A massive storm system that left several people dead in Missouri was moving toward the Mississippi Valley and US south on Saturday, prompting the National Weather Service to warn of “the potential for violent, long-track tornadoes.”

Residents in 20 states spanning over more than 500,000 square miles are in the path of the severe storm threat this weekend. The system has been blamed for strong winds that fanned wildfires in Texas and Oklahoma and spawning reports of multiple tornadoes in the Midwest late Friday and early Saturday.