At least 50 million people from Illinois to the Gulf Coast were at risk Thursday of tornadoes, large hail or wind damage in what forecasters said was expected to be the first significant tornado outbreak of the season.
The tornado risk covered at least seven states. The cities under threat included Little Rock, Ark.; Memphis, Tenn.; and Joplin, Mo., which was all but leveled by a tornado that killed 161 people three years ago.
The National Weather Service issued a tornado watch for central and western Arkansas and southern Missouri through 6 p.m. ET, but forecasters said the severe weather risk would extend well into the night.
The weather service confirmed a tornado Thursday morning outside St. Louis. It was an EF-1, classified as relatively weak. It cut a half-mile path about 100 yards long, the weather service said.
The Mississippi and Ohio Valleys are entering the most dangerous part of the year for tornadoes. Because of the unusually long and cold winter, the country is behind is average pace for twisters this year. Seventy have been reported, compared with an average of 244 by early April.
On Wednesday, severe thunderstorms dumped hail the size of golf balls on St. Louis, and drivers were stranded by flash floods in parts of the city. About 100 homes had minor damage, and the weather service was working to confirm whether a tornado had touched down and determine its strength.
Hail also damaged cars in Kansas.
The Weather Channel said that the tornado risk on Thursday was highest in Arkansas and southern Missouri. Severe storms were possible in parts of Arkansas, Kansas, Kentucky, Illinois, Iowa, Louisiana, Mississippi, Missouri, Oklahoma, Tennessee and Texas.
The storm system is expected to push east on Friday, threatening parts of the Mid-Atlantic and Southeast.