Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 432 AM EST Fri Nov 29 2019 Valid 12Z Fri Nov 29 2019 - 12Z Mon Dec 02 2019 ...CA to Great basin and Rocky Mountains... ...More heavy snow expected for the ranges of California... Day 1... The mid level low will move slowly northeast out of the Great basin and dissipate, with the low level cyclogenesis occurring east of the Rockies by Sat morning. Strong pre-frontal low level convergence and ascent focused by upper divergence maxima poleward of the jet axis leads to snow concentrating on the ranges from the Mogollon Rim in AZ to the northern Wasatch/Uintas in UT/Tetons in WY, San Juans in CO, and Beartooth Range in MT. The heaviest snow is likely in the highest elevations of the ranges most orthogonal to the flow: The Mogollon Rim, The Wasatch, Uintas, and the San Juans. In these ranges, WPC probabilities are high for 12 inches. Saturday night the next upper low and surface reflection approach the west with a renewed surge of moisture as an upper level jet moves onshore into CA from the eastern Pacific. Moist confluent low-level flow and well defined upper divergence maxima will produce snowfall once again, this time focused into the Sierra/Shasta/Siskiyou ranges of California, where WPC probabilities are high for 12 inches. Lighter accumulations are likely spreading northeast into the Cascades of Oregon on Day 2 and then the Blue Mountains of OR and ranges of northern NV on day 3. Days 1-3 snow totals of 3-4 feet are expected in the CA Sierra Nevada mountains. ...Central/Northern Plains into the Great Lakes... ...Major Winter Storm likely beginning Friday night... A closed mid-level circulation ejects northeast into the Central Plains Friday night. Beneath the mid-level low, surface cyclogenesis is expected in the lee of the Rockies, with a deepening surface low lifting northeast into the Plains before moving eastward towards the Great Lakes by the end of the forecast period. Moisture wraps around the circulation and produces a pronged period of low-mid level warm advection, resulting in mixed precipitation with snow changing over to sleet, freezing rain and rain across northeast Nebraska to northwest IA, eastern SD, southern MN, and central WI and MI. Further north, deeper into the cold air, a longer period of snow results in heavy snow from western SD to southeast ND, north central MN, and northern WI. Greater uncertainty remains as dry air aloft may impinge on the system over northeast ND to northern MN and the UP of MI. WPC probabilities indicate moderate chances for 12 inches from the Black Hills to areas near the ND/SD line, north central MN and northern WI. ...Mid Atlantic/Northeast... Days 2/3... Surface low pressure decays as it moves the Great Lakes. A triple point low develops near the mid Atlantic coast and moves off the New England coast. Low to mid level warm advection wraps north of the sfc low to produce mixed precip types with sleet and freezing rain expected in the middle Appalachians on day 2 and then into northeast PA/southeast/Southern New England Day 3. Further inland, chances are greater for prolonged snow and sleet across upstate NY and New England inland from the southern coast. There is potential for locally heavy snow in the Catskills to the Berkshires and Worcester Hills of MA, depending on the outcome of differing forecasts. Phasing differences among the ECMWF/UKMET/GFS lead to north-south differences on the axis/amounts of QPF and associated precip type amounts/duration. The snow probabilities are skewed further north in NY/VT following inclusion of some 21z SREF members and the SREF 700 mb forecast closed low further north than the ECMWF/UKMET/NAM/ECMWF Ensemble Mean. Petersen