Severe thunderstorms with possible tornadoes are forecast from the Plains states to the Mississippi, Ohio and Tennessee Valley regions.
The National Weather Service's Storm Prediction Center in Norman, Oklahoma, said the storms are expected to begin Friday afternoon from northern Texas across Oklahoma, southern Missouri, northern Arkansas, southern Illinois, southern Indiana, western Kentucky and western Tennessee.
The chance of storms appeared to be slim for much of North Texas Friday, though a few storms may develop far northwest.
Storms forecast for Saturday are likely to bring hail, high winds and a tornado threat.
Friday, the greatest chance for tornadoes appears to be in western Kentucky and northwestern Tennessee, while there is a slight chance for tornadoes in the remainder of the 142,000 square mile area that includes more than 11.2 million people.
Flood watches and advisories were issued for much of the region.