The National Weather Service (NWS) has issued a flood watch for millions of residents across five states due to an approaching storm system expected to bring heavy rainfall. The system, which has already caused severe thunderstorms and tornadoes in Oklahoma, is moving eastward, affecting the U.S. South and Great Lakes regions. Flood watches are in place for southeastern Louisiana, Mississippi, northern Alabama, central Tennessee, and Wisconsin. Major cities like Birmingham, Jackson, Nashville, and Green Bay are expected to experience significant impacts.