Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 329 AM EST Sun Jan 16 2022 Valid 12Z Sun Jan 16 2022 - 12Z Wed Jan 19 2022 ...Tennessee Valley and Southern Appalachians through the Interior Northeast... Days 1-2... Significant low pressure system is beginning to organize across the Southeast this morning noted by an expanding shield of precipitation and a robust baroclinic leaf structure in satellite. This low will begin to lift east northeast from AL/GA this morning, and then move rapidly northeast along the I-95 corridor to be over central PA by Monday morning, and then lift into Ontario Canada by Tuesday. As this low shifts northeast, it will deepen rapidly in response to rapid height falls in advance of a closed mid-level low, and impressive upper diffluence within a strongly coupled jet structure. At the same time, warm and moist advection will intensify from the Gulf of Mexico and the Atlantic as 850mb winds rise towards 50 kts and a theta-e ridge wraps cyclonically within the WCB into a mature TROWAL. At the surface, cold high pressure centered over New England will be displaced eastward, but maintain a pronounced wedge down the coast and east of the Appalachians. This cold air will be entrenched initially, but is likely to rapidly erode within the pronounced WAA. This will produce an environment favorable for snow changing to sleet/freezing rain, and then rain, east of the Appalachians, but heavy snow is likely to be the predominant p-type in the terrain and to the west, with rain changing to snow over the TN and OH VLYs through CAA behind the system. Significant accumulations of snow and ice are expected. On D1 /Sunday/ the low will rapidly deepen and spread precipitation from the lower TN VLY into the Carolinas and northward as far as Upstate New York and Southern New England late. Across the Carolinas, impressive 290-295K isentropic ascent characterized by mixing ratios approaching 6g/kg will produce heavy precipitation which may briefly begin as snow everywhere away from the coast. However, rapid acceleration of the warm nose atop the reinforced surface wedge will produce a significant to damaging ice event, focused along the Piedmont. Low wet-bulb temperatures and modest cool advection within the wedge will support a long duration of freezing rain across this region, and WPC probabilities are more than 50% for 0.25" of accretion. It is possible some isolated locations may exceed 0.5" of accretion. Surrounding this axis, lighter freezing rain is likely as far east as the western coastal plain, and as far north as northern VA where WPC probabilities for 0.1" are moderate. In the terrain of the Southern Appalachians and as far north as WV, the impressive isentropic upglide will be enhanced by upslope flow to produce heavy snowfall, and WPC probabilities for more than 8 inches of snow are high in the higher terrain. Additionally on D1, the deformation axis will begin to sharpen and mid-level fgen will increase, into which intense moist advection will occur. Across the TN and OH VLYs, this will manifest as an axis of heavy rain changing to heavy snow through dynamical cooling. Even on D1, determining the snowfall from this band is challenging as accumulations will be highly rate dependent. During the most intense lift snowfall rates will likely exceed 1"/hr and potentially 2"/hr as theta-e lapse rates and overlapped -EPV suggest the potential for upright convection from western TN into eastern KY. The corridor of heavy snow is likely to be pretty narrow here, but guidance has been insistent that heavy snow will occur, and WPC probabilities for 4+ inches are moderate to high from central TN through the Cumberland Plateau and into eastern KY. This deformation axis is then likely to strengthen and pivot northward well NW of the 850mb low center, and there is high confidence in a band of extremely heavy snowfall, with rates of 2+"/hr, lifting slowly across WV and into western PA and Upstate NY. Some model soundings indicate the warm nose barely exceeding 0C in parts of this region, but it is likely dynamic cooling affects will overwhelm that and keep this region all snow. This region should experience the greatest accumulation, and WPC probabilities for 12 inches are 50-80%. The 90th percentile from both the NBM and the WSE are over 20", and isolated pockets of 2 feet of snow are possible. Lastly, for the I-95 corridor from Washington, D.C. through Boston and into New England, the forecast is the most tricky. Pronounced WAA ahead of the system will likely produce a burst of heavy snow along and NW of the urban corridor. Even along I-95 itself and inch or two of snow is likely before a rapid changeover to rain occurs. However, across the interior Mid-Atlantic and into New England, heavy snow will likely be more productive to accumulations before a changeover to sleet and freezing rain occurs. The guidance again has trended a bit warmer late D1 into D2, which has led to an increase in freezing rain probabilities for 0.01" all the way into southern VT and NH. However, this more robust WAA will also manifest as a period of heavy snow, which the WPC prototype snow band tool suggests could reach 2+"/hr across Southern New England, especially in the terrain of the Berkshires, Greens, and Worcester Hills, and then into the mountains of NH and ME. For this reason the WPC probabilities for 6+ inches have increased to 50-80%, with locally in excess of 1 foot of snow likely in the higher terrain of the White Mountains of NH and ME. As the low pulls away late Monday, NW flow behind the system will lead to some enhancement downwind of Lake Erie and Lake Ontario where additional accumulations of snowfall will contribute to the event total. Also, some upslope snow is expected to contribute a few inches of additional accumulation in WV and across the Northern New England mountains. ...Upper Midwest to the Northern Rockies... Day 3... An Alberta clipper will dive southeast across ND and towards the Great Lakes on D3 /Tuesday./ While this wave is likely to move rapidly to the east, it is accompanied by modest upper diffluence and mid-level height falls, as well as significant warm/moist advection to drive PWs to +1 to +1.5 standard deviations above the climo mean. At the same time, the column will remain quite cold which supports SLRs that will be above January median values (CIPS climatology). While there remain some latitudinal differences in the axis of the heaviest QPF, it is more likely that a stripe of moderate QPF leading to a swath of heavy snowfall will occur from northern MN through the U.P. of MI and possibly into WI and the L.P. WPC probabilities for 4+ inches are above 70% in this region, and local amounts over 8 inches are possible. Farther west, the tail-end of the associated cold front will sink southward into western Montana where upslope snow will increase around Glacier National Park. Weiss ~~~ Key Messages for Winter Storm Affecting the Mid Mississippi Valley and Ozark Region to the Eastern U.S. January 15-17 ~~~ -- Heavy snow rates greater than 1"/hr are possible across portions of the Tennessee Valley and along and just west of the Appalachians Sunday into Monday. Significant impacts to travel are likely. -- Ice accumulation will expand from northeast Georgia through the interior Mid-Atlantic states on Sunday. The most damaging icing is likely across the Piedmont of the Carolinas, including Charlotte, NC and Greensboro, NC, which will result in dangerous travel, power outages, and tree damage. -- Heavy snow is expected along the Appalachians into the interior Northeast. Significant snow totals are likely across western North Carolina northward through western Pennsylvania and New York into Northern New England. -- Along the Atlantic coastline, strong winds and coastal flooding are likely. Some locations may reach moderate flood levels, with inundation affecting some coastal roads and structures.