Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 230 PM EST Mon Jan 13 2025 Valid 00Z Tue Jan 14 2025 - 00Z Fri Jan 17 2025 ...Northern Rockies/Plains... Day 1... Deep northwesterly flow, with embedded energy aloft, will support periods of snow and a stripe of mostly light accumulations from the northern Rockies southeastward into the mid Missouri Valley. Additional accumulations after 00Z are expected to be no more than an inch or two for most locations. However, WPC probabilities indicate that orographic enhancement may support some totals of at least 4 inches in the north-central Wyoming and south-central Montana mountains and in the Black Hills. ...Great Lakes... Days 1-3... An occluded surface low will move east of the Great Lakes through the St. Lawrence Valley, leaving deep cyclonic flow in its wake. Lake effect snows will continue to develop and intensify, with locally heavy amounts beginning to accumulate in the typical snowbelts this evening. Lake effect snows will continue into Wednesday before an approaching shortwave diving southeast through central Canada introduces some warm advection/synoptic-driven snows into the region by late in the day. Some of the heaviest lake effect snows are expected to fall east of Lake Ontario where a single band is expected to drift slowly south across the region over the next day. WPC probabilities indicate that accumulations greater than a foot are likely from this evening into late Tuesday across the region. Other areas impacted will likely include the northwestern Pennsylvania and far western New York counties, where localized two-day totals over a foot can be expected. Lesser accumulations are expected elsewhere, including the northern Michigan snowbelts. ...Pennsylvania... Day 1... Steep lapse rates in addition to an amplified shortwave moving through the base of the broader scale trough may support snow showers and potential squalls east of Lake Erie on Tuesday. The NAM and GFS continue to show Snow Squall Parameter values greater than 2, especially across western to central Pennsylvania tomorrow afternoon/evening. Accumulations will generally be light, but a brief period of intense, wind-driven snow may create hazardous driving conditions. For the Day 1-3 period, the probability of significant icing of at least 0.10" is less than 10 percent across the CONUS. Pereira