There were reports of tornado damage southwest of Oklahoma City, Capt. Paul Timmons of the Oklahoma Highway Patrol said. A semitruck blew over in the storm, he said, and low visibility was making it difficult to reach the hardest hit areas.
"There's debris just everywhere, and there's a lot of water on the roadways," he told CNN's "Erin Burnett OutFront."
A photo from CNN affiliate KFOR
showed a storm ripped off parts of an exterior wall of the Norman Hotel in Norman, Oklahoma, leaving the parking lot filled with insulation and debris. The front desk clerk said she called guests as the storm approached and moved them into a storm shelter.
The National Weather Service declared a flash flood emergency late Wednesday for the first time in Oklahoma City history.
The main airport in the city canceled all flights for the rest of the night Wednesday.
"There will be no more flights tonight at WRWA," the Will Rogers World Airport tweeted.
Hospital on generator power
The Norman Regional Hospital had some minor damage and is on generator power but is fully operational, state officials said.
CNN meteorologist Sean C. Morris said 1 to 5 inches of rain fell in the Oklahoma City area through midafternoon, while an additional 3.07 inches fell at Will Rogers World Airport in less than an hour in the evening.
Flooding will be a major concern overnight, according to the National Weather Service.
No injuries or fatalities have been reported so far in the state, the Oklahoma Department of Emergency Management said.
Jeff Theis, who shot video of a dark funnel cloud near Hardy, Nebraska, said he saw a tornado destroy a farm in the community. Hardy is near the state line with Kansas.
Weather spotters reported seeing tornadoes in central Oklahoma near the town of Amber, and in Kansas northeast of the town of Lincoln, according to the National Weather Service.
In Bridge Creek, Oklahoma, about 30 miles southwest of Oklahoma City, forecasters warned residents to take cover as the Weather Service issued a tornado emergency and said a large, destructive twister was in the area.
At Will Rogers World Airport in Oklahoma City, officials said they were evacuating passengers and employees to a pedestrian tunnel.
"We are seeing some pretty ominous-looking skies," CNN meteorologist Jennifer Gray said.
The storms are part of a larger system going through Kansas, Oklahoma and Texas, Gray said, with the possibility of gusting winds, damaging hail and tornadoes.