Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 441 AM EST Wed Jan 18 2023 Valid 12Z Wed Jan 18 2023 - 12Z Sat Jan 21 2023 ...Western U.S./Great Basin/Four Corners.. Days 1-3... Sharpening mid-level trough off the WA/OR coast this morning will move ashore this afternoon, preceded by a cold front about 6-9 hours earlier. Though PW amounts will be near climo and IVT values will, at best, be up to 300 kg/m/s into the Olympics and Coastal Range early (before weakening), the sharp height falls will couple with upslope into the terrain to yield more than 6 inches of snow at higher elevations in the Cascades D1. Farther south, digging jet and a bit more divergence aloft will fight against waning moisture flux, but will still yield about 6"+ in the NorCal ranges and especially the Sierra. A broader area of light snow is expected for much of interior OR, western ID, and northern NV as the fronts moves southeastward. By the start of D2, the elongated trough will split into a northern (Canada) and southern (Great Basin) entity, though each losing their moisture tap to the Pacific. Generally light snow is forecast for much of Nevada into Utah and northern Arizona, where WPC probabilities of at least 30% for more than 4 inches are confined to higher elevations above 6000-7000ft. Even as the low closes off at 500mb, it will still be wont for moisture. That will not happen until later D3, once some limited Gulf moisture pushes north-northwestward by the end of the period. With the surface boundary still in tact and a closed low in the mid-levels, even limited moisture will wring out several inches o snow over the White Mountains in AZ, San Juans, Sangre de Cristos, and into the Raton Mesa where WPC probabilities of at least 4 inches are low to moderate, but still generally under 60%. ...Central Plains through the Western Great Lakes... Days 1-2... A closed mid-level low over southeastern CO at the start of the forecast period will lift northeastward D1 and deepen further as it reaches the Corn Belt early Thursday. This evolution will occur in concert with a sharpening and amplifying 250mb subtropical jet streak positioned south of the parent longwave trough, placing intensifying and impressive/favorable LFQ diffluence over the Central Plains this afternoon. The overlap of these features will help drive lee cyclogenesis as the surface low moves east-northeast today then picks up more speed to the northeast tomorrow, reaching the Great Lakes by Thursday night. Downstream of this low and associated mid-level trough, WAA will intensify drawing anomalous moisture out of the Gulf of Mexico, evidenced by PW anomalies of +1 to +2 sigma surging northward into the Upper Midwest on moist isentropic lift with mixing ratios around 5g/kg on the 295K surface. This will likely lift along the WCB into an impressive TROWAL across the High Plains and Central Plains, with a more modest TROWAL persisting all the way into the northern Great Lakes. Snow rates this morning through the afternoon over NE should reach 1-2"/hr per the WPC Snowband Probability Tracker tool and this is supported by model soundings that have all the right ingredients -- strong lift into and through a sizable DGZ with a nearly isothermal layer beneath it, a fully saturated column, and modest winds that should not blow apart aggregates. This pivoting band of snow will lift northeastward in tandem with the surface and 850/700mb lows through the Corn Belt and into the western Great Lakes, though the parameters will be less impressive that far downstream. Total accumulations may well exceed a foot over central Nebraska with more than 6 inches likely in a stripe northeastward into the Upper Lakes. WPC probabilities of at least 8 inches in the D1-2 period are high (>70%) over much of central/eastern NE and moderate (>40%) over parts of IA and into northeastern WI and the U.P. of Michigan. To the south, strong WAA well north of the surface warm front will ride up and over a sub-freezing boundary layer, favoring freezing rain and some sleet. Ice accretion of at least a few hundredths is possible over a wide area (KS to Lower Michigan), but WPC probabilities of at least 0.10" are >50% with some probability of more than 0.25" across northern KS/northwestern MO/southeastern NE/southwestern and southern IA. ...Northeast... Days 2-3... As the vertically-stacked low pressure over the Great Lakes Thursday afternoon matures, a triple point low will start to form over the central Appalachians/Mid-Atlantic as the occlusion stretches out west-east. The triple point will become the new dominant low as it deepens just south of Long Island and close to or over Cape Cod early Friday. Combination of leftover cold northeast flow around northern Quebec high pressure and a moist easterly flow around the low circulation will favor light to modest wintry precip for much of the Northeast. First will be in association with the low's passage near Cape Cod and second will be with the old parent low and dying occlusion/trough along the I-90 corridor. Warm and moist advection downstream of the primary low will expand moisture from the Mid-Atlantic into the Northeast beginning Thursday afternoon, and while a warm nose exceeding 0C at 850mb is likely to lift as far north as central New England, wet-bulb cooling near the surface as Canadian high pressure slowly retreats to the north and east should result in a swath of light to moderate sleet and freezing rain through Friday morning, with locally some heavier ice accretions possible surrounding the terrain of the Catskills and Adirondacks. Northeast of this axis, the column should remain cold enough for all snow, especially as the secondary low develops east of Cape Cod resulting in cold ageostrophic flow to offset the WAA, with CAA eventually becoming dominant on Friday as the low pulls away. Still sufficient moisture and ascent along the inverted trough as the column cools D3 is likely to cause p-type to transition back to snow as far south as northern CT/RI, with some snow developing south of Lake Ontario and Lake Erie as well due to LES development. Front-end burst of snow may yield a bit more snow than previously forecast, and overall totals have crept up just a bit especially over northern VT/NH over the Green and White Mountains. WPC probabilities for at least 4 inches of snow during D2-3 are high over the Adirondacks eastward into central NH, and above 10 percent (from Rt 2/I-495 west to near Albany) north through central Maine. Freezing rain will start over northwest PA and the Southern Tier of NY early Thursday, then expand northeastward through D2 across much of NYS generally north of I-84 with many areas changing to plain rain. As the triple point starts to develop, freezing rain chances increase over Upstate NY east of I-81 later Thursday especially where colder temperatures will be slower to warm (e.g., Catskills, Adirondacks, and Berkshires). WPC probabilities for at least 0.10" icing late D1 through early D3 are moderate (>40%) over eastern NY between I-87 and I-81. Fracasso/Weiss Key Messages for the Central Plains, Great Lakes, and Northeast Winter Storm: --A winter storm will develop over eastern Colorado tonight and track into the Great Lakes by Thursday with a mixture of heavy snow, sleet, and freezing rain. Snow and some ice will expand into the Northeast on Thursday and continue into Friday. --Intense snowfall rates greater than 2"/hr are possible at times within the heaviest snow bands from northeast Colorado into central Wisconsin. This could result in snowfall exceeding 15 inches in parts of Nebraska. --These heavy snowfall rates combined with winds gusting up to 35 mph will result in dangerous travel due to blowing snow with near-zero visibility at times. --An icy wintry mix of freezing rain and some sleet is likely from northern Kansas through southern Iowa. This will produce slippery roads/sidewalks, with isolated power outages and tree damage also possible where the heaviest ice accumulates.