Millions of people in the southeastern U.S. spent Monday preparing for the 13th straight day of severe weather Monday as major cities from Florida to the Carolinas were smacked by powerful thunderstorms and flooding rain.This latest threat came after more than 20 people were killed, including children and first responders, after a relentless barrage of tornadoes and historic flash flooding across several states.HOW TO WATCH FOX WEATHERSevere Thunderstorm Warnings and Tornado Warnings were issued across portions of the Southeast before sunrise ahead of the Monday morning commute.In Leon County, Florida, local offices began cleaning up damage from a likely tornado in Tallahassee Monday afternoon.A photo showed crews cleaning up fallen trees and debris from the side of a road. Severe weather was possible from the Southeast to the mid-Atlantic throughout the day and into Monday evening, but the risk wasn’t near as dire as it was over the past two weeks.However, it only takes one storm to turn lives upside down. So, be sure to download the free FOX Weather app and enable notifications to be alerted to severe weather or changes to the forecast in your area.SEE IT: DRONE VIDEOS SHOW PARTS OF KENTUCKY TOWNS DISAPPEAR UNDER FLOODWATERSNearly 18 million people in six states, from Florida to Virginia, were at risk of severe weather on Monday. However, NOAA’s Storm Prediction Center (SPC) placed nearly 7.5 million people in the Southeast and the Carolinas in a Level 2 out of 5 threat on its severe thunderstorm risk scale.This threat zone included cities such as Tallahassee in Florida, Savannah in Georgia, Columbia in South Carolina and Raleigh and Fayetteville in North Carolina.FEARS OF RIVER FLOODING RISE IN MISSISSIPPI, OHIO VALLEYS AMID MANDATORY EVACUATIONSThe FOX Forecast Center said that ahead of the cold front moving across the region, torrential precipitation is likely to cause rivers and streams to overflow their banks, leading to a risk of flash flooding.NOAA’s Weather Prediction Center (WPC) said there’s a risk of flooding from Alabama to Virginia. However, the WPC placed parts of southern and south-central Alabama and the western Florida Panhandle in a Level 2 out of 4 risk.