Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 339 AM EST Mon Dec 10 2018 Valid 12Z Mon Dec 10 2018 - 12Z Thu Dec 13 2018 ...Southern Appalachians to the Southeast Coastal Plain... Day 1... Surface low pressure will move away from the Outer Banks of NC beneath an initial shortwave, while two additional vorticity lobes drop into the longwave trough from the northwest. The third and final shortwave, which is the most potent, will swing across the Gulf Coast and off the coast of GA, spawning weak surface cyclogenesis tonight. While this surface feature will contribute little to any wintry precip, a strengthening upper jet will provide ascent within the left exit region, which will combine with increasing 1000-500mb RH to produce the potential for light to moderate snow showers through this afternoon. Weak frontogenesis and modest deformation in the vicinity of the 700mb trough axis will provide mesoscale support for lift, which may be strong enough within the increasingly saturated DGZ to produce a swath of an inch or two of snow, despite marginal low-level temperatures. WPC probabilities are low, less than 20 percent, for 4 inches across the area, but 1 inch, with isolated amounts up to 2 inches, are possible. This is most likely in the terrain from NE GA into SW NC, as well as in central NC where ascent within the deformation axis is subtly more robust. ...Pacific Northwest... Days 1-3... A series of shortwave with associated Pacific Jet energy and enhanced moisture will advect into Washington/Oregon through the forecast period. The first, and weakest, will move onshore this morning accompanied by a surface cold front. This feature is progressive and moisture is quick to spill across the Cascades and into the ranges of ID/MT, so snow is likely in the mountains from the Olympics of Washington through the Tetons of Wyoming. The highest snow totals are likely in the Cascades where higher moisture, better jet diffluence, and stronger upslope flow due to 700mb winds nearly orthogonal to the mountains, combine to produce heavy snow, and WPC probabilities are moderate to high for 4 inches of snow, with lesser probabilities downstream to the south and east from there. A stronger and shortwave with stronger jet dynamics and more significant Pacific moisture will stream into WA/OR beginning Tuesday. High 1000-500mb RH will spread across WA/OR and into the northern Rockies producing snowfall across elevations as low as 2000 feet. 850-700mb flow will become aligned perpendicular to the Cascades, which combined with intense moist advection as noted by PWAT anomalies approaching +2 standard deviations above the mean, and jet level diffluence will likely produce more than 2 feet of snow in the Washington Cascades, with 8-12 inches likely in the higher terrain of the Oregon Cascades and Bitterroots of Idaho. Although the best moisture will begin to shift east by day 3, continued snowfall in the higher terrain will make event totals of more than 4 feet likely in the Cascades of Washington, with 1-2 feet probable across the high terrain of ID/OR. With snow levels down to 2-3 kft, the valleys also have the potential to see a few inches of accumulation as well. Persistent subfreezing air pooled east of the Cascades in WA/OR will continue to result in pockets of freezing rain through Day 2. Low probabilities for a tenth inch of ice are present along the Columbia River east of the Gap for Monday and in the lee of the Cascades in WA Tuesday. Weiss