Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 252 AM EST Tue Dec 19 2017 Valid 12Z Tue Dec 19 2017 - 12Z Thu Dec 21 2017 ...Heavy rain possible along the Pacific Northwest coast, with heavy snowfall likely in the Cascades and northern Rockies... ...Heavy rainfall and flash flooding possible across portions of the Lower Mississippi Valley into the Tennessee Valley... Coastal rain and elevation snows across the Northwest will increase in coverage today as a strong cold front pushes onshore this morning. Heavy snowfall is likely across the Washington and Oregon Cascades as well as portions of the northern Rockies. Winter storm warnings are in effect in these ranges where as much as 1 to 2 feet of snow is forecast. As the cold front moves eastward through the Western U.S. it will usher in colder temps which may change rainfall in the lower elevations to snow or a rain/snow mix, including Seattle. By Wednesday, the moisture supply weakens but generally light accumulating snow will spread into much of the Great Basin, the Rockies, the northern Plains, and eventually the upper Mississippi Valley. Expect rain or a rain snow mix by Thursday morning across portions of the central Plains just to the north of a developing surface low. Meanwhile, showers and thunderstorms are expected to develop to the north of a warm front lifting across the Gulf Coast states. Heavy rainfall will be possible along the boundary from northeast Texas into southern Tennessee today, where WPC is highlighting a marginal to slight risk for excessive rainfall/flash flooding. Isolated severe thunderstorms are also possible, as per the latest convective outlook issued by the Storm Prediction Center. Heavy rainfall will spread eastward across the Tennessee Valley and into the lower Mid-Atlantic on Wednesday into Thursday as a surface low develops along the front and sweeps eastward through the Deep South. Elsewhere, light rain and snow will be possible across the Northeast associated with a frontal passage today. The activity should wane by this evening but may linger into Wednesday across portions of the Great Lakes as westerly flow aids in some mainly light lake effect snow showers downwind of the lakes. Santorelli Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php