See how a heat dome will trap dangerous temperatures over the central and southern U.S.

The summer heat just got turned up a notch − and trapped under a lid. About 100 million people will be living with 100-degree days, according to AccuWeather RealFeel® Temperatures. The cause? A massive late-July heat dome that will move back and forth across much of the United States through the end of the month.

The National Weather Service said July 23 that the remainder of the week features a "strengthening heat dome over the Mississippi Valley today that gradually builds east towards the East Coast by week's end." A combination of mid-upper-90s temperatures and humid conditions are a recipe for "oppressive heat indices that range between 110-115 degrees for many areas along and east of the Mississippi River."

Where the heat is going

According to AccuWeather, temperatures will rise to 100 degrees almost every day for the next week or two in many locations. Before this week, several of these locations had not yet topped 100 degrees this year.

"The bottom line is that this has the look of a long-lasting heat wave with limited rainfall," AccuWeather senior meteorologist Chad Merrill said in a news release. "Kansas City, Missouri, has not hit 100 degrees since Aug. 25, 2023, but is positioned to do so on multiple days during the upcoming heat dome."

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What causes a heat dome?

A heat dome occurs when a persistent region of high pressure traps heat, said William Gallus, professor of in meteorology with the Department of Geological and Atmospheric Sciences at Iowa State University.

"The heat dome can stretch over several states and linger for days to weeks, leaving the people, crops and animals below to suffer through stagnant, hot air that can feel like an oven," Gallus said in an article in The Conversation.

Tips on keeping cool

• Drink water, more than usual, no matter how active you are. Don't wait until you're thirsty. Make sure your pets have plenty to drink.
• Avoid sugary, alcoholic or caffeinated liquids. They can increase body fluid loss.
• Stay in air-conditioned buildings as much as possible.
• Take cool showers or baths.
• Limit outdoor activity, especially in the middle of the day.
• Use drapes or shades on windows to keep out the sun.
• Limit oven and stove use to keep in-home temperatures lower. 
• Wear light-colored, loose-fitting clothing to improve perspiration evaporation.

What else can you do?

• Check on well-being of friends and neighbors – do they need help in keeping cool?
• Never leave people or pets in a closed car, even with partially open windows. The interior can rapidly reach dangerous temperatures.
• Seek immediate medical care for people with symptoms of heat illness.

CONTRIBUTING John Bacon and Shawn Sullivan

SOURCE National Oceanic and Atmospheric Administration, Centers for Disease Control and Prevention, Federal Emergency Management Agency, AccuWeather and Windy.com