PITTSBURGH (KDKA/AP) – An organized complex of storms is expected to affect the Pittsburgh area during the overnight hours, and a tornado watch has been issued until midnight.

The watch includes Allegheny, Beaver, Fayette, Washington, Westmoreland, and Greene Counties.

Meteorologists are warning that the 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. 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.

The National Weather Service has issued a Flash Flood Watch for much of Western Pennsylvania between 10 a.m. and 6 p.m. Thursday.

According to KDKA-TV Meteorologist Dennis Bowman, look for partly sunny skies, warm and humid conditions with thunderstorms possible in the mid to late afternoon time frame.

Today’s high will be around 82 degrees.

Storms will become more likely tonight and into the overnight hours and could produce large hail, damaging winds and flooding downpours. An isolated tornado is also possible.

The overnight low will be about 69 degrees.

For Thursday, expect heavy rain and a thunderstorm or two into the afternoon hours with rain totals potentially hitting 2 to 3 inches. Tomorrow’s high will be 74 degrees.

Remember that in high winds, the safest place is as low as you can get with as many walls as possible between you and the storm.

Never drive through a flooded road either. As the saying goes, turn around, don’t drown.

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.

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