Record-breaking heat wave threatens millions across U.S.

More than 24 million people from the Northern Plains to New England to parts of Texas are under a heat advisory, with at least six states setting record highs on Monday. 

Boston is under a heat emergency as temperatures rose above 90 degrees for the third consecutive day. Many flocked to cooling centers and neighboring parks. Tuesday is expected to be another scorcher in the city, with temperatures expected to be above 90 degrees for a fourth straight day. 

"In the last couple of days we've had 18 calls that we've flagged as directly heat-related. But on top of that we've seen a 31% increase in call volume for the last 48 hours and today's on track to do that as well," said Jim Hooley, the chief of Boston Emergency Medical Services. 

It was a similar story in Wisconsin, where libraries offered an escape. "We changed our policy to allow drink in the library so people can bring their water bottle in or fill their water bottle at the library," said Madison Public Library spokeswoman Tana Elias. 

At least 15 public schools in Minneapolis, Minnesota, announced Monday they will move to remote learning from Tuesday through Thursday as the extreme heat is expected to continue throughout the week, according to CBS Minnesota.  

Severe drought conditions across the West helped fuel two wildfires in Arizona, forcing hundreds to evacuate homes outside of Phoenix. More than 60,000 acres have burned. As of Monday, the Telegraph Fire was 0% contained, according to KPHO-TV. 

The heat and humidity also produced scattered severe thunderstorms from Alabama to Texas, where a woman's body was recovered after floodwaters swept her car off the road. More than 100 homes were flooded outside of Houston.