Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 358 AM EST Sat Jan 15 2022 Valid 12Z Sat Jan 15 2022 - 12Z Tue Jan 18 2022 ...Ozarks to the Tennessee and lower Ohio Valleys... Days 1-2... Surface low tracking southeast across Missouri will weaken as secondary low pressure strengthens over MS/LA. This secondary low will become dominant early today in response to rapid height falls beneath a closed mid-level low collocated with increasing upper diffluence in the RRQ of a strengthening jet streak to the east. As this surface low intensifies, WAA will spread the theta-e ridge northwestward as a TROWAL begins to develop, driving increased moisture into AR/TN/KY and surrounding areas. As the mid-level closed low shifts eastward, a deformation axis is likely to sharpen and pivot across the TN VLY and into the OH VLY this evening through Sunday aftn. Within this axis, precipitation will become heavy, and intense ascent will likely lead to dynamical cooling of the column to change precipitation from rain to heavy snow. The guidance has trended a little south tonight, and the latest PWPF reflects this shift. There remains some uncertainty into how much snow can accumulate, as it will likely be rate dependent and may fluctuate at times, based on intensity, from rain to snow. However, impressive ascent within the deformation axis and cross-sections indicating the potential for upright convection suggest snowfall rates of 1-2"/hr are possible. WPC probabilities are as high as 40% in the AR mountains D1 and then as high as 50% D1.5-D2 from NW MS through much of TN and into eastern KY. Significant impacts from heavy snowfall are possible. ...Southern Appalachians through the Interior Northeast... Days 1-3... A significant to major winter storm is expected to develop today and then lift up the coast through Tuesday morning. The primary low will develop near the Gulf Coast later today before lifting northeast just inland from the coast on Sunday, and then likely lifting west of I-95 Monday into southern Canada. This low will likely deepen rapidly as it ejects northward in response to a favorable and intensifying coupled jet structure and steady height falls/mid-level divergence associated with the closed low. Precipitation will become widespread and heavy as impressive warm and moist advection produces IVT above the 90th percentile, combined with a robust theta-e ridge which will wrap into an intense TROWAL. Pronounced 290K isentropic ascent will aid the deep layer ascent, with local upslope mechanisms further providing lift. Together, this suggests the entire east coast from SC to Maine will experience heavy precipitation, with snow, sleet, freezing rain, and rain all likely. The guidance has shown a subtle shift westward tonight, likely due to the further south track of the mid-level low pulling the surface wave back inland. At the same time, the preferred high-res guidance of the NAM, which usually better handles surface wedge setups (CAD) like this, is a bit stronger with the cold surface layer. With impressive 850mb LLJ within the WAA spreading inland, this will likely produce a major ice storm across the Piedmont of the Carolinas, where WPC probabilities for 0.25" are above 70%. Some guidance suggests freezing rain accretions could reach 0.75", but accretion efficiency should be quite low due to heavy rain rates, and the WSE and NBM 90th percentiles are both at or below 0.5". Still, significant and damaging ice is likely here. With the further west and subtly warmer solutions tonight, freezing rain is also becoming more of a concern across the Mid-Atlantic and as far north as the Catskills and Litchfield Hills of CT, but WPC probabilities for 0.1" of freezing rain as less than 10%. Northwest of the freezing rain, a large swath of heavy snow is expected from the Southern Appalachians through the Laurel Highlands of PA D1-2. In the terrain, the column will remain cold enough for snow, and as isentropic upglide becomes influenced by upslope ascent, snowfall will likely be prolific despite modest SLRs. WPC probabilities for more than 6 inches are above 70% in this region, with local maxima over 12" likely, highest in the NC mountains. North and west of the terrain, synoptic forcing will be most impressive, and a deformation axis characterized by impressive mid-level fgen and -EPV will produce heavy snowfall rates which may reach 2"/hr. The guidance continues to track slightly west with this axis, but confidence is high in a region of heavy snow and significant accumulations. WPC probabilities for 6+ inches are above 70% D2 in western PA and far western NY, reaching more than 80% into Upstate NY D3. Many locations within this band will likely receive more than 12 inches of snowfall. Further east the forecast is less clear-cut as warm advection/isentropic snow is likely to be significant, at least at onset, but may feature a changeover to sleet, freezing rain, or rain. For the I-95 corridor from Washington, D.C. to Boston, a thump of heavy snow is likely for a few hours at onset, but total snowfall accumulations should be modest, and WPC probabilities for 2+ inches are generally 30-50%. NW of the big cities, heavier snow is likely, especially in the terrain, and WPC probabilities feature a high risk for 4+ inches in a large area from central MD through much of interior New England where locally more than 8 inches is possible in the highest terrain. Weiss ~~~ Key Messages for Winter Storm Affecting the Mid Mississippi Valley and Ozark Region to the Eastern U.S. January 15-17 ~~~ -- Heavy snow is expected across portions of the Mid-South and Tennessee Valley. Narrow, localized areas of higher snow totals in excess of 6 inches are possible, but there remains uncertainty in the location and potential of these higher snow bands. -- Ice accumulation is likely from northeast Georgia through southern Virginia. The most damaging and significant icing potential is across the Piedmont of the Carolinas and this is likely to result in dangerous travel, power outages, and tree damage. -- Heavy snow is expected in and near the Appalachians, and across much of the interior Northeast and New England, well inland from the major cities. Significant snow totals are likely across western North Carolina northward to western New York. -- 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.