Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 159 PM EST Wed Jan 25 2023 Valid 00Z Thu Jan 26 2023 - 00Z Sun Jan 29 2023 ...Lower Great Lakes, Central Appalachians, and Northeast... Days 1-2... Surface low pressure moving through the Ohio Valley beneath an opening mid-level wave will traverse northeast tonight into Upstate New York. As this occurs, the energy will begin to transfer to a triple point over the Mid-Atlantic, with secondary low pressure developing and then moving northeast through New England. Although mid-level height falls and PVA will be weakening through D1, still impressively coupled upper jet streaks will result in strong ventilation aloft to support the deepening of this secondary low as the first low over NY occludes and fills. Strong warm and moist advection downstream of these dual-lows will spread heavy precipitation northward through the Mid-Atlantic, Upstate NY, and New England, before the dry slot races into the region shutting off heavy precipitation everywhere by Maine during Thursday morning. While this precip will manifest initially as heavy snow in most areas due to fgen driven by WAA, this same WAA will surge a warm nose northward, which when combined with the dry slot will turn precip over to sleet, freezing rain, and then rain most areas south of Northern New England. Even across northern NH and ME, the guidance has trended farther north with the secondary low, resulting in a warmer solution, and overall snowfall has trended downward today, but freezing rain has increased. WPC probabilities for more than 6 inches are above 50% only for northern NH and the northern 2/3 of ME, with locally 12 inches possible near Caribou. At least 2 inches of snow is likely in most areas before the transition, however. With the warmer solution now likely, a longer duration of freezing rain both due to the warm nose and the dry slot entering the DGZ could result in notable ice accretions, with WPC probabilities for 0.1" reaching 20-40% for parts of the southern Adirondacks and southern Greens, into the Monadnock region of NH and northward through the White Mountains. As the system departs late D1 into D2, CAA on NW flow will develop behind it. Although the 850mb-sfc delta-T is modest, strong omega into the subtly deepening inversion should yield periods of heavy LES in the favored N/NW snow belts downstream of Lakes Michigan, Erie, and Ontario. The long duration of this favorable setup will produce moderate to heavy snowfall accumulations, with WPC probabilities for more than 4 inches reaching 40-80% both D1 and D2, with the highest accumulations possible along the Chautauqua Ridge. Additionally, the NW flow will result in favorable upslope snow development across the Central Appalachians of WV, with a secondary maximum possible along the spine of the Green Mtns in VT. WPC probabilities for 4+ inches are around 50% D2, with storm total snowfall of around 10 inches possible in WV. ...Northern Rockies and Northern High Plains... Days 2-3... Broad troughing will encompass most of the CONUS through the weekend, while spokes of vorticity rotating through the flow drive ascent into the northern tier. Two distinct shortwaves will work together to produce a swath of heavy snowfall late Friday through Saturday, with snow accumulating rapidly from the terrain of the Northwest through the Northern Plains. A lead shortwave will drop out of Saskatchewan into Minnesota Thursday night into Friday, driving a clipper-type low pressure along the international border. While this feature will produce some light snow ahead of its associated warm front, accumulations should generally be modest. However, the trailing cold front will begin to sink slowly southward into the United States and bank against the terrain from the Northern Rockies through the Black Hills on Friday, pushing farther south into the Central Rockies during Saturday. As this occurs, a second shortwave will eject from British Columbia and drop almost due south into the Pacific Northwest and then along the front, interacting with the low-level baroclinic gradient to produce a secondary wave of low pressure. This surface low will move southeast along the front, aiding in ascent, which will become even more impressive as upslope flow intensifies on NE flow in the post-frontal regime thanks to the surface low surging southeastward into the Northern Plains Saturday. Additionally, mid-level fgen will increase along the front, and the placement of the most intensely sloped fgen appears to correlate well with the deepening DGZ as the column cools, suggesting snowfall will become intense into the terrain with snowfall rates eclipsing 1"/hr likely, as snow levels fall quickly from around 2000 ft early D3 to the surface by the end of the period. There is high confidence in this becoming an extreme snowfall event into the terrain, with multiple feet of snow likely in some areas. WPC probabilities for more than 8 inches are high both D2 and D3 from the Northern Rockies and Bitterroots southeast into the Absarokas, NW WY ranges, Wind Rivers, and into the Park Range of CO. Locally, more than 3 feet of snow is likely in the higher terrain. Moderate accumulations are also possible in the lower elevations and valleys as the intense dynamics overcome more marginal thermals initially, with the column cooling impressively by D3 to further support snow at low levels. A secondary maxima of snowfall accumulation is likely across the Northern Plains, generally SD/NE where the surface low tracks along the front, and a jet streak downstream of the amplifying trough over the West intensifies to 120kts, leaving favorable RRQ diffluence atop strong 850-700mb fgen and at least modest theta-e advection downstream of the surface low. Regional soundings indicate the column will be quite cold at this time, leaving a deep DGZ with a cross-hair signature due to the low-level fgen. This should produce fluffy SLRs and efficient snowfall accumulation despite the progressive nature of this system, and WPC probabilities for more than 6 inches are 40-60% along the SD/NE border. ...Lower Great Lakes/Ohio Valley/Central Appalachians... Days 1-2... Low pressure ejecting across New England will trail an accompanying cold front eastward across the Ohio Valley and then offshore the Atlantic Coast by Thursday morning. Behind this front, secondary shortwave energy embedded within the amplifying longwave trough across the east will cause additional subtle height falls, which combined with periods of PVA will likely produce scattered snow showers and snow squalls Thursday evening into Thursday night. While there is still some uncertainty into the coverage of convective-type snow, the snow squall parameter is progged to reach well above +1 from Lower Michigan through much of the Ohio Valley and into the Central Appalachians. This is due in part to an overlap of 0-2km theta-e lapse rates falling to slightly below 0, with at least pockets of SBCAPE reaching 100-200 J/kg, where low-level streamlines indicate additional moisture transport from the Great Lakes. The high-res guidance suggests scattered to widespread snow showers with at least modest 1"/hr snow probabilities evident in the HREF, from lower Michigan through northern Tennessee and stretching as far east as western Pennsylvania Thursday evening. While total snowfall within any of these snow showers or squalls will be minimal, the combination of heavy snow rates with gusty winds of 20 kts or more could produce brief periods of severely reduced visibility resulting in hazardous travel. Weiss Key Messages for Jan 25-26 Winter Storm... --A winter storm will track through the eastern Great Lakes overnight. Bands of heavy snow are expected over northern New York and New England with snowfall rates of 1-2"/hr. Some icing is possible, especially over New England. --As a second area of low pressure will develop over Southern New England and move into the Gulf of Maine by early Thursday, snow will change to a wintry mix and some rain along the coast. Heavy snow is forecast for interior locations where over 10" is possible. --Strong winds up to 50mph are possible over some coastal locations, which may result in power outages and scattered tree damage. --Snow-covered roads and reduced visibility due to blowing snow will make travel difficult across impacted areas.