Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 354 PM EST Sun Dec 08 2019 Valid 00Z Mon Dec 09 2019 - 00Z Thu Dec 12 2019 ...Upper Mississippi Valley and Great Lakes... Days 1-3... A northern stream trough will shift east across the Dakotas tonight behind a strong cold front pushing south down the Plains. Beneath this impulse, a surface low will continue to develop, enhancing lift along a narrow but robust band of mid-level frontogenesis. The rapid progression of this wave is a limit to accumulation, but the dendritic growth zone in the low levels will maximize snow. Southern stream energy is drawn into this trough late tonight/early Monday which enables a bit of a pivot to the banding just southwest of Lake Superior. Moisture enhancement from the Lake should further the potential for heaviest snow there which Day 1 WPC probs are moderately high for 8 inches, particularly over the northern shore of WI. Behind this system, intense CAA on NW and then W winds will drive increasing lake effect snow in the favored regions downwind of all the lakes. The heaviest LES through D3 is likely across the U.P. MI, where WPC Day 1.5 and 2.5/3 probabilities for 8 inches in 24 hours are moderate. ...Central and Southern Rockies into Texas... Days 1-2... The southern stream portion of a positively tilted longwave trough split from northern stream energy tonight with the southern stream mid-level low tracking from southern CA through Texas through Tuesday. A tropical moisture feed into this system from the Pacific (currently depicted in GOES-17 WV imagery) will increase the risk for heavy precipitation to the highest terrain of the Four Corners states for Day 1 into 2. Snow levels of 8-10kft will lower to 6-8kft as the low approaches. WPC probabilities are moderate on Day 1 for an additional 8 inches mainly over the highest terrain of CO and NM. Monday night, the southern stream low will be just south of NM with lingering snow over the southern Rockies in NM with low probabilities for four inches over the Sacramento Mtns. More notably Tuesday morning is the risk for accumulating snow across southeast NM and west TX on the back side of the cold front. Moist mid levels over run colder low levels allowing some snow development. ...Lower Mississippi Valley through New England... Days 2-3... A cold front will drop southeast through the southern Plains and TN/OH valleys Monday night before shifting off the Mid-Atlantic/New England coasts late Tuesday. Meanwhile, while a southern stream shortwave lags back across Texas. As this shortwave begins to lift northeast, height falls and mid-level divergence will lift atop the front producing anafrontal moisture. As the cold air continues to filter southeastward, the column will cool, and forcing may become intense through 850-600mb frontogenesis overlapped with right entrance diffluence of an intensifying 170+kt upper jet streak. The combination of ascent and a cooling column will cause precipitation to changeover from rain to snow on the back side of the frontal precipitation Tuesday from roughly AR across the TN Valley to the central Appalachians. Despite this mainly occurring during the day Tuesday, some accumulations are possible in this swath, particularly where rates are enhanced. Day 2 WPC probabilities for 2" are above 10% in a portion of the Ozarks in AR then in places along the TN/KY border. Farther east, terrain and latitude aids snow fall with Day 3 WPC probs moderate for four inches in the central Appalachians and into the Catskills/Berkshires/southern Green Mtns which is mainly Tuesday night. Into Wednesday morning, there is a risk for accumulating snow down to the I-95 corridor northeast from DC, particularly in New England. However, downsloping effects east of the Appalachians complicates this forecast, so more attention will need to be paid to this area. For Days 1-3, the probability of significant icing (0.25-inch) is less than 10 percent. Jackson