At least eight people died in Garland, near Dallas, five of them when their cars were blown off a motorway. Three bodies were found in other towns.
In west Texas the problem was snow – high winds caused drifts that have made a number of roads impassable.
The storms across the South have been unusually powerful for winter.
According to Texas reports, churches were destroyed, cars mangled and trees toppled across a 20 mile zone from south of Dallas up to suburbs in the north-east.
In December 27 briefing, Lt. Pedro Barineau, of Garland police, said 600 buildings had been damaged.
The Red Cross is setting up shelters for those with damaged homes.
Police said all street and highway lights had been knocked out, leaving officers working in the dark overnight.
Two people were found dead at a petrol station in Copeville, and a third was killed in Blue Ridge, reports in local media said.
Kevin Taylor, a church pastor in Glenn Heights, south of Dallas, described to WFAA how his church began collapsing around him.
While extreme weather in the US around Christmas is not unknown, meteorologists say that unseasonably high temperatures in some areas contributed to the severity of the storms.
The forecast for the eastern US is of continuing high temperatures – Washington DC pushed close to 70F on December 27.
A year ago, a tornado hit south-eastern Mississippi, killing five people and injuring dozens more.
A storm on Christmas Day in 2012 which included several tornadoes damaged homes from Texas to Alabama.
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