At least 14 people have died and scores of homes have been destroyed in tornadoes across six states in the US south and midwest.
Emergency declarations were issued in Mississippi and Tennessee, the two states hardest hit by severe weather this week.
The storm system triggered more than 20 tornadoes in Arkansas, Illinois, Indiana, Mississippi, Tennessee and Michigan.
One large tornado tore a 160km path through northern Mississippi, demolishing or heavily damaging more than 100 homes and other buildings.
Mississippi Governor Phil Bryant declared a state of emergency in areas affected by the storm, saying 14 tornadoes had touched down in his state.
He said seven people were killed there and one person was missing.
Six storm-related fatalities were reported by authorities in Tennessee and an 18-year-old woman was killed in Arkansas when a tree crashed into her house.
The US east coast, meanwhile, is enjoying unseasonably warm weather.
Temperatures in New York's Central Park peaked at 22C yesterday, the warmest Christmas Eve since records began in 1871.
