Summer scorcher: Excessive heat headed to Northeast

The hottest weather of the season is spreading across the eastern half of the U.S. -- with cities in the Northeast in the bull's-eye on Wednesday -- after baking the Midwest with extreme temperatures early in the week.

Chicago's actual temperature hit 99 degrees on Tuesday, breaking the city's daily record of 97 degrees. The heat index -- what temperature it feels like with humidity -- soared to a scorching 115 degrees in Chicago on Tuesday.

On Wednesday, 55 million people from Missouri to New York City are on alert for heat.

The heat index is forecast to soar to 105 degrees in Philadelphia and Washington, D.C.; 102 degrees in Cincinnati, Ohio, and Louisville, Kentucky; 104 in Richmond, Virginia; 103 in Nashville, Tennessee; and 96 in New York City and Pittsburgh.

The scorching temperatures will end in the Northeast on Thursday, but will linger in the Ohio Valley and Tennessee Valley through Friday.

Record highs are possible in Nashville, Cincinnati and Raleigh, North Carolina, by the end of the week.

There are hundreds of deaths each year in the U.S. due to excessive heat, according to CDC WONDER, an online database, and scientists caution that the actual number of heat-related deaths is likely higher.

Last year marked the most heat-related deaths in the U.S. on record, according to JAMA, a peer-reviewed medical journal published by the American Medical Association.

Click here for tips on how to stay safe in the heat.