The storms, feeding on unseasonably warm air, left a trail of destruction in rural communities from Alabama to Illinois, just as Christmas reached its crescendo.
More than a dozen tornadoes were reported in six states, with the southern state of Mississippi hardest hit. Seven people were confirmed dead there and another 60 injured, with one person missing, said the state's emergency management agency.
"We are experiencing some flash flooding today, with storms coming through right now in five counties. And damage assessments are still ongoing," the emergency agency's Brett Carr told AFP.
Mississippi Governor Phil Bryant declared a state of emergency in seven counties after the storms caused widespread damage.
Six fatalities were confirmed in neighbouring Tennessee, including three people found dead Thursday in a car submerged in a creek, according to the fire department in Columbia, Tennessee.
One person was reportedly killed in Arkansas.
Georgia's governor declared a state of emergency in counties affected by the severe weather.
Officials were inviting volunteers to help clean up or make donations as people who fled returned to their homes to see what, if anything, was still standing.
Television footage and pictures posted on social media showed homes flattened across several states, with possessions and Christmas presents strewn on the ground or left in a messy heap. Power lines, trees and mobile phone towers were also toppled.
Therese Apel, a reporter at Mississippi's Clarion-Ledger newspaper, spoke with a north Mississippi family hard hit by the violent weather.
