Tornadoes have killed at least five people and wreaked massive damage in Oklahoma near the battered city of Moore, still recovering from a major twister two weeks ago.
The dead included a mother and child who were killed in a car on Interstate 40 near Oklahoma City.
At least 12 people were also injured in the storm, with the number expected to rise.
Several people were hurt in crashes and overturned vehicles, according to a Oklahoma Highway Patrol spokeswoman, who described the situation as heartbreaking.
Forecasters lifted an emergency for parts of the tornado-prone midwestern state, though flash flood warnings, severe thunderstorm warnings and tornado watches remain in place.
In Missouri, which borders the Mississippi River, Governor Jay Nixon declared an emergency, urging Missourians to closely monitor weather conditions, so they can take shelter or move to higher ground if needed.