NORMAN, Okla. (AP) -- Forecasters cautioned millions of people in Middle America to keep an eye to the sky Wednesday and Thursday amid threats of hail, high winds and strong tornadoes.

The Storm Prediction Center, in a midday update to its forecast Wednesday, upgraded to its second-highest advisory level - a moderate risk - while stressing that a significant tornado or two could form in a narrow stretch from northern Oklahoma to central Missouri.

Strong storms swamped Indianapolis, Cincinnati and Charleston, West Virginia, at midday Wednesday and forecasters said more severe weather could form as far away as the plains of West Texas.

Collapsed shed near Pretty Prairie, Kansas last week, when storms with high winds caused damage across the area. (AP Photo/The Hutchinson News, Travis Morisse)

Indiana State Police said high winds toppled a tractor-trailer on Interstate 69 near Evansville, while utilities reported a number of power outages after wind gusts reached 70 mph.

Fewer than 1 million people were in Wednesday's "moderate risk" area between Wichita, Kansas, and Jefferson City, Missouri, but 34 million were under at least a slight risk of seeing damaging winds, large hail and possibly a tornado.

Emergency managers in Kansas and Illinois huddled separately to address the approaching storms. Kansas officials warned that hail could be the size of baseballs, while Illinois officials told residents they should be prepared to seek shelter if bad weather arrives.

Heat, humidity and the approach of a cold front and a jet stream could cause severe weather on Thursday, too. Forecasters said 57 million people were at an "enhanced risk" of seeing storms nearby, including residents in Chicago, Detroit and St. Louis, as well as those in Memphis, Tennessee, and Little Rock, Arkansas.

Areas that don't see strong storms Thursday could see heavy rain instead.

The Storm Prediction Center said Wednesday's storms in the southern Plains could be significant - defined as having 2-inch hail, 75 mph winds and tornado damage rated at EF2 or higher.

Emergency officials said a tornado touched down briefly Tuesday night in southeastern Kansas. Parts of Missouri and Indiana also had severe weather Tuesday.

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