Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 407 PM EST Sat Jan 18 2025 Valid 00Z Sun Jan 19 2025 - 00Z Wed Jan 22 2025 ...Great Lakes... Days 1-3... Deep low pressure centered over northern Ontario will maintain cyclonic flow with Arctic cold over the Great Lakes through at least Tuesday. NWly flow prevails through Sunday before backing Wly Sunday night through Monday. 72hr snow probabilities for >12" are over 50% in the western U.P., northern and southern western shores of the L.P. and over 80% for the eastern U.P. The shift to westerly flow over the eastern Great Lakes allows single banding to develop by Monday afternoon with heavy snow persisting into Tuesday night for Buffalo and the South Towns as well as northern sections of the Tug Hill where Day 3 snow probabilities for >8" are over 60% (80% for the Tug Hill). ...South-Central Plains... Day 2... The approaching longwave trough that will cause wintry weather along the Gulf Coast (see section below) will cause snow banding east from the southern CO/northern NM Rockies in a similar manner to the banding that occurred today east of the Rockies (not the heavy snow in the terrain last night) on Monday. Day 2 snow probabilities for >2" are 20-60% over SW KS extending into western OK. ...Central Appalachians, Mid-Atlantic and Northeast... Days 1-2... ...Significant winter storm to produce a swath of heavy snow from the Central Appalachians through the northern Mid-Atlantic and New England Sunday through Sunday night... The main guidance changes today on the Northeast winter storm are with a slower timing. RAP based guidance (HRRR) has incrementally slowed with each recent run since yesterday while the NAM based models remain on the slower end. Meanwhile a slightly farther east solution is seen for New England in the EC and 18Z HRRR models which would allow heavier snow a bit farther east than before. The Arctic cold front will push off/to along the Northeastern Seaboard tonight while stalling over the Carolinas as the surface wave tracks toward Cape Hatteras through Sunday morning. This wave rapidly develops as it tracks north along the frontal zone through Sunday night, reaching the Canadian Maritimes by 12Z Monday. Strong upper- level divergence increases off the Mid-Atlantic Coast through Sunday morning as the SWly 250mb jet exceeds 150kt with diffluent left- exit region aiding the rapid surface/coastal development. As 850-700mb WAA increases over the Mid- Atlantic, it will clash with the approaching Arctic air- mass to the north, resulting in a band of heavy snow that initially starts out over the Allegheny Highlands early Sunday morning, then spreads over the Laurels in PA across northern VA and MD through midday (which is a later timing based on the slowed guidance). Eastern PA through southern NY see great increases in 850-700mb FGEN through the afternoon allowing snowfall rates to increase through this swath. The 12Z HREF mean hourly snowfall hits 1"/hr over the northern half of NJ by 00Z Monday with 1"/hr rates in a rather wide swath tracking north through the night until it clears eastern Maine at 12Z. The storm races north through New Brunswick early Monday morning with lingering periods of snow throughout parts of Maine tapering off by midday Monday. Day 1.5 WPC snow probabilities for >6" snow are 40-70% over central WV terrain and over southern PA through Maine with 50% probs for >4" over far northern VA/northern MD, the Philly and NYC metros as well as a stripe of southeast New England generally west from Providence, RI. Hazardous driving conditions are anticipated in these areas on Sunday. With bitterly cold temperatures spilling in for the upcoming work-week, snow and instances of re-freezing/black ice are likely to stick around for much of the week in wake of this winter storm. ...Texas, Gulf Coast, and Deep South... ...Disruptive winter storm growing in confidence across the Southern U.S. next week... Days 2/3... Exceptionally cold/dry air plunges through Texas tonight, expanding east over the Gulf Coast through Sunday night. This sets the stage for wintry precip to develop in South Texas Monday as an approaching trough axis over the Four Corners allows a low level inverted trough to develop over the western Gulf, causing an onshore flow to develop and ride over the low level cold along and inland of the lower TX coast. This trough continues to develop through Monday night with precip expanding up through central and into North Texas and southwest Louisiana by 12Z Tuesday. Snow can be expected over central TX and north while an icy wintry mix can be expected over south-central and interior lower TX then along the TX and LA Coasts. The trough axis reaches the TX Coast Tuesday evening with rapid eastward expansion expected to the wintry mix through the day Tuesday, likely reaching the FL Panhandle by Tuesday evening. Moderate to locally heavy snow banding is expected from roughly north of the Houston metro east across southern LA into southern MS on Tuesday with notable snowfall for these areas. Day 3 snow probabilities for >4" are 30-60% through this zone. The conceptual model is for a stripe of sleet and freezing rain to be south of the snow. As currently depicted those stripes look to both be on or inland from the Gulf Coast. Day 3 ice probabilities for >0.1" are currently around 10% in both southern LA and just south of I-10 in south-central TX, north of Corpus Christi. Details with p-ptype are better done with higher-res models which offer output in the Days 1/2 timeframe, so expect greater detail to these wintry accumulations at that time. Please be sure to visit WPC's Medium Range discussion and our Key Messages for more details on the impacts that will extend through much of the Southeast through Wednesday. Jackson ...Winter Storm Key Messages are in effect as linked below... Southern/Gulf Coast Winter Storm... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png Northeast Winter Storm... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png