Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 243 PM EDT Thu May 17 2018 Valid 00Z Fri May 18 2018 - 00Z Sun May 20 2018 ...A persistent wet pattern from the Deep South to the Mid-Atlantic will continue into the weekend with heavy rainfall resulting in flash flooding possible... ...Slight risk for severe thunderstorms and excessive rainfall across the portions of the Great Plains... The never-ending wet pattern across the Eastern U.S. will continue as a stubborn boundary remains anchored over the Mid-Atlantic, but should finally begin to drift ever so slowly northward through the Tennessee and Mid-Mississippi Valleys by this weekend. The best chance for heavy to excessive rainfall will be from the central Appalachians into the Mid-Atlantic tonight and Friday as tropical moisture out of the Gulf of Mexico and western Atlantic interacts with the front. WPC has highlighted this region in a slight risk for excessive rains/flash flooding. Elsewhere, expect continuing showers and thunderstorms across much of the Northwest underneath a broad upper level trough, with localized enhanced activity possible along the favored terrain as well as near a frontal boundary dropping into the northern High Plains. By this evening, however, things should begin to ramp up across the northern and central High Plains as moisture streaming northward interacts with two frontal systems -- the first being the front dropping southward through the northern Plains, and a second lifting northward into the southern/central Plains. Expect rain and strong to severe thunderstorms to develop initially across the High Plains, moving eastward into the central Plains by tomorrow. In addition to the severe weather threat, heavy to excessive rainfall will be possible along the sagging northern stream boundary, particularly across parts of the northern Plains tonight, and the central Plains/High Plains tomorrow into Saturday. See the convective outlooks issued by the Storm Prediction Center for details on the severe weather, and consult WPCs graphics for additional information about excessive rainfall. Santorelli Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php