Two people have been killed by strong tornadoes that swept across portions of southern Oklahoma.
Emergency services said one twister hit the town of Wynnewood as a number of violent storms raked the Southern Plains and the Ozark mountains.
The emergency director in Garvin County, Bud Ramming, said a man believed to have been in his late 70s was found dead in a home.
Elsewhere, police in Johnston County said a man died near the town of Connerville.
Some of the weather was so bad that forecasters declared a tornado emergency for the towns of Roff and Hickory.
The tornado emergency notice said: "You are in a life-threatening situation. Flying debris will be deadly to those caught without shelter."
One person sustained minor injuries in Murray County, where the tornado moved slowly through a rural area on its path that would take it south of Roff, said Gary Ligon, Murray County's emergency management director.
He said the injured person was taken to hospital.
The Storm Prediction Centre in Norman, Oklahoma, warned that twisters as far north as Nebraska and Iowa could develop.
The bad weather should settle in the Ohio and Tennessee valleys on Tuesday, forecasters said, while another storm system should bring bad weather to the area from north Texas to near St Louis on Wednesday.
Monday's series of violent thunderstorms dropped tornadoes near Oklahoma City and in the rolling hills south of the city, destroying some homes and barns. Oklahoma Highway Patrol closed Interstate 35 near Wynnewood for 15 minutes so the storm could pass, temporarily shutting down the primary route between Oklahoma City and Dallas.
Meteorologists said twisters with wind speeds above 110mph were possible from eastern Oklahoma to central Arkansas. Hail 2in or more in diameter was possible from eastern Texas to south-eastern Kansas.
About 41 million people from Houston to Sioux City, Iowa, were at risk for some type of stormy weather.