(AP) — Meteorologists are warning that a gigantic line of powerful thunderstorms is likely to threaten one in five Americans.

The storm system packing tree-toppling winds is forecast to rumble Wednesday from Iowa to Maryland.

The continuous line of storms might even spawn an unusual weather event called a derecho (duh-RAY’-choh). That’s a massive storm of strong straight-line winds spanning at least 240 miles. A historic derecho last year killed 13 people and knocked out power to 4 million residents.

Bill Bunting, operations chief at the National Weather Service’s Storm Prediction Center, says Wednesday’s storms are likely to produce tornadoes, dangerous lightning and large hail. Power outages are likely, to be followed by oppressive heat. Cities at risk include Chicago, Indianapolis, Cincinnati, Detroit, Baltimore, Washington, Milwaukee, Pittsburgh, Columbus, Ohio, and Louisville, Ky.