Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 310 PM EST Tue Jan 14 2025 Valid 00Z Wed Jan 15 2025 - 00Z Sat Jan 18 2025 ...Great Lakes & Central Appalachians... Days 1-3... Multiple days of lake-effect snow are expected through the end of the workweek as Arctic high pressure over the Plains advects frigid air over the lakes, resulting in heavy snow downwind. Clipper disturbances crossing the lakes will shift the bands north and south, as well as introduce variability into how strong the bands are. The first disturbance will be exiting the lower lakes at the start of the D1/Wednesday period this evening. Ongoing single banded lake-effect will shift southward into the disturbance. Once the flow becomes more northerly tonight behind the disturbance, the single bands should shift west and break into much weaker multi- bands as the flow becomes more perpendicular to the lakes. This southward shift will be short-lived however as the center of the Arctic high builds into the Ohio Valley and the next disturbance approaches the upper lakes from the heart of Canada. Thus, expect a rapid northward shift in the bands from west to east as southwesterly flow builds in behind the high. The second disturbance will be the stronger of the two as it moves into the upper lakes Wednesday into Wednesday night. It stays plenty cold enough for all snow. For most areas away from the lakes the snow should remain light...however where the lakes can locally enhance the moisture associated with this disturbance, heavy snow is possible. For the lower lakes, the flow will remain northwesterly into Wednesday before the northward shift occurs Wednesday night into Thursday. With this disturbance approaching the lake-effect should remain rather disorganized as it shifts north. On Thursday, the light snow with the disturbance will have overspread all the lakes, though behind it Lake Superior will increasingly favor the lake-enhanced areas of the northern U.P. Further, heavier snow lifted by the central Appalachians will begin to impact much of West Virginia and western Pennsylvania Thursday morning. The upslope snow will continue into Thursday night before a new high pressure area building in increases subsidence and dry air and ends the snow. Warm advection ahead of a third disturbance will effectively end much, if not all of the lake-effect during the day Friday. This third disturbance should remain almost entirely north of the lakes, though some light snow may spread into the Arrowhead late Thursday night. WPC Probabilities show a low (10-30%) chance of 6 inches of snow or more for the D1/Wednesday period southeast of the lower lakes. There's also a low (10-30%) chance of 4 inches of snow across the central Appalachians, particularly along the Allegheny Front of PA/MD/WV. ...Northern High Plains... Day 3... An Arctic front at the leading edge of some of the coldest air of the season will race southward out of Canada into the northern High Plains of MT/WY/ND on Friday. Supported by the RER of the jet, a shearing shortwave, and the contrast of warmer air ahead of the front, this "blue norther" scenario appears highly favorable for snow squalls, and more persistent heavy snows into the Little Belt, Big Snowy, Bighorns, and the Absarokas. The front will enter the Montana Hi-Line Thursday evening, racing south down the Plains through Montana and clearing Wyoming by Friday evening. Along with the front and favorable forcing, expect snow squalls to develop with the front with a first peak in intensity (based on the NAM) in the predawn hours Friday morning across central Montana. The Snow Squall Parameter nears the top of the scale (a 5) at this time. A secondary peak follows it during the day Friday in Montana as the initial wave tracks down the Wyoming Plains. The snow squalls will be characterized by rapid changes in visibility at the initial onset of the snow as clear conditions can drop to near whiteout in a matter of seconds. Once the squall clears visibility improves about as fast as it initially deteriorated. For most of the Plains, total accumulations are expected to be minor, however since the snow will be heavy, brief periods of hazardous driving conditions are likely. In the aforementioned mountains, expect a more prolonged period of heavy snow as upslope effectively wrings out the moisture, resulting in multiple inches of accumulation. For the Day 1-3 period, the probability of significant icing of at least 0.10" is less than 10 percent across the CONUS. Wegman ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png