Updated at 11:40 a.m. ET: A storm system that dumped up to a foot of rain in parts of Louisiana, causing isolated flash flooding, is now threatening to spawn tornadoes there and in Mississippi, the National Weather Service warned.
Large hail and wind gusts up to 70 mph were also possible in parts of both states through Wednesday night.
Flash floods were reported earlier in Louisiana, Arkansas and eastern Texas in the wake of "widespread heavy rain" on Tuesday and overnight.
"Some roads are already flooded and closed ... and additional heavy rainfall will continue to produce more flash flooding" in northwest Louisiana, the National Weather Service stated. "This is an extremely dangerous situation."
In Natchitoches, La., an apartment complex was flooded and the Cane River Lake, which flows through the city, was well over its banks in downtown.
Chris Dolce, a meteorologist with weather.com, said that a "significant flood threat is unfolding across the south-central states."
Dolce cited Shreveport, La., Fort Smith, Ark. and Little Rock, Ark., as among the cities in the threat zone.
In Little Rock, police set up barriers in several flooded city streets Wednesday morning.
Dolce advised motorists to not attempt to drive on flooded roadways. "Roughly two-thirds of flood-related deaths occur in vehicles," he added.
Mark Ressler, lead meteorologist with The Weather Channel, said that up six inches of rain was likely across Arkansas and Louisiana.
Strong thunder
In central Arkansas, strong winds Tuesday afternoon caused extensive damage in the town of Morrilton and contributed to at least one injury when a tree fell on a home, authorities said.
Officials in southeast Kansas also closed several bridges, and workers in Missouri shut down a rural roadway after rain sparked flash flooding there. In Arkansas and Oklahoma, the Forest Service has closed campsites in low-lying areas to avoid another catastrophe like the flash flood that killed 20 people at a remote campground in 2010.
Storms rattled Tulsa, Okla., Tuesday morning with thunder so strong that it registered on seismic equipment.
Meteorologist Pete Snyder felt his home shake and several concerned residents called the National Weather Service to ask if there had been an earthquake.
"We have seen quite a bit of thunder on all of our seismic stations across the state," said Austin Holland, a research seismologist with the Oklahoma Geological Survey. "We can confidently say there were no earthquakes large enough to be widely felt."
On Tuesday, record highs were set in dozens of central and eastern cities, from International Falls, Minn., on the Canadian border to Lexington, Ky., in the South, according to the National Weather Service.
In Chicago, the high at O'Hare International Airport reached 85 degrees Fahrenheit, smashing the previous record of 76 degrees set in 1921, according to Accuweather.com.
It was the sixth day in the past seven that temperatures in Chicago reached 80 degrees or more. Brian Edwards, a meteorologist at AccuWeather.com, said the warmth was likely to linger into next week across much of the country.
"It looks like things will continue like this for several more days and then trend downward a little bit as we go into the weekend," Edwards said. "But even next week, temperatures will still be well above normal."
Reuters, The Associated Press, msnbc.com staff and weather.com contributed to this report.
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