Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 406 AM EST Fri Jan 14 2022 Valid 12Z Fri Jan 14 2022 - 12Z Mon Jan 17 2022 ...Upper to Mid Mississippi Valley... Day 1... Amplifying mid-level trough will continue to drop quickly southeast across the Central Plains before closing off near the ArkLaTex Saturday aftn. This feature combined with modest but favorable upper level diffluence will drive a surface low quickly southward from ND into MO. The favorable synoptic ascent will be enhanced by WAA and moist isentropic upglide to produce a swath of heavy snow, with snowfall rates approaching 1"/hr. Despite the rapid motion of this system, the deep layer ascent into an impressive moisture anomaly noted by PWs of +2 standard deviations will create a 12-hr period of heavy snow, with lowering and deepening DGZs to enhance the SLR. A nearly continuous band of heavy snow is likely across this region, and WPC probabilities are more than 90% for 4+ inches from near the Buffalo Ridge of SD southeast through far northeast MO. Across Iowa the WPC probabilities maximize at more than 50% for 8 inches, and both the WSE and NBM 90th percentiles suggest isolated maxima of 12 inches are possible. Along the western edge of the heavier snow, a wintry mix, including freezing rain is expected. While widespread heavy ice accumulations are not expected, WPC probabilities continue to indicate that at least some light accumulations are likely from central North Dakota to eastern Nebraska and western Iowa. ...Ozarks to the Tennessee and lower Ohio Valleys... Days 2-3... A mid-level low closing off near the ArkLaTex Saturday morning will dig nearly to the Gulf Coast as an amplified positively tilted trough swings through the region. At the same time, a strengthening jet streak to the east will begin to arc poleward placing intense upper diffluence within the RRQ of this jet streak over the Southeast. This evolution will spawn secondary surface low development, with impressive moist advection emerging from the Gulf Coast advecting a theta-e ridge within the WCB into a blossoming TROWAL. The interaction of the lead surface low beneath the closed mid-level low, and this secondary development will drive increasing mid-level deformation which guidance suggests will become intense. Within this deformation axis, theta-e lapse rates fall to near 0C/km, indicating the potential for CSI and intensifying snowfall rates, which both the HREF probabilities and WPC snow-band prototype tool indicate will exceed 1"/hr. There remains some uncertainty into the exact placement of this band, and how quickly the dynamic cooling associated beneath it will transition p-type from rain to heavy snow. However, guidance continues to increase the threat for heavy snow tonight, and WPC probabilities for 4+ inches are as high as 30% from northern AR across much of TN on D2, and above 50% in eastern KY D3. WSE plumes still feature a large spread, and where the most intense snowfall rates occur as this axis pivots eastward, more than 6 inches of snowfall is likely in some places. ...Eastern Seaboard from Georgia to Maine... Days 2-3... A closed mid-level low pivoting across the Gulf Coast will begin to lift northeast late D2 as it moves over GA/SC, and then races northeast to be over NJ by the end of D3. This will be accompanied by an increasingly coupled jet structure downstream of the pivoting trough, with intense ascent through diffluence aiding in the development and then strengthening of a coastal low pressure moving northeast D2-3. The guidance has come into much better agreement with the placement of this low lifting just onshore the coast, roughly along the I-95 corridor. As this low initially develops, mid-level confluence over the Mid-Atlantic will help reinforce a wedge of high pressure down the east side of the Appalachians in a typical Cold Air Damming (CAD) setup. As the low strengthens, moisture will surge northwestward on robust 290K isentropic ascent characterized by mixing ratios of 4-6 g/kg, and a theta-e ridge blossoming into an impressive TROWAL around the low pressure center. The combination of impressive ascent and deep moisture will produce a significant to major winter storm from the Southern Appalachians through Upstate New York. For snowfall, heavy snow on D2 should be confined to the Southern Appalachians where moist upglide will encounter an atmospheric column that is only cold enough for snow in the higher terrain of NE GA, Upstate SC, and the NC mountains. WPC probabilities on D2 are 40-60% for 4+ inches as the snow does not really begin until late in the period. However, by D3, the low begins to race northward pulling ample moisture with it an spreading snowfall across much of the east coast. While the low track along I-95 will allow WAA to cause p-type to transition to rain from snow well inland, this will not be the case in the higher terrain where WPC probabilities for 6 inches are above 70% from near Asheville, NC through the Laurel Highlands of PA, and it is likely many locations will receive more than 1 foot of snow. To the east, while a changeover to sleet, freezing rain, and rain is likely creating a sharp gradient in snowfall, impressive leading WAA will likely produce several inches of snowfall within the I-95 metropolitan areas before changeover. Further to the west, an impressive deformation axis is likely to pivot from eastern KY through the eastern OH Valley and into Upstate NY, producing a high likelihood for at least 6 inches of snowfall. Southeast of the heavy snow and across parts of central GA, much of SC and NC, and along the I-95 corridor of VA into MD, freezing rain is also likely. While freezing rain accumulations will generally be light in VA/MD, there is increasing signal for a significant to locally damaging ice storm across the SC and NC Piedmont. While some of the guidance tonight is indicating in excess of 0.75" of accretion, this is likely overdone due to runoff from heavy rain rates and a lack of strong surface dry air to maintain low sub-freezing wet-bulb temperatures. Still, the WSE and NBM 90th percentiles indicate the chance for 0.5" of freezing rain centered near Charlotte, NC and arcing both SW and NE from there, which matches the location of the highest WPC probabilities for more than 0.25" of accretion. Weiss ~~~ Key Messages for Winter Storm Affecting the Northern Plains to Upper Midwest January 14-15 ~~~ -- A fast-moving winter storm will drop through the Northern Plains and Upper Midwest today, continuing into the Mid-Mississippi Valley through Saturday morning. -- A swath of heavy snow with totals of 8-12 inches is expected with this system from southwestern Minnesota through far northeastern Missouri. Snowfall rates may reach 1"/hr at times. -- This snow will combine with gusty winds to produce slippery, snow covered roads and significantly reduced visibility. Travel will likely become hazardous to dangerous at times. ~~~ Key Messages for Winter Storm Affecting the Mid Mississippi Valley and Ozark Region to the Eastern U.S. January 15-17 ~~~ -- Confidence is increasing that a significant winter storm will create considerable impacts from this weekend into early next week. There is still uncertainty on the details, so check back on the forecast for updates. -- Heavy snow is expected from the Mid Mississippi River Valley, to the Appalachians, and well inland from the major cities, leading to difficult travel. There remains potential for heavy snow getting into the I-95 metropolitan areas. -- Significant icing is likely in parts of the Carolinas, and adjacent parts of northeast Georgia and central Virginia. This will likely lead to power outages and damage to trees. -- Close to the immediate Atlantic coastline, strong winds and coastal flooding will be possible.