As many as eight persons were killed after a line of deadly storms caused flooding in Texas, Missouri, Arkansas and Mississippi, the authorities said.
The storms were expected to roll into Alabama later Sunday and then move northward along a line that extends to Michigan, reports the CNN.
Seventeen million people are under some risk of severe storms, including hail, strong winds and tornadoes.
The storm system could become more potent on Monday as it heads to Pennsylvania and New York.
The authorities in four states so far have confirmed eight fatalities linked to severe weather, but that toll could rise.