Storms slammed the North Texas college city of Denton with hail as large as baseballs, leading to reports of broken windows and other damage. The National Weather Service in Tulsa noted reports of hail up to the size of ping pong balls and strong wind gusts.
Arkansas saw one-inch hail, and falling tree limbs knocked out power in western parts of the state.
Forecasters said the unsettled weather could spawn other tornadoes later Thursday, notably in southern Missouri, southern Illinois, Arkansas, and western portions of Kentucky.
"That's where we think (potential of) tornadoes - some potentially strong - will be the greatest," said Bill Bunting, the forecast operations chief for the National Oceanic and Atmospheric Administration's Storm Prediction Center in Norman, Okla. "This will continue to evolve with time."
Tornadoes, hail and winds in some cases gusting to 75 mph were possible as part of the storm.
Afternoon tornado warnings were issued in central Missouri, but there weren't any immediate signs that a funnel had touched down or caused damage.
No injuries resulted from the twister that hit University City just west of St. Louis shortly before 5:30 a.m., damaging about 100 homes in winds that reached up to 110 mph, National Weather Service meteorologist Jayson Gosselin said. That system also carried heavy rain - up to 5 inches in parts of Missouri, prompting flash flooding that damaged dozens of homes and forced at least two water rescues.
In University City, a densely populated St. Louis County area, the city opened a shelter for evacuees. Gov. Jay Nixon declared a state of emergency.
Rainfall was heavy over much of Missouri and western Illinois. The National Weather Service said portions of Johnson County, Mo., had more than 5 inches of rain, causing flash flooding that forced evacuation of some homes in the Warrensburg area. Highway T in Johnson County was closed after rushing water washed out three culverts.
At least two drivers had to be rescued from water that swamped their cars. Even a three-person rescue team was briefly imperiled when flood debris clogged their jet skis. They eventually floated to safety.
Heavy rains also flooded some roads in Indiana, and conservation officers say they have rescued at least eight people.
Associated Press writers Maria Fisher in Kansas City, Mo., and Alan Scher Zagier in University City contributed to this report.