WASHINGTON — A gigantic line of powerful thunderstorms could affect one in five American on Wednesday as it rumbles from Iowa to Maryland packing hail, lightning and tree-toppling winds.

Meteorologists are warning that the continuous line of storms may even spawn an unusual weather event called a derecho, which is a massive storm of strong straight-line winds spanning at least 240 miles. Wednesday’s storms are also likely to generate tornadoes and cause power outages that will be followed by oppressive heat, said Bill Bunting, operations chief at the National Weather Service’s Storm Prediction Center in Norman, Okla.

The risk of severe weather in Chicago, Indianapolis, Cincinnati and Columbus, Ohio, is roughly 45 times higher than on a normal June day, Bunting said. Detroit, Baltimore, Washington, Milwaukee, Pittsburgh and Louisville, Ky., have a risk level 15 times more than normal. All told, the area the weather service considers to be under heightened risk of dangerous weather includes 64 million people in 10 states.

“It’s a pretty high threat,” Bunting said, who also warned that the storms will produce large hail and dangerous lightning. “We don’t want to scare people, but we want them to be aware.”

Wednesday “might be the worst severe weather outbreak for this part of the country for the year,” said Jeff Masters, meteorology director at Weather Underground.