Tornadoes raked America's southern Plains region, overturning cars on an Oklahoma City motorway and destroying dozens of homes.
No deaths were immediately reported from the tornadoes that hit Oklahoma and rural parts of Kansas and Nebraska.
The worst damage seemed to be in the Oklahoma City area, where a twister destroyed homes at Bridge Creek, Amber and Blanchard and it appeared another tornado touched down later when a second storm came through the area.
"We have damage reports, so we do strongly think there was a tornado on the south side of Oklahoma City," meteorologist Michael Scotten of the National Weather Service in Norman said after the second storm that hit at around 8.40pm local time.
That storm flipped vehicles on Interstate 35 and left power lines strewn across the motorway. Twelve people from a mobile home park in south Oklahoma City were taken to hospital.
In Grady County a zoo about 25 miles south west of Oklahoma City was hit by a tornado. All animals were accounted for.
The storms dumped up to 6ins of water in the southern part of the city, prompting a flash flood emergency for the first time in its history. Road crews were waiting for the storms to abate to set up barricades and evaluate troublespots.
The ambulance service responded to water rescues all over the Oklahoma City metro area, with two ambulance crews needing assistance after getting stuck in high water.
The Storm Prediction Centre had warned that bad weather would come to Tornado Alley and more storms were possible later in the week, with flooding a major concern.
In Oklahoma, Grady County emergency management director Dale Thompson said about 10 homes were destroyed in Amber and 25 in Bridge Creek. As the storm moved to the east, forecasters declared a tornado emergency for Moore, where seven schoolchildren were among 24 people killed in a storm two years ago.
When the first of the storms moved through, schools kept pupils in safe places.
At Will Rogers World Airport in Oklahoma City, people were twice evacuated into a tunnel outside the security zone.
In Nebraska, 10 to 15 homes were damaged near Grand Island, and between Hardy and Ruskin, near the Kansas line.
At least nine tornadoes were reported in Kansas, the strongest of them in the sparsely-populated north-central part of the state.