Summer scorcher: Dangerous heat hits 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, heat advisories are in effect from St. Louis, Missouri, to New York City. An excessive heat warning was issued in Philadelphia, where the heat index could hit 105 degrees.

The heat index is forecast to rise Wednesday to 106 degrees in Baltimore, Maryland; 103 in Washington, D.C.; and 97 in New York City.

The final tennis major of the year, the U.S. Open, which is underway in New York City, is operating under an "extreme weather policy," with stadium roofs partially closed and extended breaks for players.

Washington, D.C., hit 100 degrees on Wednesday, shattering the city's daily record.

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

On Thursday, the heat index is forecast to climb to 104 degrees in Nashville, Tennessee, and Columbia, South Carolina; 102 degrees in Raleigh, North Carolina, and Louisville, Kentucky; and 105 degrees in Greenville, Mississippi.

Record highs are possible Thursday in cities including Nashville and Louisville.

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.