Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 342 AM EST Thu Nov 15 2018 Valid 12Z Thu Nov 15 2018 - 12Z Sun Nov 18 2018 ...An early-season winter storm bringing ice and snow from the Midwest/Ohio Valley will intensify as it shifts east to the Appalachians and Interior Northeast today before turning northeast Friday... ...Appalachians and Northeast... Days 1-2... Upper level low pressure over the mid-Mississippi Valley translates east to the central Mid-Atlantic coast through tonight before shifting northeast along the New England Coast Friday. A tropically connected warm conveyor belt ahead of the upper low will lead to 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 cold air damming wedge down the east side of the Appalachians into tonight. A wintry mix line is expected near the Fall Line and inland with a swath of mainly snow along the northern extent of the precipitation shield from the low. The extent and strength of the warm nose remains uncertain with 00Z guidance increasing confidence of a large isothermal layer near 0C developing over the interior northern Mid-Atlantic. 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 for higher elevations from eastern West Virginia to the Pocono mountains of PA. Farther north, eastern New England likely see a swath of freezing rain along and just inland from the coast as the low develops and becomes vertically stacked. A weaker warm nose in New England should result in 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 enhance along the Mid-Atlantic coast and lift northward to New Jersey this evening. The surface low is expected to hug the coast as it tracks along the Northeast coast into Atlantic Canada on Friday. A TROWAL is forecast to setup northwest of the low is expected to enhance snowfall amounts from central PA to northern New England, with moderate WPC probabilities for 8 inches over this area. The major cities from Washington DC to New York City are too far southeast for significant snow or ice. There is an expectation for some snow/sleet/freezing rain across these cities with the initial warm advection precipitation, but low-level winds becoming east-southeast will quickly transition precipitation type to rain. Boston has a greater chance of snow with moderate WPC probabilities for 2 inches on coastal eastern MA. ...North and Central Rockies and Great Plains... Days 1-3... A shortwave will dig into the Northern Rockies tonight bringing modest Pacific jet energy and moisture into the north-central CONUS. At the surface a cold front will dive southward across the Northern Plains and bank against the mountains producing a sharpening baroclinic gradient and upslope low-level flow late tonight in MT and Friday, then farther south to the central Plains and CO Saturday. A wave of low pressure will develop along this temperature gradient over the northern Plains tonight and Friday while Pacific moisture leads to 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 toward the Laramie mountains of Wyoming. Also, a band of snow can be expected to extend from MT/ND Friday to MN/IA with a preference given to the 00Z ECMWF/FV3 which are between the farther south GFS and farther north NAM. Pereira/Weiss/Jackson