In all, almost 66 million people are at risk of severe weather on Wednesday and Thursday, according to the Storm Prediction Center, USA Today reported.
Cities at greatest risk for bad weather on Wednesday include Milwaukee, Chicago, Louisville, Indianapolis, Columbus, Cleveland, Cincinnati, and Pittsburgh, said AccuWeather meteorologist Alex Sosnowski.
The storms could include a derecho, a fierce blast of wind that can bring gusts to 80 mph or more and damage to a large area over hundreds of miles, Sosnowski said.
Even if a derecho does not develop, AccuWeather predicts that powerful thunderstorms will still be quite numerous across much of Illinois, Indiana and Ohio.
An infamous derecho last June roared along a 700-mile path from the Midwest to the Mid-Atlantic, leaving millions powerless for several days. Washington, D.C., was among the hardest-hit cities.
In addition to the severe weather, flooding will be possible as heavy rainfall is expected with the storms that develop and move into these areas, the National Weather Service reports.
Overnight tonight and into Thursday, damaging winds, large hail and a few tornadoes are possible in the Mid-Atlantic region, including Philadelphia, Baltimore and Washington, D.C., the Weather Channel forecasts.
The weather service is forecasting a 90% chance of heavy rain and thunderstorms in the Philadelphia area on Thursday, which is bad news for the already waterlogged golf course at the US Open in Ardmore, Pa., just outside of the city. A flood watch is in effect for the area.