Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 403 AM EST Sun Dec 15 2019 Valid 12Z Sun Dec 15 2019 - 12Z Wed Dec 18 2019 Day 1... ...Central Plains across the Mid-Mississippi and Ohio Valleys to the Central Appalachians... The first of back-to-back waves to impact the region is shifting across the central High Plains this morning. Warm air advection this wave will support precipitation spreading east from eastern KS into MO this morning with a deformation band setting up over east-central MO into IL this afternoon and crossing the Ohio Valley and the central Appalachians tonight. Locally heavy snow in the band could accumulate quickly with WPC probabilities for six inches on Day 1 around 20% just north of St. Louis. This deformation band persists across IN and southern OH before enhancing a bit over the central Appalachians late tonight with lower probabilities for four inches as this weaker leading system quickly shifts east. A wintry mix developing to the south is expected with moderately high probabilities for a tenth inch of ice in a swath from southeastern KS across southern MO and into southern IL (tracking just south of St. Louis) and a separate area of lower probabilities for a tenth inch in the Allegheny Highlands on the WV/VA border. ...Great Basin to the South-Central Rockies... A well-defined mid-level shortwave moving east-southeast into Nevada this morning will cross the south-central Rockies tonight. Increasing southwesterly winds ahead of the wave will increase moisture along a low-to-mid level baroclinic zone centered along the Utah-Arizona and Colorado-New Mexico borders, supporting light to moderate snows across the higher terrain. Heavy snows are expected to continue into tonight across southern Colorado and far northern New Mexico. Areas expected to be impacted by the greatest totals include the San Juan and Sangre de Cristo mountains where Day 1 WPC probabilities are high for additional accumulations of 12 inches. Day 2... ...Central Plains into the Northeast... The long wave trough shifting from the west maintains a positive tilt on Day to over the south-central Plains with the surface low developing over the TN valley Monday night at tracking through the central Mid-Atlantic into Tuesday. This track is again a bit farther south with the 00Z guidance consensus compared to the previous. Strong low-to-mid level frontogenesis will support comma head wintry precip on a similar track to the leading wave...from eastern KS Monday across south-central MO across south-central IL Monday night. Day 2 WPC probabilities are low for for additional accumulations of 4 inches or more centered across central MO and central IL. Meanwhile, southerly low level flow ahead of the surface low will provide a focus for a wintry mix over the northern Mid-Atlantic to southern New England as the front brought by the first wave lifts north as a warm front Monday night. The most impactful weather on Day 2 looks to be a sleet/freezing rain mix over much of PA south into the Potomac Highland of MD/WV/VA and east across northern NJ where probabilities are moderately high for over a tenth inch of freezing rain. Day 3... ...Northeast... The surface low ahead of the still positively tilted trough over the central CONUS lifts northeast from the central Appalachians Tuesday morning. The swath of north snow north of this track remains fairly uncertain, particularly with the 00Z ECMWF tracking much farther south than the 00Z GFS/UKMET. At this time interior Upstate NY and north-central New England remains under the greatest threat for four inches as indicated by low Day 3 WPC probabilities. Jackson