Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 237 PM EST Thu Feb 6 2025 Valid 00Z Fri Feb 07 2025 - 00Z Mon Feb 10 2025 ...Pacific Coast through the Northern Rockies... Days 1-2... ...Winter Storm impacting the West as a potent low tracks into the Pacific Northwest tonight and the northern Rockies by Friday... A stormy pattern across the West will finally come to an end late this week as the upper-level pattern resets to a much drier pattern. A cutoff low centered near Washington State will rejoin the polar jet and open up into a potent (but less anomalous) trough. An associated surface low off California coast will move ashore tonight, tracking northeastward into northern California/Nevada, southwest Oregon, and into Idaho by Friday morning (eventually transitioning to a leeside low over Colorado this weekend). The upper level trough will be weakening through late Friday night, however the surface low will tap into quite a bit of Pacific moisture which will be moving into the Pacific Northwest. The low- turned-trough will start out positively tilted, but as it moves into the interior Pacific Northwest it will become more neutrally tilted as it is fully absorbed into the polar jet. Global model initializations (12z Thu runs) depict significant a significant feed of moisture off the Pacific with more than 500 kg/ms of Integrated Vapor Transport (IVT), which still tops the 99th percentile (per NAEFS climatology). Snow levels will rise ahead of the low, but lower both behind it and to the north of the track. NBM snow levels into Oregon and Idaho are indicated to be between 3-4k ft. Into the southern Sierras, snow levels will rise to above 8k ft through the first half of Thursday night, then fall to around ~5k ft through midday Friday behind the cold front. For the northern Sierras, snow levels will stay between 6-8k ft for the bulk of the snowfall, falling to between 3-5k ft by Friday morning (as snowfall begins to taper off). WPC probabilities for the Sierras are moderate to high (50-70%) for snow amounts above 24 inches through mid-day Friday. Into far northern California, probabilities are moderate to high (50-70%) for snowfall amounts of over 12 inches for the same time period. While snow levels will be lower, there will not be nearly as much moisture available to convert to snowfall (as the IVTs are focused farther south of the front). From late tonight into Friday morning, the snow will spread northeastward into Idaho, Montana, and Wyoming as the shortwave and associated surface low pivot to the northeast (while generally weakening). Meanwhile, a 1030+ mb Arctic High will remain in place from Alberta and Saskatchewan and into the Northern Plains. This high will supply plentiful cold air to keep all of these areas as snow. Plentiful moisture from the warm side of the system will follow the cold conveyor belt into the northern Intermountain West, facilitating widespread snow. The greatest snow in this area will be into the Sawtooths of Idaho, Bitterroots of Montana and into the Wind River Range and Yellowstone/Teton N.P. areas of Wyoming. WPC probabilities in this area are moderate to high (50-70%) for 12 inches or more of snow through late Saturday night. WSSI values will get into the Extreme range (substantial disruptions to daily life, travel not advised) for the southern Sierras with the heaviest snow through tonight. Moderate to major impacts (considerable disruptions to daily life, avoid travel if possible) are expected from the Klamath mountains of northern California into southern Oregon as well as through the Sawtooth and Wind River Ranges. Churchill/Wegman ...Northern Plains through the Great Lakes and Ohio Valley... Days 1-3... A potent shortwave will eject from the interior Northwest Friday evening and begin to race east within flat/progressive flow entrenched across the CONUS. This shortwave trough will remain positively tilted through its evolution across the region, but have plenty of vorticity and accompanying PVA to produce impressive omega. This will additionally be enhanced by a potent subtropical jet streak exceeding 150kts which will both additionally enhance deep layer lift but also enhance IVT (progged to exceed the 90th percentile of the CFSR climatology into the Plains/Upper Midwest). This moisture will be additionally enhanced by a surge of WAA/isentropic ascent, especially along the 285-295K surfaces, downstream of the parent shortwave. This WAA will manifest as well as a stripe of enhanced fgen, especially within the 700-600mb layer. This will additionally improve ascent, and there is likely to be a region of intense omega driving heavy snowfall rates as the system moves rapidly eastward. At the same time, the DGZ is progged to deepen considerably in response to this WAA, which is reflected by SREF DGZ 100mb depth probabilities reaching 50% from eastern SD through the Twin-Cities of MN and into central WI. With the column being quite cold, this could result in fluffy and efficient dendrites that can accumulate rapidly. However, it should be noted that the best fgen lies at the top of this DGZ, so may not really manifest as an efficient "cross-hatch" signature for snow growth, and with the column so cold large aggregates will be difficult to achieve. Still, an above-climo SLR seems likely, and snowfall rates of 1-2"/hr should be supported within this translating swath of snow. The progressive nature of the system combined with the slightly less than ideal snow growth, will somewhat limit total snowfall, but this will still be an impactful event, and in some areas may be the heaviest snowfall of the season to date. Current WPC probabilities are 50-70% for 6+ inches across central MN and into western WI D2, with generally 3-6" expected on either side of the core of this snowfall from SD into MI. Farther to the southeast, the WAA will lift a warm nose more robustly immediately downstream of a developing surface low into the Upper Midwest and Ohio Valley, primarily after 12Z Saturday. This will cause a stripe of mixed precipitation to develop from Illinois through eastern Ohio, resulting in another round of accumulating sleet/freezing rain across these areas. WPC probabilities for 0.01" of ice are moderate (50-70%) from eastern IL through eastern OH, with locally more than 0.1" possible (10-30%) in IN/OH. Weiss ...Mid-Atlantic and Northeast... Days 1-3... The first in a series of winter weather events will be exiting this evening as a coastal low moves progressively eastward from New England. As this low departs this evening, it will continue to toss moisture back to the west on cyclonic flow, resulting in areas of still moderate snowfall across eastern New England and much of Maine. However, the progressiveness of the mid-level pattern will keep this system moving quickly away from the area, and snowfall associated with it should end before daybreak Friday. Additional accumulations of 2+ inches is likely across northern Maine as shown by WPC probabilities reaching 50-70% in the higher elevations. Behind this low, a potent cold front will race quickly southeast in its wake, leading to pronounced CAA and briefly impressive low- level convergence. This will have a two-pronged impact to winter weather across the Northeast. The first will be an increased threat for convective snow showers and snow squalls Friday morning, especially across Upstate NY and into Northern New England. Although the timing of the front is not ideal for impressive squalls, there is forecast to be enough overlap of low level RH, fgen, and instability to support scattered to widespread convective snow showers and isolated squalls. This is supported by simulated reflectivity in the high-res CAMs as well as HREF probabilities showing low-end chances for 1+"/hr rates. Accumulations with any squalls will be minor, but briefly heavy rates and lowered visibility could cause hazardous travel. Then behind this front, CAA causes a drop in 850mb temperatures to support sfc-850mb delta T nearing 20C across Lake Ontario (Lake Erie is now completely ice covered). EL's approaching 15,000 ft over Lake Ontario will help support a single band of lake effect snow (LES) with rates at times reaching 1-2"/hr beginning Friday aftn and persisting into Saturday aftn, with some upstream connection to Lake Huron aiding in the threat. This will likely lead to some heavy snowfall accumulations, especially across the Tug Hill Plateau, where WPC probabilities for 4+ inches of snow D1 are above 70%, with an additional 30% chance on D2, leading to LES total snowfall as much as 12 inches in isolated areas. A more significant system begins to take shape the latter half of D2. Despite the generally flat and progressive flow across the CONUS, a potent shortwave trough ejecting from the Plains will move into the Ohio Valley by 00Z/Sun, and then continue rapidly east across New England on Sunday. This will be accompanied by an impressive upper jet streak progged to reach 170kts as it arcs across the northern tier of the CONUS. Downstream and to the south of this overlapping synoptic ascent, a wave of low pressure will begin to develop along a warm front as it lifts northward in response to intensifying WAA, leading to an expansion of precipitation late D2 and moreso on D3 from the Ohio Valley northeast through the Mid-Atlantic and into New England. High pressure ahead of this developing low will retreat to the east Sunday, with the resultant isentropic ascent maximizing along the 290-300K surfaces. Mixing ratios within this lift are quite impressive, 4+g/kg, which will push a plume of PWs exceeding the 90th percentile according to the NAEFS climatology into the Mid- Atlantic and Southern New England. As this low skirts east off the NJ coast, it will likely intensify south of New England, and the resultant ageostrophic flow and CAA in its wake will halt the progression of the accompanying warm nose, keeping mixed precipitation confined to the Mid-Atlantic states. While there is still quite a bit of latitudinal uncertainty in the placement of this low and accompanying WAA strength, confidence is increasing that a significant winter storm will move quickly across the region D3. This will result in a swath of moderate to heavy icing, generally across the Central Appalachians and Mid-Atlantic states where WPC probs for more than 0.1" of ice are above 70% (10-30% for 0.25" in the Laurel Highlands and MD Panhandle). Farther north, however, the threat for heavy snow has substantially increased as impressive 850-600mb fgen overlaps with the ridge of elevated theta-e to support heavy snow rates and possible banded structures. Despite the fast moving system, WPC probabilities are high (>70%) for 6+ inches from near the Capital District around Albany eastward to the Boston metro area and SW ME. Locally double-digit snowfall is possible, especially across the Worcester Hills of MA and Monadnock Region of NH. Weiss ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png