Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 413 AM EST Thu Dec 20 2018 Valid 12Z Thu Dec 20 2018 - 12Z Sun Dec 23 2018 ...Northwest to Northern/Central Rockies... A deep low pressure center associated with a well-defined mid-upper level trough is forecast to track into British Columbia on Thursday. A strong trailing cold front is expected to push across the Pacific Northwest, bringing snow levels appreciably lower late Thursday into early Friday. Locally heavy accumulations are possible for portions of the Olympics, Cascades as well as the northern Rockies from central to northern Idaho and northwest Montana. Models show this system moving quickly to the east, supporting a brief period of locally heavy snows across the western Wyoming ranges on Friday. Shortwave ridging is expected to support drier conditions across the Northwest late Friday into Saturday before the next system approaches the coast late Saturday, with probabilities for heavy snows increasing once again along the Cascades by early Sunday. ...Appalachians/Northeast... As it shifts east from the central into the eastern U.S., a full-latitude trough is forecast to deepen further as northern stream energy dives south from the upper Midwest toward the Gulf Coast on Friday. Cold air filtering in behind an area of surface low pressure lifting to the north from the central Appalachians into the Northeast is expected to support a changeover from rain to snow across the mountains before precipitation comes to an end late Friday into early Saturday. Vigorous mid level energy moving through the backside of the trough along with strong northwest flow interacting with residual moisture may support some significant snowfall totals along the spine of the central and southern Appalachians Friday evening into early Saturday. Uncertainty remains with respect to the depth and timing of the cold air, as well as how much moisture will remain once it arrives. The warmer GFS shows a fair amount of disparity with the NAM, ECMWF, SREF Mean - all of which show heavier totals along the central into the southern Appalachians. WPC probabilities indicate at least a Slight Risk for amounts of 4 inches for areas within this region during the Day 2 period. As the system lifts into Canada Friday night, rain should mix with and then change to snow across portions of the interior Northeast. Then as the low pulls further away, a continued period of cold air advection may promote additional snow showers, raising the potential for locally heavier totals downwind of lakes Erie and Ontario before drier air filters in late Saturday. The probability for significant icing (0.25 inches or more) is less than 10 percent. Pereira