Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 234 PM EST Tue Jan 17 2023 Valid 00Z Wed Jan 18 2023 - 00Z Sat Jan 21 2023 ...Western U.S... Days 1-3... Shortwave riding aloft the first half of D1 will give way to an approaching sharp but low amplitude wave moving eastward from the Pacific. This trough is progged to lift onto the WA/OR coast Wednesday night and then dig sharply southeast while amplifying into a closed low over the southern Great Basin Friday morning. A modest north-to-south oriented jet streak will dive southward as well atop this feature, providing modest enhancement to deep layer ascent through LFQ diffluence. A weak surface trough is progged to move onshore beneath the mid-level low, with this trough splitting into two weak lows, one moving across the Canadian border while a second low moves across the Great Basin. The associated frontal structure will also pivot onshore, led by a warm front Wednesday morning and associated IVT eclipsing 250 kg/m/s, and followed closely by a cold front Wednesday evening. The most significant precipitation is likely to be just ahead of the cold front, when snow levels are progged to rise to 3500-4000 ft within the WAA ribbon, but generally only west of the Cascades. Farther to the south and east, snow levels will be 1500-3000 ft, but slightly higher along the CA Coast. WPC probabilities on D1-D2 feature a high risk for more than 6 inches of snow stretching across the Cascades of WA and OR, and then extending down through the Shasta/Trinity/Siskiyou region and into the Sierra, where locally 1-2 ft is likely at the higher elevations. Late D2 into D3 the heaviest snow extends into the Great Basin and Four Corners where WPC probabilities for more than 6 inches reach as high as 20-40% as far south as the Mogollon Rim. ...Four Corners northeast through the Western Great Lakes... Days 1-3... A closed mid-level low will be positioned over the Four Corners at the start of the forecast period /tonight/ and then deepen further as it shifts eastward towards the Central Plains, reaching -3 standard deviations below the climo mean by Wednesday night according to the NAEFS ensemble tables. This evolution will occur on concert with a sharpening and amplifying 250mb subtropical jet streak positioned south of the parent longwave trough, placing intensifying favorable LFQ diffluence over the Central Rockies by tonight. The overlap of these features will help drive lee cyclogenesis Wednesday, with this surface low then shifting east-northeast into the Central Plains Thursday, and racing more quickly into 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 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. Forecast soundings have become more favorable for high SLR as the WAA beneath the DGZ helps to drive a near-isothermal layer beneath a deepening DGZ with impressive ascent producing a favorable crosshair signature from eastern CO through southern MN/WI. Farther north, the DGZ appears to be more elevated, but the colder column could still support higher SLR. Additionally, the guidance has come into very good agreement in placement and evolution of the important synoptic and mesoscale features, with CSI likely supporting impressive 1-2+"/hr pivoting snow bands beneath the best deformation over CO/NE on D1, with more transient but still impressive WAA/fgen bands lifting northward through IA/MN/WI/MI D2. These rates are additionally supported by the WPC prototype snowband tool. While there remains some uncertainty into the exact placement of the warm nose driven northward on the impressive WAA, and as to how far northeast the dry slot will pivot, overall the forecast is well aligned, noted by WSE snowfall standard deviation of just 4 inches near the heaviest bands/accumulations. This, combined with pWSSI reaching above 50% for MAJOR impacts in the High Plains, leads to high confidence that a significantly impactful snow event will occur, with at least moderate impacts due to freezing rain south of the heaviest snow axis. D1 WPC probabilities for more than 6 inches of snow are above 70% for much of the Colorado Rockies, the San Juans, and the Sangre de Cristos, but the heaviest snow is likely from the NE High Plains of CO, including the I-25 urban corridor, through eastern NE. Here, upslope flow into the Front Range/Palmer Divide/Pine Ridge, and the likelihood of pivoting banded structures with rates of 2"+/hr will likely result in some areas receiving nearly 18 inches of snowfall, with a surrounding broad shield of 6+ inches. During D2 the system becomes more progressive, but still impressive banding and large scale moderate ascent drives WPC probabilities for more than 6 inches to 50-80% from far northeast NE through the northern L.P. of MI. Locally, banding here could result in as much as 10 inches of snow, especially just south of the Twin Cities and up towards the Door Peninsula of WI thanks to lake enhancement. The entire system pulls away by D3 but residual moderate accumulations are likely south/southeast of the Great Lakes due to modest LES. South of the heavy snow axis, a stripe of significant freezing rain is likely with WPC probabilities above 70% for 0.1" stretching from north-central KS through southern IA, and locally more than 0.25" is likely in a few locations. Farther downstream, light accretions of up to 0.1" are possible from southern WI through the L.P. of MI. ...Northeast... Days 2-3... A vertically stacked low pressure over the Great Lakes Thursday aftn will occlude out to a triple point near NJ Thursday night, and then develop secondary low pressure near Cape Cod Friday morning. This secondary low will deepen in response to divergence/height falls downstream of the parent shortwave and accompanying LFQ upper diffluence as a zonally oriented jet streak extends from the Southeast to the Mid-Atlantic. While this secondary area of low pressure will likely move progressively to the east, the occlusion/inverted trough will hang back across New England through D3 resulting in continued ascent to produce an extended period of light to moderate wintry precipitation. Warm and moist advection downstream of the primary low will expand moisture from the Mid-Atlantic into the Northeast beginning Thursday aftn, 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 CT/RI, with some snow developing south of Lake Ontario and Lake Erie as well due to LES development. For freezing rain, WPC probabilities for more than 0.1" develop D2 across much of Upstate NY and northern PA, primarily on the leading edge of the precipitation shield before extending northeastward into the Catskills, Adirondacks, and Berkshires into D3. Probabilities exceed 70% in some of the terrain and surrounding foothills, with locally more than 0.25" possible. North of the freezing rain, moderate to heavy snow is likely late D2 and during D3, when WPC probabilities for 4+ inches exceed 70% for parts of central New England. Local maxima at least double this are possible where both the leading WAA snow and the trailing CAA/inverted trough snow can accumulate, which may occur across portions of NH and southern ME, but these finer details will need to be reasoned out with future forecast cycles. Weiss Key Messages for the High Plains through the Great Lakes 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. --Intense snowfall rates of 1-2"/hour are possible within the heaviest snow bands from northeast Colorado and southern Nebraska to northwestern Iowa. This could result in snowfall totals above 10 inches. --The combination of heavy snowfall rates and gusty winds will likely result in hazardous travel conditions due to blowing and drifting of snow creating reduced visibility. --An icy wintry mix of freezing rain and some sleet is likely from northern Kansas through southeastern Nebraska and into southern Iowa. This could result in slippery roads/sidewalks and isolated power outages.