Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EST Sun Dec 15 2024 Valid 00Z Mon Dec 16 2024 - 00Z Wed Dec 18 2024 ...Wintry mix expected for portions of the central Appalachians, northern Mid-Atlantic, southern New York and southern New England this evening and overnight... ...Showers and thunderstorms with locally heavy rainfall possible across the Ohio Valley through Monday... ...Modest atmospheric river activity to bring unsettled weather to the Pacific Northwest and northern Rockies going through early this week, including heavy rains for the coastal ranges and accumulating snow for the higher elevations... An upper-level disturbance will continue to cross through the central Appalachians and into portions of southern New York and southern New England going through this evening and overnight which will bring an area of generally light precipitation. Sufficient cold air is in place at least early on to facilitate areas of light snow along with some sleet and freezing rain. Locally a few inches of snow will be possible across the interior of the northern Mid-Atlantic, along with a glaze of ice before temperatures warm up and allow for a changeover to rain later tonight and early Monday morning. A chilly rain can be expected elsewhere tonight across the Mid-Atlantic. Meanwhile, farther to the west, a new storm system and an associated frontal system will be ejecting across the Plains and Midwest by Monday. This will drive a threat for areas of showers and a few thunderstorms across much of the Ohio Valley later tonight and Monday as increasingly moist and somewhat unstable air lifts north in close proximity to the advancing cold front. Some of these showers and thunderstorms should extend southwestward down into areas of the Lower Mississippi Valley. Generally the rains are expected to be beneficial and not excessive with locally 1 to 2 inches of rain anticipated through Monday. This storm system will quickly lift across the Great Lakes region, and the trailing cold front should then cross through much of the Mid-Atlantic and Northeast by later Tuesday. Just ahead of the cold front should be the pooling of much milder air, and temperatures across much of the eastern half of the country should see high temperatures as much as 10 to 20 degrees above normal Monday and Tuesday. Much colder air though will be arriving across the northern Plains and Upper Midwest on Tuesday behind an Arctic cold front, and temperatures here will be trending back below normal by the middle of the week. A new round of modest atmospheric river activity associated with onshore Pacific flow and the arrival of a couple of frontal systems will keep areas of the Pacific Northwest and the northern Rockies unsettled going through the early part of the week. Locally heavy rainfall can be expected for the coastal ranges with locally a few inches of rain possible across northwest Oregon and western Washington. Meanwhile, heavy accumulating snow is expected for the higher elevations. This will include the Cascades, Sawtooth, Bitterroots and Tetons where multi-day accumulations through Tuesday of 1 to 2+ feet will be possible. Some lesser accumulations will be possible across the northern Sierra Nevada as moisture gradually settles farther south. The remainder of the country including the Southwest and adjacent areas of the southern U.S. should see generally dry weather through the first half of the week with temperatures above normal. Orrison Graphics available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php