Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 345 AM EST Fri Feb 14 2025 Valid 12Z Fri Feb 14 2025 - 12Z Mon Feb 17 2025 ...Central NYS... Day 1... WNW flow will continue a lake effect snow band off Lake Ontario into the Syracuse metro this morning/afternoon and diminish later this evening as it lifts northward. Day 1 snow probs for >4" are 40-90% in a narrow corridor. ...Sierra Nevada through the Rockies... Days 1-2... Atmospheric River has moved well inland, but the lagging upper trough will continue onshore flow with instability enhancing precip over the Sierra Nevada as the low opens into a slow-moving trough. Snow levels have fallen back to 5000-6000ft under the trough axis (Sierra) and around 4500ft (Shasta/Trinity) bringing light snow to lower elevations. Snow should taper off later this evening from west to east across CA. Day 1 snow probs for an additional 8" are moderate(40-70%) across much of the Sierra. Moisture has reached the Rockies with a strong influx from the SW and the trough axis will pass the Four Corners early Sat, favoring moderate to heavy mountain snow with gradually lowering snow levels as colder air moves in. Snow rates will ease over the next 12-24 hours with additional snow over the Wasatch and Tushar in Utah and western CO Rockies northward into the Tetons and eastward into the San Juans and Sangre de Cristos. 48-hr snow probs for >12" are over 80% for these areas. ...Pacific Northwest... Days 2-3... The next multi-vorticity lobe upper low in the northeast Pacific will send a cold front into the Pac NW late Sat into early Sun. Snow levels around 3000-4000ft for the northern-southern Cascades (respectively) will give light to moderate snow to the passes that continue into early Monday as the trailing main upper low finally reaches the coast. WSW flow will spread moisture across eastern OR into ID, western MT and western WY where the favored terrain will see moderate snow totals. Cold temperatures over the northern Great Basin/northern Rockies will favor at least some snow to most valley floors. 48-hr snow probs for >12" are >50% for this higher terrain. ...Upper Midwest/Great Lakes/Northeast... Days 1-3... Eastward extension of the western trough will bring WSW flow to the Upper Midwest later today with embedded vort maxes atop an inverted surface trough. Thump of snow with this inverted trough is on warm air advection, but sufficient cold air is there to allow heavier snow with >10:1 SLRs over much of WI tonight. WPC snow probs for >4" are >50% from southeastern MN across central WI. A broader area of light snow is expected over much of the region, setting up a somewhat prolonged but interrupted period of light snow. Into D2, the western trough will push into the Plains and start to amplify in concert with the downstream upper jet (>160kts) across the Great Lakes. Moisture will continue to spread eastward across Lower Michigan, southern Ontario, then into the Northeast. With a trend toward a slower/sharper upper system, QPF has increased over much of the Midwest and into Lower Michigan where WPC probs for at least 4 inches of snow on Sat are >50% over most of Lower Michigan, but are at least 25% from southern Iowa and across northern IL. On the eastern side, WAA will promote light snow over much of NY with WPC probs for >4" snow highest over the Poconos into the Catskills, but also at least 30% over the higher elevations above 1500ft or so. To the south, cold air at the surface along/east of the Appalachians (NC through VA into central PA) will hang tough as overriding precip from the west encroaches, favoring freezing rain and perhaps some sleet on Saturday. With a sfc low track from the Mid-South through OH, warm air will eventually win out due to strong 50kt SW 850 flow. WPC probs for at least 0.10" icing are >50% over southwestern VA northward into central PA, where there are also probs for >0.25" icing (30-50%). By D3, the surface low is forecast to track from just west of PIT toward western/central NY with some coastal redevelopment over southeastern MA (Cape Cod) that may move into the Gulf of Maine. A lot of details have yet to be resolved, but this scenario will likely result in all snow close to the Canadian border where warm air intrusion is least likely, a mix of snow to sleet/freezing rain over much of the area between the I-90 and I-80 corridors, and rain farther south and southeast as warmer air surges up the coast. The timing of the many players will modulate the amount of each ptype, but for now the highest probabilities for at least 8 inches of snow are over the Adirondacks into northern VT/NH and interior Maine. Several inches of snow are probable (40-60% chance) along ad north of the I-86/Rt 17 to I-84 corridor (NY to CT). Icing is most likely in areas that are able to stay coldest the longest, including central PA, the Poconos, Catskills, and Berkshires where WPC probs for at least 0.10" icing are >50%. Again, the details of the system are still in flux which will influence where and as what type the precipitation may fall. Fracasso ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png