Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 442 PM EST Wed Jan 18 2023 Valid 00Z Thu Jan 19 2023 - 00Z Sun Jan 22 2023 ...Central Plains, Upper Midwest, Great Lakes, and Northeast... Days 1-3... A wide ranging and impressive winter storm will spread heavy snow, along with sleet and freezing rain, from the Central Plains through New England by the end of the week. The event is ongoing this morning is being driven by a deepening closed mid-level low positioned over eastern CO aligned with the LFQ of a subtropical jet streak beginning to shift northeast around the associated longwave trough. Tonight and Thursday, this closed low, with anomalous height reaching -2 standard deviations from the climo mean at 500mb and 700mb according to the NAEFS ensemble tables will shift northeast and slowly begin to fill as it reaches the Great Lakes by the beginning of D2 /00Z Friday./ At the same time, the upper jet streak will maintain favorable LFQ diffluence atop the greatest height falls to support at least modest deepening of the surface low moving into the L.P. of MI Thursday night. The slow increase in forward speed leading to a more progressive system through D1 will likely result in lesser snow accumulations across the Upper Midwest and Great Lakes than across the Plains, but the evolution will still support impressive banding beneath a potent mid-level TROWAL wrapping cyclonically around the low coincident with a stripe of theta-e lapse rates <0C/km. Some pivoting bands are still likely across NE/SD early D1, but in general the nature of banded structures should transition to more laterally translating WAA bands shifting northward across IA/MN/WI and into MI. Snowfall rates within these bands will likely reach 2"/hr or more at times as reflected by the WPC prototype snowband tool which will result in rapid accumulations in some areas. WPC probabilities on D1 are above 80% for 6 inches across northeast NE and northwest IA, with a secondary maxima in northeast WI and into the eastern U.P. of MI where some lake enhancement and frictional convergence will enhance snowfall. Locally as much as 12 inches of snow is possible in these areas, with more than 6 inches likely in a broad swath extending as far NW as Duluth and Minneapolis, and as far east as Alpena, MI. Thursday night into Friday, this primary low will occlude out to a triple point near the Mid-Atlantic coast as it becomes vertically stacked over the Great Lakes. This will shift the greatest ascent eastward, with impressive WAA developing out of the Gulf of Mexico and Atlantic coast surging moisture northward into New England noted by PW anomalies approaching +3 standard deviations around 00Z Friday. This WAA will lead to intense moist isentropic upglide along the 295K surface, which will help expand precipitation across PA, NY, and into New England due to high mixing ratios around 5g/kg. As this precip surges northward, it will encounter a slowly retreating Canadian high pressure such that the warm air overrunning the cold air will initially produce mixed precip including sleet and freezing rain across PA/NY and southern New England, with snow likely across central and northern New England where the column is colder. In the presence of this strong omega, a thump of WAA heavy snow is likely early D2, with snowfall rates likely reaching 1"/hr or more at times, but at the same time the heavy precip rates should limit total freezing rain accretion due to runoff. However, as the secondary low (stemming from the triple point) shifts eastward, a dry slot will spread across New England to remove ice from the DGZ and lead to light freezing drizzle for a time on Friday aftn. Thereafter, the stacked low over the Great Lakes will shift eastward, and the associated occlusion will result in an inverted trough extending from the departing surface low to the weakening low approaching from the west. This should have a two-pronged result: one will be an increase in ascent both due to low-level convergence and steepening mid-level lapse rates, with the second being a cooling of the column in response to CAA behind the departing wave and an increase in the ageostrophic cold air drainage from the north. While the guidance is not exceptionally robust with additional snowfall, these setups can sometimes over-perform, and the WSE plumes indicate a lot of spread late D2 for this potential, with a continued upward trend noted in the temporal means. For snow, WPC probabilities for more than 6 inches peak above 50% for the eastern Adirondacks, Greens, much of NH, and coastal ME, where locally more than 8 inches is possible, driven by both the lead WAA thump of heavy snow, and the more moderate, lower density snow on Friday aftn/eve. It is possible these probabilities may shift southward into the Worcester Hills and northern MA with later iterations if the cold air draining can be more intense. WPC probabilities for freezing rain exceeding 0.1" are 30-50% late D1 into D2 for parts of northern PA, southern Upstate NY, and into the Catskills/Berkshires. Locally 0.25" of ice accretion is possible, but the relatively modest warm nose, impressive precip rates, and slightly colder trend in the models indicates ice accretions should generally be moderate. Finally, as the entire system ejects eastward late D2 into D3, flow will become northwesterly as CAA develops in earnest. This will bring colder 850mb temperatures across the Great Lakes yielding at least modest lake effect (LES) snow in the typical NW snow belts. The progged 850mb-sfc delta-T is not very strong and inversion heights in the various regional soundings are rather low. This should keep LES generally light, but WPC probabilities D1 into D2 feature a low to moderate risk for an additional 4+ inches across the U.P. of MI, far northwest L.P., and then downstream of Lake Erie towards the Chautauqua Ridge, and the Tug Hill Plateau east of Lake Ontario. ...Western U.S./Great Basin/Four Corners.. Days 1-3... A sharpening mid-level trough will move onshore the Pacific NW coast this evening and then dive southeastward and close off over the southern Great Basin Friday morning. This feature will then continue to deepen as it shifts towards the Four Corners by Saturday, leaving downstream divergence into the southern High Plains/Texas Panhandle where it will overlap with impressive diffluence along the LFQ of a subtropical jet streak. Moisture noted by PW anomalies across the West are near normal to even slightly below, but the robust deep layer ascent aided by both warm front/cold front convergence will wring out all available moisture, falling as snow above 3000ft during the heaviest precipitation across CA and west of the Cascades, with 1500-3000 ft snow likely farther to the east. On D1, WPC probabilities for more than 6 inches of snow are confined to the Sierra and some spillover into the Great Basin terrain, but are highest in the Sierra where locally 12 inches of snow is possible. As the mid-level low intensifies and begins to interact with the upper jet D2 into D3 across the Four Corners and southern High Plains, a surface low pressure is progged to develop across southern CO. Downstream of this low, moisture advection will intensify with moist isentropic ascent maximizing around 290K and then surging northwestward into the High Plains. Although PW anomalies D3 are still modest, peaking only around 0.5 standard deviations above the climo mean, the guidance continues to become deeper with this low development which could result in some enhanced snowfall from the High Plains of NM and CO eastward into central KS by Saturday evening. The trends will need to be monitored as if this low can deepen further it could result in heavier snow through some banding on the north side, and at this is reflected by significant spread in snowfall among the WSE plumes in the vicinity and ECMWF EFI values of 0.9 for snow. Although some heavier snow is likely D2 beneath the upper low from the southern Wasatch through the Kaibab Plateau and along the Mogollon Rim, where WPC probabilities for 6+ inches reach 20-30%, the heaviest snow is likely on D3 as the surface low consolidates. WPC probabilities for more than 6 inches reach as high as 60% in far northeast NM and southeast CO, with 10-30% probabilities extending far north and east into the TX/OK Panhandles and western KS. Weiss Key Messages for the Central Plains, Great Lakes, and Northeast Winter Storm: --Low pressure will continue to produce heavy snow across the Central Plains tonight, before spreading into the Upper Midwest and Great Lakes through Thursday, and across the Northeast Thursday night into Friday. --Intense snow rates greater than 2"/hr are possible at times within the heaviest snow bands from Nebraska through northern Michigan. This could result in additional snowfall of 6-12 inches, producing event total snowfall in excess of 15 inches in some areas. --These heavy snow rates combined with winds gusting up to 35 mph will result in dangerous travel due to blowing snow with near-zero visibility at times. --Heavy snow rates of 1+"/hr will create dangerous travel early Friday morning across New England, with lighter snows continuing through much of Friday. --Freezing rain may lead to ice accumulations of more than 0.1", especially in the Catskills, Poconos, and Berkshires, leading to slippery travel and isolated power outages.