– The Associated Press

CHICAGO – An unusually massive line of storms packing hail, lightning and tree-toppling winds was rolling through the Midwest on Wednesday and could affect more than one in five Americans from Iowa to Maryland.

Meteorologists warned about the possibility of a weather event called a derecho (Spanish for “straight ahead” or “direct”), which is a storm of strong straight-line winds spanning at least 240 miles. The storms are also likely to generate tornadoes and cause power outages that will be followed by oppressive heat, said Russell Schneider, director of the National Weather Service’s Storm Prediction Center in Norman, Okla.

The weather service issued tornado warnings in several counties in Iowa, Illinois and Wisconsin.

“We’re becoming increasingly concerned that a major severe weather event will unfold,” Schneider said. “The main thing is for folks to monitor conditions and have a plan for what to do if threatening weather approaches.”

For the first time this year, the center was using its highest alert level for parts of Iowa, Illinois, Indiana and Ohio. Airlines canceled more than 120 flights at O’Hare airport in Chicago.

The storms were expected to push into northwest Indiana early Wednesday evening.

The Northern Indiana Public Service Co., the region’s largest utility, said it was increasing staff at its customer call center and scheduling extra work crews to handle any power outages.

All told, the area the weather service considers to be under heightened risk of dangerous weather includes 74.7 million people in 19 states.