Heat wave maps: How long the dangerous temperatures will last

Thirteen states in the East from Massachusetts to Kentucky are on alert for more extreme heat before the dangerous temperatures move to the South and the West.

Record highs were shattered in New England on Thursday, including 99 degrees in Manchester, New Hampshire; 98 degrees in Hartford, Connecticut; and 97 in Augusta, Maine.

On Friday, the temperature is forecast to hit a scorching 98 degrees in Louisville, Kentucky; 84 degrees in Indianapolis; 96 degrees in Pittsburgh and Washington, D.C.; 95 in Philadelphia; and 92 in New York City.

In Philadelphia, this could be the longest stretch of 90-degree temperatures this early in the season in 30 years.

This weekend, the heat will spread into the South.

Washington, D.C., may hit 100 degrees this weekend, which would be the first time the city reached triple digits in June in 12 years.

Record highs are possible in Little Rock, Arkansas, by Monday.

The dangerous heat is also returning to the West this weekend, with heat alerts in effect in California, Arizona and Utah.

Temperatures could climb to 110 degrees in Palm Springs, California; 114 in Phoenix; and 107 in Las Vegas.

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.

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