Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 402 PM EDT Sun May 17 2015 Valid 00Z Mon May 18 2015 - 00Z Wed May 20 2015 ...There is a slight risk of severe thunderstorms from parts of the Upper Mississippi Valley/Upper Great Lakes to parts of the Western Ohio Valley/Lower Mississippi Valley... ...Heavy rain possible over parts of the Lower Mississippi Valley... ...Temperatures will be 15 to 30 degrees below average over parts of the Upper Midwest... ..Snow over parts of the Northern Plains... A storm over the Upper Mississippi Valley will move northeastward into Southern Canada by Monday night/Tuesday morning. A plume of moisture from the Western Gulf of Mexico will extend northeastward to the Great Lakes and will move into the Northeast by Tuesday morning. On Monday night into Tuesday, another plume of moisture will start to extend northward over the Southern Plains. Showers and thunderstorms will develop from parts of the Upper Mississippi Valley/Upper Great Lakes and extend into the Ohio Valley/Mid-Atlantic to parts of the Lower Mississippi Valley/Southern Plains that will be displaced eastward to the Mid-Atlantic/Southeast to the Southern Plains/Central Gulf Coast by Tuesday. In addition, rain will develop over parts of the Northern Plains that will move into parts of the Upper Great Lakes by Tuesday. Furthermore, snow will develop over parts of the Northern Plains Sunday evening into early Monday morning. By Monday evening into Tuesday, showers and thunderstorms will develop over parts of the Southern High Plains into parts of the Southern Rockies associated with the return plume of moisture. Meanwhile, an upper-level low just of the California Coast will move eastward to the Great Basin/Southwest by Tuesday. The energy will aid in the development of rain with embedded thunderstorms over parts of the Northwest southeastward into the Great Basin/Central Rockies through Tuesday morning. Some portions of the highest elevations of the Rockies will develop snow through Tuesday, too. Ziegenfelder Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_wbg.php