Updated June 29, 2026 at 12:53 PM PDT
A heat dome is bringing oppressive heat to the eastern half of the contiguous U.S. this week, with forecasters warning people in those areas to take precautions to cope with extreme high temperatures. The phenomenon occurs when high pressure lingers and traps warm air near the surface, while also suppressing clouds and precipitation.
Temperatures this week will be in the 90s and low 100s in much of the Central and Eastern U.S., the National Weather Service said Monday.
"The combination of high humidity values will produce heat indices between 105-115 degrees across portions of the southern Plains, Mid-Mississippi Valley, and eventually into parts of the Mid-Atlantic," the NWS said.
Heat alerts were in effect for more than 69 million people from Texas to Maine on Monday.
In Iowa, forecasters predicted the heat index would reach as high as 110 Fahrenheit.
In Nashville, temperatures could feel like 115 degrees. "Mamma mia, it's hot. There's no doubt about it. We're going to be a summer night city all week," the city's NWS office wrote.
But for many, summer nights won't be much fun.
The NWS said overnight cooling will be "limited," with many areas only going down to the mid or upper 70s. "The combination of prolonged daytime heating and limited nighttime relief will increase the risk of heat-related illnesses, especially for vulnerable populations and those without adequate cooling."
Climate change, brought on primarily by the burning of fossil fuels, is intensifying heat waves.
What is a heat dome?
"It almost acts like a lid on a pot," the National Weather Service's Alex Lamers told NPR, discussing how a heat dome works. Lamers is the operations branch chief at the Weather Prediction Center.
"If you've made grilled cheese in a pan and you put a lid on there, it melts the cheese faster because the lid helps trap the heat," Lamers said. "It's a similar concept here: You get a big high-pressure system in the upper parts of the atmosphere and it allows that heat to build underneath over multiple days."
That's right: days, or even weeks.
A heat dome is basically a massive area of high pressure and warm air that "parks" over a region. The exact location is generally linked to the jet stream — a narrow band of strong wind that blows from west to east.
When a heat dome sits over a large land area, it can form a sort of feedback loop, Lamers said, adding that high pressure typically means dry weather, which can help drive the heat even higher.
Extreme heat has shattered records in many parts of the U.S. in recent years, and scientists warn that we're likely to see even more dangerous temperatures in the future — a trend linked to human-caused climate change. Both 2023 and 2024 were designated the hottest years ever recorded, while 2025 was the third-hottest.
The most intense heat waves would be impossible without human-caused global warming.
Health alerts again mark the start of summer
Almost exactly one year ago, a heat dome brought record high temperatures to many of the same areas as this year, sending summer to a scorching start. Another heat dome happened at the same time in 2024, too.
The current bout of extreme heat is expected to peak on Thursday or Friday, but the heat "may finally begin to abate from west to east this weekend," the NWS said.
The extreme temperatures and humidity will be especially threatening for people with respiratory issues, the elderly, and people taking certain medications.
Nausea, headache, confusion and dizziness are among the symptoms of heat-related illnesses — which can include heat exhaustion, heat rash and heat stroke. Heat stroke is the most severe and can lead to death if not treated quickly.
Anyone who has to be outside should consider advice from Dr. Jess Weisz, a pediatrician at Children's National Hospital in Washington, D.C., who told Morning Edition these tips in 2025: "Taking lots of breaks, especially if they're being physically active, drinking a lot of water, also using sun protection" such as sunscreen and a hat.
Heat domes affect different parts of U.S.
Heat domes have struck other regions in recent years. In early June of 2024, the Western U.S. suffered high temperatures that were 20 to 30 degrees hotter than normal. In 2023, a dome that formed over a huge swath of the Great Plains, South and Midwest in late August brought a rash of record temperatures from New Orleans to Chicago. For the second year in a row, there were more than 600 heat-related deaths in Arizona's Maricopa County in 2024 and Phoenix saw a record 113 consecutive days of temperatures at or above 100 degrees.
And in 2021, a heat dome broiled Oregon, Washington and other parts of the Northwest in the early summer, causing hundreds of deaths.
The extreme heat in the U.S. comes as other countries deal with record highs. A heat wave struck Western Europe last week, with France reaching highs above 100 degrees. The scorching heat has moved into Central and Eastern Europe.
World Health Organization Director-General Tedros Adhanom Ghebreyesus said on Sunday that more than 1,300 excess deaths have been recorded since June 21 that are linked to the extreme heat in Europe.
"Driven by climate change and global warming, the phenomenon of the 'once-in-a-generation' heatwave is now occurring nearly annual. We were warned," he wrote.
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