Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 327 PM EST Fri Nov 23 2018 Valid 00Z Sat Nov 24 2018 - 00Z Tue Nov 27 2018 ...Western U.S.... Day 1... A potent mid-level shortwave with a subtropical moisture connection will drop across the Mountain West during Day 1, delivering another round of heavy precipitation, with heavy snowfall likely in the mountains from the Cascades southeast through the Colorado Rockies. Snow levels are expected to momentarily rise before a mid-upper level low dropping south from Vancouver Island pushes colder air across the region from the north into Saturday morning. This low will continue to drop southeast across the northern Intermountain West and Rockies through Saturday night, leading to enhanced moisture spilling across the terrain. Strong upper forcing along the leading edge of the upper jet will support heavy snow accumulations along the ranges of northern Idaho, western Wyoming, Utah, and Colorado. Storm total accumulations will likely to exceed a foot across a wide area. WPC probabilities are high for 8 inches of snow across most of these ranges, with amounts decreasing below 7000 feet. Snows are expected to diminish from west to east as the trough swings further east into the Plains Saturday night. ...Central Plains to the Great Lakes... Days 2-3... An upper level jet streak will eject eastward early on day 2 /Saturday night/ producing a band of light to moderate snow across the western Central Plains. WPC probabilities are moderate for 4 inches of snow from the Black Hills of South Dakota through central Nebraska. As this first jet streak lifts away, a more intense jet maximum associated with the polar jet will drop southward into the Plains and phase with subtropical jet energy Sunday night across the lower Missouri Valley. This coupled jet structure combined with rapid height falls in response to a shortwave digging southward will spawn cyclogenesis Sunday morning which will then rapidly intensify as it lifts northeast into the Great Lakes on Day 3. The guidance differs in the track of this surface low, which creates a lower confidence forecast in placement of snow amounts. Regardless of the track, very strong dynamics associated with this system will produce a swath of heavy snowfall from the central Plains into Michigan. The guidance has setup into two camps, the GFS/CMC/UKMET favor a northward track of the surface low, while the NAM/ECMWF are significantly further south. The 00Z ECMEnsemble mean was more towards the northern solution however, so that lends credence to a UKMET/GFS/CMC/ECens preferred blend for WWD today. There will very likely be a band of 8" or more of snow as strong frontogenesis and deformation combine N/NW of the surface low track. Snow rates will likely exceed 1"/hr at times noted by -EPV coincident with strong forcing into the DGZ, and many models suggest periods of 4"/3hrs, which combined with strong winds in the tightening pressure gradient will produce near blizzard conditions for portions of the region. As the upper low tilts negatively and closes off, this will further enhance the jet-level forced synoptic ascent, and WPC probabilities are high for 4 inches of snow from extreme NE KS into northern IL. In this same stripe, the potential exists for more than 8 inches as well where the longest duration of heavy snow pivots overhead. As the low moves into Michigan on D3, a TROWAL may develop as the WCB wraps cyclonically around the occluding system. This could enhance snow totals across the L.P. of Michigan, where WPC probabilities have increased for 4 inches of snow. ...Southern and Central Appalachians into New England... Days 1-2... Isentropic lift along with mid-upper level dynamics associated with an upper trough moving across the Tennessee Valley will support widespread precipitation across the region Saturday morning for the southern Appalachians, lifting northward into New England late Sunday. With a cold air wedge remaining in place, precipitation is expected to begin as a wintry mix with some light icing expected before strong southerly winds erode the low level cold air. Some light snow is possible in upstate New York and New England as well, most likely in the higher terrain. While the probabilities for ice accumulations of 0.25 inches or more remain less than 20 percent, WPC probabilities support increasing confidence that it will be an icy morning for several areas, with WPC probabilities 50 percent or greater for measurable ice from western North Carolina to southwestern Pennsylvania. The best chance for more than 0.1 inches of accretion will be in the NC mountains, with the best chance for a few inches of snow focused in the Adirondacks of New York. Weiss