Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 324 AM EST Fri Nov 29 2024 Valid 12Z Fri Nov 29 2024 - 12Z Mon Dec 02 2024 ..Great Lakes... Days 1-3... *** Heavy Lake Effect Snow event through Monday *** Mid-level low remains stalled over James Bay into Saturday before will meander nearly in place through Saturday before drifting southeast to northern New England through Sunday night. Shortwave troughs cycle around this low and over the Great Lakes through this time with the subsequent long-wave trough axis swinging through the Great Lakes on Sunday. WNWly flow prevails across the Great Lakes through Saturday night with intense lake effect snow (LES) bands until then before veering flow to the NW spreads across the lakes through Sunday. Cold air advection will create 850-sfc temperature gradients that are as much as +20C or more at times due to lake temps that are well above normal. This will result in lake-induced instability of more than 1000 J/kg and inversion levels peaking above 10,000 ft. With impressive ascent off the lakes, expect multi-bands off Lake Superior and Lake Michigan, and intense single bands shifting off of Lakes Erie and Ontario. Some of these bands may additionally contain lightning and rates of 2-3"/hr, especially east of Lakes Erie and Ontario generally just south from Buffalo and around Watertown, NY. The heaviest snow accumulations are likely east of these eastern lakes, where 24hr WPC probabilities for more than >12" are high (50-90%) through Saturday night. On the preferred snow belts in WNWly flow on southern shore of Lake Superior and in the northern higher lands of the L.P. Day 1 WPC probabilities for >12" are moderate (30-50%) and moderate for >6" for Day 2. The veering flow on Sunday likely disrupts the intensity of the banding as they pivot south. Day 3 PWPF for >6" are generally around 40% for single areas of the U.P. and northern L.P. and 50-70% for the southeast Lake Erie shore and the Tug Hill. The arctic-sourced cold and LES off the Great Lakes through the weekend is the subject of the Key Messages linked below. ...Missouri to West Virginia... Day 2... Clipper-type trough currently approaching Vancouver Island will shift southeast around a large- scale longwave trough centered north of the Great Lakes to produce a swath of moderate to heavy snow over the lower Missouri Valley Saturday morning to the south- central Appalachians Saturday night. Although the amplitude of this feature will be modest, it will be favorably placed in conjunction with the left exit region of a WNWly jet streak. A residual baroclinic zone be in place from a stalled front, providing focus for the surface trough to strengthen, albiet modestly. The overlap of the accompanying fgen with the isentropic ascent and synoptic lift should result in an expanding swath of precipitation over central Missouri Saturday morning, falling as moderate snow in the better bands. The wave is progressive, but antecedent cold conditions and moderate snowfall rates should accumulate efficiently. Day 2 PWPF for >2" is 30% in a west-to-east swath over St. Louis and around 50% in southern WV. The probability of significant ice across the CONUS is less than 10 percent. Jackson ...Winter Storm Key Messages are in effect... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png