Cities from Chicago to New York are facing well-above-average heat, with the National Weather Service (NWS) issuing advisories for much of the northeastern US, as well as parts of the South including Arkansas, Tennessee, Mississippi, and Louisiana.
The unusual June heat—driven by a high-pressure heat dome parked over the eastern half of the country—is expected to persist through at least Wednesday, bringing a dangerous mix of soaring temperatures and high humidity.
"Take frequent breaks, stay hydrated, and protect pets and outdoor animals," warned the NWS office in Wakefield, Virginia, urging residents to avoid prolonged exposure.
Tragedy in New York as storms compound the crisis
In upstate New York, the extreme weather turned deadly after thunderstorms toppled trees in the early hours of Sunday. Three people were killed, including twin 6-year-old girls, after a large maple tree crashed through their home in Oneida County. A neighbour helped the mother escape through a window, according to local reports.
A separate treefall nearby killed a 50-year-old woman, while thousands lost power amid heavy rains and downed power lines. New York Governor Kathy Hochul declared a state of emergency in 32 counties, citing both the storm damage and the looming heat risk.
‘I just want to sit in my air conditioning'
In the Midwest, heat indexes soared above 39°C (102°F) over the weekend. In Madison, Wisconsin, even the city's annual naked bike ride was described as "sticky and sweaty" amid the sweltering conditions.
Lynn Watkins, who runs a daycare in Sun Prairie, Wisconsin, said she had already decided to cancel all outdoor activities this week. "I can't stand being outside when it's like this," she said. "I just want to sit in my air conditioning."
The heat index reached 40°C in Pittsburgh and hovered around the same in Columbus, Ohio. In Chicago, a Seattle Mariners pitcher collapsed from heat exhaustion, while a Cincinnati Reds shortstop became ill during a game in St. Louis.
Philadelphia declares heat emergency
In Philadelphia, officials declared a heat emergency through midweek, opening cooling centres and launching a "heat line" staffed by medical professionals. Forecasts suggest the heat index could climb to 42°C (108°F) on Monday.
Elsewhere, crowds gathered at Rehoboth Beach in Delaware as residents sought relief before temperatures become "too hot to manage," according to beachgoers. At the PGA Travelers Championship in Connecticut, spectators lined up for water and shade, with some trying—and failing—to cool down on ventilated benches.
"It didn't work," said Karin Skalina, who had retreated from the sun-soaked bleachers. "We're just trying to follow the shade."
Record-breaking stretch could last all week
Forecasters say the heat will intensify and shift eastward through the week. New York City is expected to see highs around 35°C (95°F) on Monday and Tuesday, while Washington, D.C., and Boston could experience temperatures nearing 38°C (100°F) by midweek.
Mark Gehring, a meteorologist with the NWS in Wisconsin, said while summer heat is not unusual, the scale of this event is striking.
"It's basically everywhere east of the Rockies," he said. "That's what's unusual—just how broad this dome of heat and humidity really is."
The extreme heat comes amid mounting concerns over how climate change is reshaping weather patterns, bringing longer and more intense heatwaves earlier in the season—and over larger areas than previously experienced.