NEW YORK — Life-threatening heat spread across the eastern half of the U.S. this week, first hitting the Midwest, then the Northeast and now the South.

Chicago hit 99 degrees on Tuesday, breaking the city's daily record of 97 degrees. Some schools in the area closed early due to the weather.

On Wednesday, the heat moved east. Record highs were shattered in Washington, D,C., which reached 101 degrees, and Greenwood, Mississippi, which reached 103 degrees.

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

The heat has now left the Midwest and Northeast.

On Thursday, the South is in the bull’s-eye, with record highs possible in cities including Nashville, Tennessee; Raleigh, North Carolina; and Louisville, Kentucky.

The heat index -- what temperature it feels like with humidity -- is forecast to soar to 100 degrees in Washington, D.C., and Charleston, South Carolina; 102 degrees in Nashville; Richmond, Virginia; Birmingham, Alabama; and Charlotte, North Carolina; 103 in Raleigh and Charleston, West Virginia; and a scorching 105 in Jackson, Mississippi.

Friday will bring one more day of record heat. By the weekend, cooler air will move in.

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.

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