and D3, and 3-day totals may exceed 4 feet in places. Elsewhere in the West, the best forcing occurs on D2 across Oregon and into Idaho/Nevada where WPC probabilities are high for 4 inches, with amounts over 8 inches possible in the highest terrain. ...Central/Northern Plains into the Great Lakes... Days 1-2... ...Major Winter Storm likely beginning Friday night... Intensifying surface low pressure will emerge from the lee of the Rockies in CO tonight and then move northeast into Iowa by Saturday night, and further east into Michigan Sunday night. As this low moves northward, it will initially spread precipitation into the Northern Plains through isentropic ascent and WAA. A band of moderate to at times heavy precipitation will likely accompany this first band of precip, and guidance has cooled a bit causing an uptick in snowfall potential from northern NE into SD/IA and southern MN. Eventually the WAA will transition p-type to rain in these areas, but at the same time the enhanced synoptic ascent combined with warm moist air wrapping cyclonically NW will cause a long duration of heavy snowfall from eastern WY/MT through ND/SD and into MN through Saturday. Snowfall rates may exceed 1"/hr at times, and there is high confidence in a swath of snow exceeding 12 inches as noted by WPC probabilities over 70% from western SD into southern ND and N-Central MN. S/E of this band, some dry air intrusion may limit snowfall amounts, while further NW the forcing into the TROWAL will not be as robust, at the same time drier air tries to advect southward from a Canadian high. Surrounding the jackpot snow region, WPC probabilities are high for 4 inches. On Sunday, the low will continue to advect eastward, with continued moist advection on its east side wrapping into the cold air to the north. Heavy snow will continue into the U.P of MI, parts of northern WI, and the northern L.P. of MI where WPC probabilities are moderate to high for 8 inches. In the northern L.P., some lake enhancement may produce more than 6 inches as well. While a large area of 12+" of snowfall is likely, there may be lollipops of 18+ where banding sets up, or where upslope flow can enhance amounts. This is most likely in the Black Hills of SD, or along the western lake shore of Lake Superior. ...Mid Atlantic/Northeast... Days 2/3... Two-pronged winter event likely beginning Sunday from Pennsylvania and points northeast. The first round will be associated with WAA ahead of a warm front lifting northeast from a low/triple point near Ohio. As the WAA lifts into the cold air, a stripe of heavy mixed precipitation is likely, starting as snow and transitioning to sleet, freezing rain, and then rain. These WAA thumps can sometimes over-perform, and soundings indicate a good potential for a burst of heavier precipitation. In upstate NY where the precipitation should remain all snow, WPC probabilities are 20-40% for 4 inches. Further SW, especially in the terrain of PA, a prolonged period of freezing rain is likely, and WPC probabilities are moderate for 0.25" of accretion. Between the highest freezing rain probabilities in PA and the heavy snow in NY, a band of sleet/snow/freezing rain is likely, but total accumulations of any individual p-type are likely to be small. Sunday night into Monday, low pressure redevelops off the Mid-Atlantic coast and lifts slowly E/NE off the Southern New England coast while a potent upper low drops southeast from the Great Lakes towards New Jersey. The combination of these features will produce large scale ascent, and the track south of New England will advect moisture westward into the cold air over New England and New York. There remains considerable spread in the low track, but it appears a significant snowfall is becoming likely across portions of Southern New England and upstate New York, with somewhat lesser but still moderate snowfall possible beneath the upper low and on the back side late on D3 and even into D4. WPC probabilities are high for 6 inches or more across much of eastern upstate NY, as well the Berkshires and Worcester Hills of Massachusetts. Weiss