Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 231 AM EST Wed Jan 15 2025 Valid 12Z Wed Jan 15 2025 - 12Z Sat Jan 18 2025 ...Great Lakes & Central Appalachians... Days 1-2... Ongoing lake-effect snow over the eastern Great Lakes will wind down today as a new shortwave moves in from the northwest, bringing light snow to the Upper Midwest/Western Great Lakes on WAA. As the warm front passes, winds will switch to NW as the cold front makes its approach but weakens across the region in response to height rises from the west. Regardless, some lake enhancement or lake effect snow is likely over much of the region but with overall light amounts over the U.P. and into western Lower Michigan. East of Lakes Erie and Ontario, system will bring in some light snow followed by a period of lake enhanced/effect snow D2 before ending D3. To the south, shortwave will swing right through the central Appalachians, maximizing upslope into eastern WV where several inches of snow are likely D2. WPC probabilities for at least 6 inches are high (>70%) especially over 2000ft. ...Northern/central Rockies/High Plains... Day 3... As an upper ridge builds across the Northeastern Pacific Thursday into Friday, downstream response will be digging troughing out of western Canada nearly due south through the High Plains via a strong cold front ("blue norther"). Though moisture will be limited, strong northerly flow will support upslope enhancement into some of the terrain over central/western Montana (esp the Little Belts and Big Snowy Mountains) southward into the Bighorns, Absarokas, and into the southeastern WY ranges. As the front dives southward, steeper lapse rates will support snow squalls along the front Friday in Montana progressing into Wyoming. Snow squall parameter per the guidance still shows values >1 (and even >3) suggesting the possibility of bursts of snow with sharply reduced visibility leading to near whiteout conditions. CAM guidance should shed a little more light on the threat over the next two days, but we have outlined this area in our Key Messages (see below). WPC probabilities for at least 4 inches of snow over Montana/Wyoming are moderate (40-70%) and mostly over the terrain with light snow (1-2") elsewhere. As the system races southward, cold front will slow a bit across the Rockies but continue to plunge through the High Plains, favoring upslope enhancement into the Front Range late Fri/early Sat. Snow will expand through the I-25 corridor into the Denver Metro area with higher amounts across the Front Range as temperatures fall into the teens, helping to increase SLRs from ~12:1 up toward ~18:1. Additional snowfall is likely past 12Z Sat. Through then, WPC probabilities for at least 4 inches of snow are at least 50% across the Front Range. For the Day 1-3 period, the probability of significant icing of at least 0.10" is less than 10 percent across the CONUS. Fracasso ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png