Parts of Oklahoma, including Moore and Oklahoma City, were issued a tornado warning Thursday afternoon.
Forecasters and authorities in the area have urged residents to take cover.
Two twisters have already touched down in north central Oklahoma while another tornado has struck Arkansas where at least one injury has been reported.
The National Weather Service noted several "wall clouds" on Thursday afternoon, one of which it said was "very impressive" that was west of Chickasha, about 35 miles southwest of Moore, the Oklahoma City suburb that was hit the hardest in the May 20 EF-5 tornado.
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Storms reportedly could produce 3-inch hail and significant tornadoes, which will generally move toward the east.
Tornado watches — meaning conditions are favorable for the formation of tornadoes, though one hasn't touched down or isn't thought to be imminent — have also been issued for large sections of Arkansas and Missouri.
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