Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 335 AM EST Tue Nov 26 2024 Valid 12Z Tue Nov 26 2024 - 12Z Fri Nov 29 2024 ...Upper Great Lakes and Interior Northeast... Days 1 and 3... A negatively tilted trough over the Great Lakes will lift over New England today. Warm air advection ahead of the surface triple point that tracks east over southern NY this morning will continue to cause freezing rain to fall in sheltered valleys mainly around the White Mtns after 12Z where Day 1 PWPF for >0.1" ice is around 20%. Otherwise cyclonic flow over the Lakes behind the system will continue to allow LES off WNWly flow to spread over northern MI and the Tug Hill east of Lake Ontario where Day 1 PWPF for >4" snow is 10-50% in preferred snow belts with upwards of 6" in the Superior coastal sections of the eastern U.P. A reinforcing northern stream trough amplifies as it crosses the Midwest Wednesday night, taking on a negative tilt as it lifts over New England Thursday, promoting rapid surface cyclogenesis over the northern Mid-Atlantic and New England coasts through Thursday night. Arctic-sourced air shifting in with this trough will reinvigorate LES of Lakes Superior and Michigan where Day 3 PWPF for >4" are 20-60% over typical U.P. and northern L.P. snow belts in WNWly flow. Deformational snow banding is expected to develop late Wednesday over/just south of the eastern Great Lakes and intensify late Wednesday night through Thursday in a narrow corridor of the interior Northeast north and behind the rapidly developing coastal low. Day 3 PWPF for >4" are rather focused on terrain with 30-60% probs over the Tug Hill, Adirondacks, Greens, Whites, and far northern Maine. However, given the expectation for banding, heavy snow is likely in at least a narrow swath northeast from far NWrn PA through interior New England regardless of elevation. Details on the location and intensity of this band will further increase as it enters the finer resolution model timeframes. Those with Interior Northeast interests over Thanksgiving should closely monitor this snow forecast as it should be a disruptive event across portions of the interior Northeast from Thanksgiving into Friday. The Arctic-sourced cold and subsequent LES off the Great Lakes through the weekend is the subject of the Key Messages which are linked below. ...Southern Sierra Nevada, central Nevada and Utah ranges... Day 1... An atmospheric river will continue to deliver a relatively narrow axis of rich Pacific moisture over the southern Sierra Nevada east through the southern Wasatch through this evening above snow levels around 8500ft. The 00Z HREF continues to indicate particularly heavy 2-3" snowfall rates will occur today on the highest southern Sierra Nevada and more like 1-2" for the central Nevada and central/southern Utah ranges into this evening. Snow levels drop to 5000ft overnight as the rates drop off after a positively-tilted northern stream trough axis passes. Day 1 PWPF for >12" are categorical over the highest central Utah terrain and 1-3' additional is expected over the highest southern Sierra Nevada. This should result in significant disruption to those in such high elevations with tree damage and power outages possible. ...CO Rockies... Days 1-2... The atmospheric river responsible for the copious amounts of heavy snowfall over the Sierra Nevada will direct highly anomalous Pacific moisture into the southern Rockies into Wednesday. Strong upslope ascent in the southern CO ranges will be coupled with potent right entrance region jet dynamics results in heavy snowfall above 7000ft in northern CO and 8500ft in southern CO today with levels dropping to around 5000ft late tonight in the north and 6000ft Wednesday morning in the south as rates taper off. Prolonged rates of 1-2" per hour are forecast by the 00Z HREF today and tonight over most western slopes of the CO Rockies with maxima in ranges such as the San Juans and Sawatch where Day 1 PWPF for >12" are categorical. The WSSI shows swaths of Moderate to Major Impacts for most CO ranges. Widespread closures and disruptions to infrastructure are also anticipated in areas expecting Major Impacts. Moderate snow is likely along eastern CO slopes and the Palmer Divide Wednesday as lee-side cyclogenesis occurs over southeast CO before dipping into the TX Panhandle. Day 2 PWPF for >4" is generally 20-50% immediately east of the southern Front Range and Sangre de Cristos including the I-25 corridor. Jackson ...Winter Storm Key Messages are in effect for the northern Plains and Great Lakes... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png