Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 307 PM EST Wed Dec 19 2018 Valid 00Z Thu Dec 20 2018 - 00Z Sun Dec 23 2018 ...Northwest to Northern/Central Rockies... Days 1-3... A vigorous Pacific low pressure system is forecast to reach Vancouver Island on Thu, with the associated cold front moving into the Pacific Northwest through Thu night. While moderate to heavy snowfall will initially be confined to the Washington Cascades on Thu, decreasing snow levels and persistent onshore flow in the wake of the cold front will gradually spread heavier snows south across the Oregon Cascades Thu night into Fri. Total snowfall through Fri will likely exceed 1 foot at the higher elevations, with perhaps some totals approaching 2 feet in the northern Washington Cascades. The snowfall potential across the Cascades is reflected in the probabilities for Days 1-2, showing potential for greater than 12" accumulations in the Washington Cascades and lesser amounts farther south. Moderate to heavy snows will also spread into the higher terrain of ID/MT/WY on Fri as the cold front moves inland quickly. Due to the relatively quick movement of the system across these areas, snowfall totals will be somewhat limited to under 1 foot, and probabilities mostly confined to the 4" and 8" thresholds. As the cold front continues east on Day 3 it may produce relatively light snows across the central Rockies, with probabilities showing the potential for at least 4" for some areas. ...Appalachians/Northeast... Days 2-3... An energetic storm system is evident on satellite imagery across the southern plains/lower Mississippi Valley this afternoon. The system is expected to initially bring widespread rain to much of the eastern third of the CONUS before incoming cold air begins to filter in beneath a deepening mid/upper-level low, changing rain to snow for portions of the Appalachians Fri night into Sat on the back side of the surface low pressure system. Favorable upper-level PVA and low-level upslope flow will contribute to a period about 6-12 hours when moderate or even locally heavy snows will be possible along the spine of the southern/central Appalachians. Potential for exceeding 4" was shown in the Day 2/3 probabilities for portions of these areas. Farther north, rain should mix with/change to snow across portions of interior New England on Sat, with areas of enhanced amounts possible in favored terrain areas and downwind of Lakes Erie and Ontario, warranting probabilities for 4" and a couple small areas of 8" across the Green Mountains and the Catskills. ...Upper Midwest/Upper Great Lakes... Days 1-3... A couple fast-moving and relatively moisture-starved frontal systems will traverse the northern Plains/Upper Midwest, the first tonight and another late Fri into Sat. A brief period of favorable frontogenetic forcing may produce light snows tonight into early Fri, with a small area of 4" probabilities indicated across northeast MN. The next system may produce a similar area of light snows late Fri into Sat. The probability for significant icing (0.25 inches or more) is less than 10 percent. Ryan