Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 346 PM EST Fri Jan 17 2025 Valid 00Z Sat Jan 18 2025 - 00Z Tue Jan 21 2025 ...Great Lakes... Days 1-3... Low pressure over the north-central Ontario along with surface ridge over western Canadian Prairies combine to push Arctic cold south down the Great Plains and east across the Great Lakes through Saturday night. The LES machine over Lake Superior begins this evening for the U.P. on NWly flow, the western L.P. overnight. Gulf-sourced moisture shifts in advance of the cold front will surge over the eastern Great Lakes Saturday before tapering off Saturday night. There's a bit of a lull Sunday over the eastern lakes with LES kicking in there Monday. Day 1 snow probs for >4" are 20-40% over the northern U.P. and 30-60% from the synoptic flow south and east of Erie and east of Ontario. Day 2 snow probs for >4" are 40-80% in the U.P. and portions of the western L.P., then higher on Wly flow for Day 3 with 40-70% over the same U.P./L.P. zones and east of Erie/Ontario. ...Colorado Rockies and High Plains... Day 1... The combination of both upper level diffluence ahead of a deep- layer trough and the arrival of a bitterly cold Arctic air-mass will continue to produce some snow squalls into this evening along the cold front from the north-central Rockies east across the Great Plains. Focused mountain snow is expected over the eastern slopes of the CO Rockies and adjacent high plains including the I-25 corridor tonight into Saturday. The base of the main trough descending from Canada has a vort lobe currently pushing south over Idaho. This will shift SSE over the Four Corners through Saturday which will aid lift over CO which has low level upslope flow post- frontal with a 1050mb high moving into MT on Saturday. Snowfall rates reach 1"/hr per the 12Z HREF on the eastern slopes of the Front Range this evening before the bands work their way south across the Raton Mesa early Saturday. Day 1 snow probs for >4" are high (60-90% over the Front Range down to Pikes Peak and 30-50% for the I-25 corridor through the length of CO. Be cautious of hazardous driving conditions in any further snow squalls and on the eastern slopes of CO terrain. ...Central Appalachians, Mid-Atlantic and Northeast... Days 2-3... ...Confidence has further increased from a non-ECMWF deterministic solution general model blend for a disruptive winter storm from the central Appalachians, central Mid-Atlantic through the Northeast with locally heavy snowfall possible along and west of the I-95 corridor... Guidance remains in good agreement on a two wave solution. The initial wave on Saturday is a shortwave in advance of the main polar sourced trough with the wave of concern the positively-tilted trough axis that lifts from the Southeast Sunday through Sunday night. AI-based guidance (ECMWF- AIFS and GFS GraphCast) have been consistent with coastal low development off the Mid-Atlantic coast Sunday lifting north past New England Sunday night. Ensembles (ECMWF-EPS, GEFS, GEPS) continue to be snowier, and pretty much all deterministics outside the ECMWF are as well. The 12Z CMC is the farther west/most interior low track while the 12Z ECMWF is the farthest east/offshore and has the lowest QPF of the deterministics. The ECMWF-AIFS, however, now has five days of consistent runs with a coastal low along the Mid- Atlantic coast. This solution has drifted north and been more potent over the more recent runs up through the 06Z. All to say that while most guidance has considerable heavy snow (in the 6-10" range) from eastern PA up through Maine, there remains some volatility and uncertainty in the heavy snow swath location and intensity. The heavier snow begins late Saturday night over eastern KY with terrain and low level fgen aiding enhanced snow over the central Appalachians through WV into PA and then northeast over the more interior central Mid-Atlantic (generally NW from the I-95 corridor of northern VA and north) with a max between eastern PA and southern Maine. Day 2 snow probs for >6" are 40-70% over central WV/far western MD into the Laurels of PA. Day 2.5 values for >6" are 30-70% from eastern PA north of Philly through eastern Maine with the highest probs from the Catskills through Mass and southern NH. There is expected to be a tight rain/snow line around the DC Metro and northeast east from I-95 through the NYC Metro and Boston Metro. Only minor adjustments in this line makes for impact changes to millions of people, so close attention should be taken in the coming days. ...Gulf Coast... Next week... A reinforcing shortwave trough rounding the Four Corners Monday brings about surface trough and eventual low development off the lower TX Coast starting later Monday. Cold air arrives into south Texas Sunday night, so overrunning flow may cause an onset of wintry mix over south Texas prior to 00Z Tuesday. Further info on this threat can be found in the medium range discussion (PMDEPD) and in newly issue key messages linked below. Jackson ...Winter Storm Key Messages are in effect and linked below... Gulf Coast winter storm threat next week next week https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png Northeast winter storm threat this weekend https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png