Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 426 PM EST Thu Nov 15 2018 Valid 00Z Fri Nov 16 2018 - 00Z Mon Nov 19 2018 ...An Early-Season Winter Storm to bring ice and Heavy Snow from the Midwest/Ohio Valley to the Appalachians and Interior New Northeast through Friday... ...Northern Appalachians and Northeast... Days 1-2... A broad upper level low pressure moving out of the mid-Mississippi Valley will be filling as it continues to translate east/northeastward across the central Mid-Atlantic coast by later tonight before it changes course and heads northeast along the New England Coast on Friday. Widespread precipitation from the central/northern Appalachians into New England will continue to be fueled by a warm conveyor belt with extends well south into a moisture-rich atmosphere. 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 the overnight hours tonight. A wintry mix line is expected near the I-95 corridor and inland with a swath of mainly snow along the northern and western side of the precipitation shield...with drier air eventually spreading eastward and northward behind the system. Uncertainty lingers as to the exact placement of the various precipitation types even though the 12Z suite of model guidance got into somewhat better agreement with respect to the extent and strength of the warm nose. The root of the uncertainty appears to be rooted in the magnitude of vertical velocities within the deep near-isothermal layer. Farther north, eastern New England likely see a swath of freezing rain along and just inland from the coast as a low pressure system 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 later 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. ...North and Central Rockies and Great Plains... Days 1-3... A shortwave trough digging into the Northern Rockies, supported by modest Pacific jet energy, will spread moisture out over the 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 through 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 a roughly equal blend of the 15/12Z ECMWF/GFS. Bann