The bodies of a man and a woman - who were missing since a tornado in Benton County, Mississippi - were found, bringing the state's death toll to 10.
Another four people died in a traffic accident in Garland, Texas.
Officials said Texas and Oklahoma could be hit with a "historic blizzard", bringing up to 16in (41cm) of snow.
Motorists were warned they could face icy roads.
Six people died in Tennessee. Arkansas and Alabama each reported one fatality.
In Mississippi, 56 injuries have been reported and more than 400 houses have been destroyed or damaged.
At least 20 tornadoes hit the state, knocking down trees and leaving thousands without power over Christmas holidays.
One large tornado reportedly touched land in Mississippi and tore a path more than 100 miles (160 km) long to Tennessee.
While extreme weather in the US before Christmas is not unknown, meteorologists say that unseasonably high temperatures in some areas contributed to the severity of the storms.
One 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.
At least 22 people are now known to have died after nearly a week of tornadoes, heavy rains and flooding in several US south-central states.