With more severe weather expected across the central US, forecasters are warning of airport delays and flooded roads as travellers return home after the Christmas holiday.
Feeding on unseasonably warm air, storms left a trail of destruction in rural communities from Alabama to Illinois.
More than a dozen tornadoes were reported yesterday in six southern states.
In Alabama, where Governor Robert Bentley has declared a state of emergency to deal with the heavy flooding, tornadoes uprooted trees and tore off rooftops, with one touching down in Birmingham, the state's most populous city.
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No one died, but Gordon said victims were rescued from the debris.
Mississippi Governor Phil Bryant and Georgia counterpart Nathan Deal also declared states of emergency in counties affected by the weather.
In Mississippi, 10 people were confirmed dead today, the state emergency management agency said on Twitter. About 60 others were injured, it said.
Among the dead was a seven-year-old boy killed when a storm picked up and tossed the car he was travelling in, fire chief Kenny Holbrook told reporters in the town of Holly Springs.
Another person was killed in Arkansas.
Forecasters at the National Weather Service warned that while the heaviest rain was over for the southeastern United States, the central and southern plains states, especially Arkansas and Missouri, were now in for severe thunderstorms.
Flash flooding could be a "serious concern" from northeast Texas into Missouri, they said.
Flood warnings and advisories remained in effect today in parts of Mississippi, Tennessee, Alabama, Kentucky and other areas in the southeast.