Such heat domes hover above a region, trapping heat and humidity closer to the ground, according to experts.
About 97 million people from the central Plains through the Southeast are under heat advisories, according to the National Weather Service.
High temperatures are expected in more than a dozen states in the Southern Plains and lower Mississippi Valley through Wednesday, the National Weather Service said.
Some places could hit 100 degrees Fahrenheit (37.8 degrees Celsius), forecasters said. But the combination of soaring temperatures and high humidity could make it feel as hot as 115 degrees Fahrenheit (46 degrees Celsius) in portions of the Southeast.
Additionally, the persistent heat will offer little relief to those affected during the evening hours, forecasters said.
“The nighttime lows are not going to get below about 80 Fahrenheit for a lot of urban areas,” Yale Climate Connections meteorologist Jeff Masters said in a statement. “So that’s really going to bump up the mortality from this heat wave. It’s going to be deadly because of all these factors: long-lasting, intense, and high humidity.”
Heat domes can make it difficult to fully recover from high temperatures at night, making it especially important to find ways to stay cool during the day and evening hours.
“It’s those night times—especially, locations and people, who don’t have access to adequate cooling, really need to be aware of the impacts that added heat stress will have and their body’s ability not to be able to cool down,” according to Zachary Labe, climate scientist at Climate Central, an independent collective of scientists, “because humidity is going to be a really key factor in boosting those heat impacts.”
The heat wave is arriving as the new school year begins across much of the country, prompting schools to consider how to keep students safe and comfortable on buses, in classrooms, and even on playgrounds.
In Central Arkansas, one district adopted a policy this year requiring students to stay indoors, including during recess, whenever the heat index reaches 95 degrees Fahrenheit.
Meanwhile, two school districts in South Carolina canceled a series of Friday evening football scrimmages involving more than a dozen high schools as an extreme heat warning was in place.
Officials urge residents in affected areas to remain indoors whenever possible and drink plenty of fluids. They also encouraged people to check on older adults, children, and others who may be especially vulnerable to extreme heat.