Extreme heat threatens to break records in 250+ cities this week

The week is already off to an extremely hot, and dangerous, start for many in the Eastern half of the U.S., as experts say hundreds of cities could see record-breaking heat.

Sunday’s temperatures were above average, but the week beginning Monday June 23, 2025, is set to be even hotter, with a heat-dome encompassing more of the U.S. The forecast has the National Weather Service (NWS) issuing warnings for large portions of the country, mostly in the central and Northeast regions. Meteorologists believe heat records could be smashed in at least 250 cities.

On Monday, locations like Boston, New York City, Washington, D.C., and Philadelphia are under extreme heat warnings, as they are expected to experience 100-degree (or hotter) heat—a number New York’s Central Park hasn’t reached since the summer of 2012, according to the NWS. Likewise, Philadelphia hasn’t warmed that much since 1994, while Boston hit 100 just three summers ago.

As the heat will stretch across the eastern half of the country, heat advisories are also in place for cities like Chicago, Kansas City, St. Louis, Detroit, Cincinnati, and Pittsburgh. 

NWS says the heat risk is level 4—the highest level of extreme heat—and while in some locations the heat is expected to break by Tuesday, in others, it will be just getting started. Temperatures in the Northeast aren’t set to break until Thursday.

And it’s not just the heat making things uncomfortable. It’s the humidity. According to Weather.com, humidity could make the already stifling heat feel even hotter, especially for Midwestern and Northeast cities. The combination of high temperatures and humidity is more dangerous than dry heat, experts say, due to the fact that our bodies aren’t able to cool themselves as effectively when humidity is also high. 

The heightened heat index will also mean that nighttime will bring little relief.

“Hot temperatures, oppressive humidity and very light winds are expected today, leading to dangerous levels of heat,” the National Weather Service in Boston said in a post on X. Tuesday will be “the hottest day of the stretch,” the agency said.

Extreme heat isn’t new, but with each passing summer, more heat records are being broken. According to a recent World Weather Attribution report, about half of the world’s population experienced around one month of extreme heat due to human-made climate change over the past year. 

“Although floods and cyclones often dominate headlines, heat is arguably the deadliest extreme event,” the report said.