Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 407 AM EST Wed Nov 07 2018 Valid 12Z Wed Nov 07 2018 - 12Z Sat Nov 10 2018 ...Northern Rockies and WA Cascades... For the Day 1 period (Wednesday), a shortwave impulse tracks towards the southeast and enhances lift and increases low level RH. A strong upper jet and low-mid convergence in place aid in ascent and produce several inches of snow in the high terrain of the Bitterroots, and ranges of western Montana and northwest Wyoming. WPC probabilities feature a moderate to high risk for 4+ inches in the Bitterroots. A drier weather pattern returns by Thursday for this region with just a few lingering snow showers remaining for the Absarokas and Big Horn mountains. On Friday, a 700 mb wave and associated cold front move onshore from the East Pacific across western WA. Pre-frontal low level to 700 mb convergence leads to precipitation, with snow in the WA Cascades expected, with several inches possible in the northern Cascades. As the wave progresses inland, light snow occur with the front as it crosses the ranges of northern ID. ...Michigan Days 1-2... A cold front progresses east across Lake Superior and the UP of MI today. Frontal convergence and then then post-frontal lake enhanced/lake effect snow occur in the UP of MI to the WI border. 1 to 3 inches of snow appears likely. Northwesterly flow across Lake Superior will provide some lake enhancement as steeper lapse rates develop owing to cold air advection. Lee shore convergence will also provide lift over this region, and WPC probabilities are slight for 4+ inches in the western UP of MI on Wed. Amounts are expected to be light on Thu as the approach of a low level ridge in the upper MS Valley results in drying aloft, resulting in less coverage/intensity of the snow showers. ...Central Plains to the Great Lakes... A mid level impulse is expected to produce 700 mb warm/moist advection and convergence, leading to a band of accumulating snow from northeastern Colorado to northern Kansas and southern Nebraska. Snowfall rates may be locally enhanced owing to some elevated instability and steeper lapse rates, promoting more vigorous lift in the dendritic growth zone. There are multiple EC and GEFS ensemble members showing potential for 4 inches or more of snowfall, with the greatest overlap of members on Day 1 near the CO/NE border. WPC probabilities are low to moderate for 4 inches of snow during the Day 1 period Wed, and low on day 2 Thu as the wave progresses steadily east northeast. By Thursday night, the disturbance tracks out of the central Plains across the Midwest states and then northeast towards the Great Lakes by Friday afternoon. A region of mid-level ascent is expected in the moisture plume from Kansas, across northern Missouri, and then over northern Illinois and southern Wisconsin. Temperatures will be cold enough for light snow on the northern edge of the precip shield, on the order of 1 to 2 inches in most cases. On Friday, the northern and southern stream waves merge and a new surface low forms and tracks across the lower Lakes. On the west side of the low, temperatures will be cold enough for snow in northern WI/northern lower MI and the UP of MI. Several inches of snow are possible in lake enhanced areas south of Lake Superior, centered on the UP of MI. The 00z NAM has the heaviest amounts owing to a slightly stronger cyclone and further west low track. ...Northern New England Day 3... A low-mid level warm front moves north across New York and New England. Well defined low-mid level warm/moist advection and convergence leads to widespread precipitation. The models indicate it is cold enough for snow at the onset across northern New England. The warm advection leads to a good chance the snow will change to rain. The uncertainty relates to how long precip remains as snow, and resultant impact on amounts. Also, the 00z nam had high QPF amounts than the 00z GFS/ECMWF/06z NAM, so the 00z run was an outlier with heavier snow and icing potential than other solutions indicate. The probability of significant icing is less than 10 percent days 1-3. Petersen