Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 442 PM EST Wed Nov 14 2018 Valid 00Z Thu Nov 15 2018 - 00Z Sun Nov 18 2018 ...An early-season winter storm to bring ice and snow from the Midwest/Ohio Valley to the Appalachians and Interior Northeast through Friday... ...Mid Mississippi Valley into the lower Ohio Valley... Day 1... A significant early-season winter storm will develop through the evening into the overnight hours as an upper low continues to deepen as it tracks from the lower Mississippi into the lower Ohio valley. The GFS continues to show 500 mb heights 2-3 standard deviations below the mean as the system lifts across the region this evening and overnight. Overall, models remain in good agreement, indicating an intense deformation band of snow which will likely setup and pivot over far eastern Missouri and southern Illinois. WPC probabilities for the 24-hr period beginning 00 UTC Thu indicate that snow accumulations of 4 inches or more are likely, with accumulations of 8-inches or more possible, over far eastern Missouri, including the St. Louis Metro, and across southern Illinois to the Indiana border. ...Appalachians and Northeast... Days 1-3... As the previously noted low ejects from the Mississippi Valley toward the Northeast, warm advection ahead of this feature is forecast to produce widespread precipitation from the Southern Appalachians through Maine. At the surface, a cold high pressure centered over New England will only slowly retreat to the east, but maintain a wedge down the east side of the Appalachians. Significant forcing and ample moisture will produce heavy precipitation along the East Coast, with mixed precipitation further inland. Expect sleet and freezing rain to the be predominate precipitation type for most of the event along the central Appalachians with the models continuing show a significant signal for freezing rain amounts of 0.25 inch or more, especially along the western Virginia and eastern West Virginia mountains. Some models continue to show some greater amounts across this area, however there remains the question of how much sleet will occur - which would temper those amounts. Further to the north, although areas as far north as southern New England may experience freezing rain, a colder overall airmass and mid-level confluence will prevent the warm nose from getting nearly as strong as it will further south, affording more sleet and snow at the onset and less freezing rain. As the upper low moves moves from the Ohio valley into the Mid-Atlantic, a surface low will develop along the Mid Atlantic coast and lift northward toward New Jersey Thursday evening. The surface low is expected to hug the coast as it tracks along the Northeast coast into Atlantic Canada on Friday. A deformation band forecast to setup northwest of the low is expected to enhance snowfall amounts from north-central PA to northern New England, with WPC probabilties indicating the likelihood of accumulations of 4-inches or more across the area. The major cities from Washington DC through Boston will likely be too far southeast for significant snow or ice. There is moderate potential that some snow/sleet/freezing rain will occur across these cities with the initial warm advection precipitation, but low-level winds becoming east-southeast will quickly transition precipitation type to rain. ...Northern Rockies... Days 1-3... A shortwave will dig into the Northern Rockies Thursday into Friday accompanied by modest Pacific jet energy. At the surface a cold front will dive southwards into the Northern Plains and bank against the mountains producing a sharpening baroclinic gradient and upslope low-level flow. A wave of low pressure will develop along this temperature gradient, while Pacific moisture causes a steady increase in column relative humidity. Lift associated with upper diffluence and the developing surface low will produce widespread elevation snows from the Northern Rockies near Glacier National Park, southward towards the Laramie mountains of Wyoming. The highest snowfall is expected near Glacier National Park as well as the Absaroka and Big Horn ranges, where snowfall totals may reach 12 inches. Pereira/Weiss