Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 326 AM EST Thu Jan 13 2022 Valid 12Z Thu Jan 13 2022 - 12Z Sun Jan 16 2022 ...North to Central Great Plains/Upper to Mid Mississippi Valley... Days 1-3... Shortwaves ejecting southward from Alberta, Canada, and the Pacific Northwest will merge into an amplified but positively tilted longwave trough over the central CONUS late Friday, with a closed low developing on Saturday. This feature is likely to dig rapidly southward towards the Gulf Coast on Sunday, and the interaction of this trough with a modestly coupled jet structure will lead to a surface low moving southeast through the period. While the surface low is not forecast to become very deep, and should weaken D3 as secondary low development occurs off the SC coast, it will be accompanied by significant moisture noted by PWs of +2 standard deviations above the climo mean. Within this moisture plume, WAA will intensify, and isentropic ascent on the 290-300K surfaces becomes robust, increasing both the coverage and intensity of precipitation. While the slope of this upglide is modest, mixing ratios on the 750mb pressure surface are above 4g/kg, suggesting very moist ascent which will likely produce a broad swath of heavy snowfall. Despite a lack of banding signal in the guidance, and HREF probabilities for 1"/hr of just 20-30%, this robust moist upglide combined with a band of mid-level fgen suggests a good potential for at least 8 inches of snow despite a generally 12-hr snow event with below January median SLR according to the Baxter climatology. With snow developing across ND late on D1, WPC probabilities are 40-60% for 4+ inches in far northern ND. By D2, the strongest ascent and best moisture overlap, leading to a large area of high WPC probabilities exceeding 4 inches from southeast ND through extreme northern MO. The guidance overnight has shifted slightly eastward, and there continues some spread due to variations in the interaction of the 500mb shortwaves, but in a corridor from southern ND through central IA there also exists a greater than 50% chance for 8+ inches of snowfall. Within this maximum corridor, local amounts to 12 inches are possible as noted by an increase in the WSE 90th percentile. By D3, confidence decreases more significantly as the trailing upper jet weakens but the leading jet arcing into the Mid-Atlantic intensifies. There may be a break in the heavy snowfall D3, and this is reflected by lower WPC probabilities for 4+ inches extending into southern MO and northern AR. However, there are some model signals for a rapidly intensifying deformation axis aided by increasing theta-e ridging from the east to enhance snowfall. Confidence in this is low and will need to be addressed with later updates, but snowfall across MO/AR and into parts of TN and KY could be more impressive than current forecasts suggest. ...Southern Appalachians and the Carolinas... Day 3... Low pressure sinking out of the Mid-Mississippi Valley will begin to weaken as a new surface low begins to form off the SC coast in a Miller-B type setup. This coastal low will gradually become dominant, especially by D4, as the mid-level closed low over the MS VLY shifts east and combines with increasing upper diffluence to produce surface pressure falls. While much of this will occur just beyond this forecast period, increasing moist advection out of the Gulf of Mexico will spread precipitation northward into the Southern Appalachians. As this occurs, a wedge of high pressure will actually become reinforced by falling precipitation, and precipitation will likely fall as snow in the higher terrain, but freezing rain in the Piedmont. There remains considerable uncertainty into the track and timing of this system, but it does appear significant accumulations of snow and ice are becoming more likely. WPC probabilities for 4 inches of snowfall are above 40% in the NC mountains and extreme Upstate SC, with WPC probabilities for 0.1" of freezing rain above 10% across the Piedmont of SC and into the Sand Hills of NC. Weiss ~~~ Key Messages for Winter Storm Affecting the Northern Plains to Upper Midwest January 14-15 ~~~ -- A fast-moving winter storm will drop southward across the Northern Plains tonight, continuing through the Upper Midwest and into the Mid-Mississippi Valley Friday night. -- A swath of heavy snow exceeding 8 inches is expected with this system from eastern North Dakota through southern Iowa. 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. ~~~ Key Messages for Winter Storm Affecting the Eastern Seaboard January 15-17 ~~~ -- Confidence is increasing that a significant low pressure system will develop across the Southeast Sunday morning and then lift northward, just inland from the coast, exiting into Canada on Tuesday. -- There remains considerable uncertainty with the exact track of this system, but heavy wintry precipitation is likely from the Southern Appalachians through Northern New England. -- Heavy snow is expected along the Appalachians and well inland from the major cities, but there remains potential for heavy snow getting into the I-95 metropolitan areas. Significant icing is likely in parts of the Carolinas. -- This winter storm will likely create significant impacts to travel and infrastructure into early next week.