A sprawling storm system crossing the U.S. on Friday overturned semitrucks on highways and fanned wildfires in Texas and Oklahoma, where officials called for evacuations in at least one town. Tornado threats loomed for the Mississippi Valley into the night and the Deep South on Saturday.

A sprawling storm system crossing the U.S. overturns semitrucks on highways and fans wildfires in Texas, Oklahoma, Missouri and Kansas. Tornadoes might hit the South.