Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 223 PM EST Sat Nov 30 2024 Valid 00Z Sun Dec 01 2024 - 00Z Wed Dec 04 2024 ..Great Lakes... Days 1-3... *** Heavy Lake-Effect Snow event into early Tuesday *** Broad cyclonic flow from the upper levels to the surface will continue across eastern Canada and across the Great Lakes through Tuesday when the trough will finally start to move out of the Northeast. With Great Lake SSTs well above average for this time of year (+8C to +14C per GLERL) and 850mb temps around -10C, the sfc-850 delta-T will remain ~20C which supports strong/intense uplift/instability. The sometimes extreme instability will support cellular convection embedded within well-formed lake-effect bands and may include thundersnow. The cyclonic flow and continued moisture influx will also favor broad atmospheric lift irrespective of the lakes. This favorable atmosphere will maximize each lake's ability to continue producing heavy lake-effect snow. The general northwesterly flow will also allow for the upper lakes to feed the lake-effect off the lower lakes. This will be a large contributor to the prodigious snow totals forecast southeast of the lower lakes. Lake Superior and Georgian Bay (part of Lake Huron) will feed into Lake Ontario's lake band, while the bulk of Lake Huron will feed into Lake Erie's lake-effect bands. Lake Erie will be increasingly dependent on Lake Huron's support as the winds shift more northwesterly and become more perpendicular to the long axis of the lake. Upslope into the terrain of far northwest Pennsylvania and far western NY for Lake Erie's bands and into the Tug Hill east of Lake Ontario will locally further enhance the lift and ability of the bands to produce incredibly heavy snowfall -- at times exceeding several inches per hour. Storm total snowfall could approach 6 feet east of Lake Ontario and 3-4 feet southeast of Lake Erie. Off of the upper lakes, the broader geography of the larger lakes makes single band formation much more difficult, so multiple smaller bands of moderate to occasionally heavy snow are likely to impact portions of the U.P. and the northwestern lower peninsula. Snowfall amounts over a foot are expected in the eastern U.P. and far western U.P. as well as east of Traverse City. WPC probabilities of at least an additional 18 inches of snow are high (>70%) near Watertown-Fort Drum through Sunday afternoon. Over southwestern NY, WPC probabilities for at least an additional foot of snow are high (>70%) just south of Buffalo along I-90. The latest Winter Storm Severity Index values show extreme impacts through Monday in the Watertown, NY area east of Lake Ontario and along the I-90 corridor from the Buffalo southtowns through Ashtabula, OH, including through Erie, PA southeast of Lake Erie. Travel will remain extremely dangerous to near impossible with numerous road closures. By Tuesday, the upper trough will finally start to shift eastward with its axis moving east of 70W by the afternoon, helping to slowly diminish the lake effect snowfall. Upstream, another weak disturbance will move out of southern Canada into the northern Plains/Upper Midwest with light snow spreading across northern MN. Amounts look fairly light at this time, and WPC probabilities for at least 4 inches of snow are around 10% through 00Z Wednesday right along the Canadian border. The ongoing lake-effect snow is a subject of the Key Message linked below. ...Southern IN/northern KY to West Virginia... Day 1... Broad upper troughing over the eastern CONUS will carry a weak disturbance out of the MS Valley this evening and across TN tonight as it slowly weakens. Attendant surface front and weak area of low pressure will promote a light west-to-east axis of snowfall along the I-64 corridor east of MO tonight. Recent guidance has trended just a tick south and lighter with QPF (thus snowfall), and WPC probabilities for at least 2 inches are low (10-40%) mostly over southern IN and somewhat across northern KY. Farther east, upslope into the central Appalachians will be the main driver for heavier amounts and WPC probabilities for at least 4 inches of snow are >70% above 1500ft or so. The probability of significant ice across the CONUS is less than 10 percent. Fracasso/Wegman ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png