The storms, feeding on unseasonable warm air, left a trail of destruction in rural communities from Illinois to Alabama, just as Christmas reached its crescendo.
More than a dozen tornadoes were reported in six states but Mississippi, in the south, was hardest hit. Seven people were reported dead there, said the state's Emergency Management Agency (MEMA).
Mississippi Governor Phil Bryant declared a state of emergency in seven counties after the storms caused widespread damage, AFP reported.
Debris from ravaged buildings and other structures littered roads, making them unpassable.
Power lines, trees and mobile phone towers were also toppled.
The worst appeared to be over, however forecasters at the National Weather Service warned of flash floods in the US southeast.
The US East Coast meanwhile is enjoying unseasonably warm weather, with temperatures in New York's iconic Central Park peaking at 72 degrees Fahrenheit (22 Celsius) Thursday, the warmest Christmas Eve since records began in 1871.