Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 134 PM EST Fri Dec 15 2023 Valid 00Z Sat Dec 16 2023 - 00Z Tue Dec 19 2023 ...Northern Plains and Upper Midwest... Day 1... Interacting shortwaves, a northern stream impulse digging out of Saskatchewan and energy emerging from the southern stream out of Kansas will produce a wave of low pressure and an axis of mixed precipitation from ND into MN. The guidance has struggled with how these impulses will phase, but it is likely that by Saturday evening a consolidated closed mid-level low will be moving across the Upper Midwest. While this will result in an amplifying pattern over the region, the best ascent will be exiting to the east by that time, suggesting much of the wintry precipitation will occur only on D1. As the wave develops, moist advection downstream will surge northward with impressive PW anomalies reaching as high as +4 sigma according to NAEFS, with this moisture then rotating cyclonically on intensifying 285K-290K isentropic ascent. This enhanced ascent will wring out this impressive moisture, with additional lift occurring as the moisture converges into the westerlies centered over Canada north of the surface warm front. A distant jet streak may at least subtly enhance associated fgen as well, and this could result in a stripe of moderate snow as dynamic cooling during periods of the most intense ascent could cause a rapid changeover from rain to snow. Snowfall rates may briefly be heavy at times, and the column will likely gradually cool with time to permit more snow coverage, especially in MN, but overall accumulations appear to be modest, and WPC probabilities maximize at just 5-10% for more than 4 inches, focused in the MN Arrowhead. ...Great Lakes to Central Appalachians... Day 3... An amplifying shortwave digging out of Manitoba will race southeast behind a major coastal low pressure moving out of New England on Monday. This shortwave will deepen quickly into a closed low near the Great Lakes Monday evening, with intense PVA swinging to its south along a strung out vorticity maxima. This feature will drive increasing ascent from the Great Lakes through the Central Appalachians, concurrent with intensifying CAA characterized by 850mb temps crashing to as low as -20C over Lake Superior, and -10C into the Mid-Atlantic. A subtle increase in mid-level moisture will accompany this feature, but better column moisture will remain in the low-levels due to lake enhancement as NW flow moves atop the still warm lakes. This should result in two areas of snowfall Sunday night into Monday, one in the favored NW snow belts off Lakes Superior and Michigan, with a secondary maxima likely in the favored upslope regions of the Central Appalachians. There remains considerable uncertainty into the strength and placement of this upper vort, but it appears that regardless of the exact evolution, heavy snow will develop due to lake enhancement (LES) and within upslope in an extremely unstable environment characterized by deep dry-adiabatic lapse rates. At this time, the greatest WPC probabilities for more than 4 inches are 10-20% across the U.P. of MI, and 20-40% in the Central Appalachians. It will be worth monitoring the evolution of this event in the next few cycles as the setup for heavy upslope snow does appear favorable, and amounts could over-perform the guidance. Weiss