Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 329 PM EST Sun Dec 15 2019 Valid 00Z Mon Dec 16 2019 - 00Z Thu Dec 19 2019 Day 1... ...Colorado/New Mexico... A well-defined mid-level shortwave will continue to support snows as it moves across the south-central Colorado and far northern New Mexico Rockies Sunday evening. Areas impacted by the heaviest amounts will likely include the Sangre de Cristo Mountains, where upslope flow and low level convergence is expected to carry the threat for heavy snows through the evening hours on Sunday. ...Central Plains to the Northern Mid Atlantic... Initial wave impacting the region will move east from the Mississippi valley/Ozark region into the the Ohio valley Sunday evening. Moist southerly flow into a low-to-mid level baroclinic zone will support a wintry mix advancing east ahead of the wave. While widespread heavy snowfall accumulations are not expected, there is the potential for locally heavier amounts. WPC guidance shows a narrow axis of higher probabilities for amounts of 4-inches or more centered across central Indiana. Low-to-mid level frontogenesis may support banded precipitation, generating higher totals across this region Sunday evening. Latest HREF guidance shows several members producing 0.05-0.10 in/hr liquid precipitation rates across this region during the 0-6 UTC Monday time period. A wintry mix of snow, sleet and ice will then spread east into the central Appalachians overnight and then east of the mountains into the northern Mid Atlantic by morning. Meanwhile, the second wave will have moved east of the Rockies by Monday morning, producing another round of winter precipitation across portions of the Plains and mid Mississippi valley. The general trend in the models over the past day has been drier, with lighter snow and ice accumulations expected from the Oklahoma and Texas Panhandle regions northeastward across the mid Mississippi valley on Monday. Day 2... ...Mid Mississippi Valley to the Northeast... Models continue to show the second shortwave amplifying over the central Plains and mid Mississippi valley late Monday before lifting through the Ohio valley ahead of an upper trough digging into the Upper Midwest overnight. Similar to the preceding wave, widespread heavy snow accumulations are not expected. However, this system is expected to produce another widespread swath of snow, sleet and freezing rain as it moves through the Ohio valley Monday night and into the northern Mid Atlantic and Northeast on Tuesday. Significant icing may be more of a concern with this second wave, with several guidance members indicating ice accumulations of 0.10 inch or more from Potomac Highlands in West Virginia northward into the Allegheny Mountains in western Pennsylvania and then eastward as far east as northern New Jersey and the Catskills. While areas farther to the north may remain mostly snow, models show the system remaining fairly suppressed and sliding quickly off of the southern New England coast Tuesday evening. This will help to limit snowfall accumulations across the Adirondacks and northern New England. Day 3... ...Great Lakes... The previously noted shortwave digging into the Upper Midwest will continue to amplify, with an upper low closing off and moving east from the Great Lakes into the Northeast on Wednesday. This will drive a strong cold front southeast through the Upper Great Lakes late Tuesday and the Lower Great Lakes by early Wednesday. Behind the front, strong northwest winds across the Upper Lakes and west-northwest winds across the Lower Lakes will support lake effect snow showers, with significant accumulations possible over the eastern U.P. and northern Lower Michigan, as well as western New York and the Tug Hill region before showers diminish late Wednesday. Pereira