Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 343 AM EST Mon Feb 10 2025 Valid 12Z Mon Feb 10 2025 - 12Z Thu Feb 13 2025 ...Great Lakes... Days 1-2... Large surface high pressure stretching from the northern Plains through Mid-Atlantic creates enough of a pressure gradient across the Great Lakes to promote continuing westerly 850mb wind up to 35 kts at times thanks to a couple passing shortwaves. The first shortwave is racing across the Great Lakes and New England this morning with a third impulse speeding across the region Tuesday morning as flat/progressive flow persists across the CONUS. Each of these subsequent shortwaves will be accompanied by a weak cold front, driving enhanced CAA across the Great Lakes to support periods of lake effect snow (LES). On D1, 850mb temps fall to -15C or more, steepening low-level lapse rates sufficiently to lift inversion levels to nearly 10,000 ft, which with ascent maximizing into the lowering DGZ will cause bands of heavy snow with rates of 1"/hr or more across the U.P. of MI, as well as SE of Lake Ontario and into the northern Finger Lakes region early D1 before flow shifts more westerly into the Tug Hill by tonight. WPC probabilities D1 are moderate (40-60%) for 6+ inches along the SE lake shore of Ontario and Tug Hill, as well as across the Keweenaw Peninsula of the U.P. of MI. ...Mid-Mississippi Valley to the Mid-Atlantic... Days 2-3... Confluent mid-level pattern will maintain progressive flow across the CONUS as the next in this parade of storms develops over the Central Plains and Ozarks Monday evening. This system will strengthen in response to vorticity energy shedding from the base of a longwave trough initially over Baja, combining with the RRQ of a southern stream jet streak which begins to amplify and arc meridionally, leaving favorable diffluence atop the mid-level PVA/divergence. The downstream confluence of the northern and southern streams should result in suppression of this wave, keeping it fast and generally flat from west to east, but will also result in significant ascent and a prolonged period of precipitation as isentropic ascent from the Gulf surges northward and spokes of vorticity continue to lag back to the SW leaving a long duration of ascent. The north-south trends with this system appear to have stabilized as most upper features settle into place with only subtle changes in the forecast expected at this point. This aims the heaviest axis of overlapping QPF and subfreezing thermals from northeast KY, through central VA into the Delmarva. Based on current guidance, this is beginning to look like a significant winter event containing snow and sleet and freezing rain, generally from the Ozarks almost due east into the Central Appalachians and the I-95 corridor from Richmond, VA to Baltimore, MD. The downstream confluence of the mid-level pattern will force this generally west-to-east axis of heavy precipitation, with 290-295K isentropic ascent and accompanying WAA fueling the elevated PWs progged by NAEFS to exceed the 90th climatological percentile as far north as the TN VLY and southern Mid-Atlantic (NC/VA border). This will supply plentiful moisture for heavy precipitation, and the sharpening baroclinicity on the WAA leading to impressive 850-700mb fgen will help to concurrently enhance ascent. This suggests that precipitation rates will be heavy at times, although in areas with snow the DGZ appears elevated and the best fgen is well below that, which when combined with modestly "warm" temperatures should keep SLRs low and this could be a heavy and wet snow for most of the area. The end of the latest 00z HREF depicts the potential for snowfall rates up to 1"/hr in the heaviest band. Current WPC probabilities are high (>70%) in northeast KY, the central Appalachians, central VA into southern MD and the central Delmarva D2 for 4+ inches of snow. Medium probs (40-70%) expand northward through northern VA, MD, DE, and southern NJ, with a very tight gradient expected along the northern and southern fringes. This places the highest potential for significant snow between Richmond, VA and Washington, D.C. Farther to the south, there is likely to be a transition zone from rain to snow which will include axes of moderate to heavy freezing rain, especially in the higher elevations around the Ozarks, as well as across portions of the Central Appalachians from eastern KY through SW VA. There remains a lot of uncertainty as to the amounts of icing, especially noting that rates could be heavy at times which don't efficiently accrete, and with the general synoptic uncertainty continuing, but there is high confidence (>70%) for warning-level icing (above 0.25") of ice in SW VA, with locally as much as 0.6" of damaging ice possible through Thursday morning. As vorticity lobes continue to shed eastward from the Baja trough, and as the developing full latitude trough (merging northern and southern streams over the Intermountain West) lags the first impulse, a second system with additional precipitation will approach this same area from the SW late D3 /Wednesday aftn./ This will again produce an axis of all p-types from the Ozarks eastward to the Mid-Atlantic, and its possible in some areas there will not even be much of a break before this second impulse. The Wednesday system developing in the Central Plains will likely be more intense with respect to the surface feature and crossing west of the Appalachians, so more warm air should flood northward downstream, but the evolution of this second wave will be somewhat dependent on the track of the first event. At this time, WPC probabilities for 0.1" of freezing rain D3 generally just feature patch 10-30% from the Ozarks into northern KY. ...Central Rockies/Plains/Midwest/Great Lakes... Days 2-3... As the longwave trough over the West passes the Rockies Tuesday into Wednesday, precipitation will expand across the central Plains. Snow over the Rockies will accumulate several inches Tuesday afternoon and overnight through early Wednesday over the Medicine Bow mountains with at least 6 inches above 10,000ft likely (probs >50%). Positive tilt and broad SW flow will favor a progressive system D3, but the potential exists for moderate to heavy snow across much of Kansas Tuesday night into Wednesday. Upper jet will extend from TX northeastward to the Midwest with speed divergence over the Plains helping to promote ascent into a moistening column on WAA from the south in the lower levels. A large high pressure to the north will supply NE winds to the region but the gradient is not expected to be strong enough to produce much blowing snow. By early Wednesday, snow could fall heavy at times over central KS as favorable FGEN aligns with sufficient lift into a potentially very deep DGZ and/or isothermal layer, but these smaller scale features are hard to pinpoint this far out. In addition, SLRs may be close to 20:1 or higher in these favorable areas but otherwise waver around 15:1 given the colder air mass. Dry air is always a concern along northern areas of the precipitation shield, but the duel jet structure developing over the central U.S. and a broad moist forecast in upper air relative humidity fields, the greatest uncertainty will likely be associated with localized forcing and banding potential. By Wednesday night, the system begins to further organize and pick up forward speed as the surface low lifts through the Ohio and Tennessee valleys. Snow is expected to expand from southern IA and northern/central MO through much of IL, southern WI, northern IN, and the L.P. of Michigan. A narrow corridor of mixed ptype is also still expected somewhere from southwest MO through southern IL and into southern IN and western OH. The event will quickly conclude by the end of D3 across the Midwest as CAA and dry air rushes into the region. However, increasing WAA will begin to spread snow, sleet, and freezing rain northward across the Lower Great Lakes and Northeast by early Thursday. WPC probabilities of at least 4 inches of snow for this large system are >50% from far eastern CO across much of KS, southern NE, northern MO, southern IA, central/northern IL, southern WI, northern IN, and much of the L.P. of MI. This includes cities such as Kansas City, Des Moines, Chicago, and Milwaukee just to name a few. Within these probabilities, 20-50% chances for at least 8 inches of snow follow along and just north of I-70 in KS and span east-northeast across the Midwest towards lower Michigan. Maximum snowfall amounts of 8 to 12 inches appear most likely. Southern side of the precipitation shield over OK and into the Ozarks will likely be more marginal, and a mix or sleet and freezing rain is likely due to the overrunning of the cold surface with warmer air aloft. There, WPC probabilities of at least 0.10" icing are low (10-30%). ...Pacific Coast... Day 3... Although still far out in time with limited confidence and extending into Day 4, it appears the West will begin to become more active again mid- to- late week as moisture begins to stream onshore from the Pacific. During D3, there remains noticeable spread as to where the best moisture plume will go (GEFS quicker and north compared to the ECENS) as it gets restricted to the SW on the periphery of a large trough over the CONUS and beneath a short- wavelength ridge over the Pacific Northwest. At the same time, a deepening trough over the Pacific will begin to track eastward, leading to a push of moisture a bit farther NW and into southern/central CA. This could result in some light to moderate snow reaching the Sierra before 00Z Thursday, but unquestionably more active weather with much heavier snow will develop beyond this current forecast period. At this time, WPC probabilities are moderate (30-60%) for 6+ inches in the Sierra, generally above what could be very low snow levels of 2000-2500 ft. Snell/Weiss/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 https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png