HILL CITY, Kansas/CHICAGO/NEW YORK - Record-breaking temperatures spread to the eastern U.S. from the Midwest on Wednesday, putting tens of millions of people under heat warnings expected to last into the July 4 holiday weekend, when Americans will celebrate the nation's 250th anniversary.

The extreme heat was expected to push "real-feel" temperatures to 100 to 115 degrees Fahrenheit (37.8 to 46.1 degrees Celsius) across much of the region, elevating the risk of heat-related illness for vulnerable populations and threatening to overwhelm power grids already strained by rising consumption from data centers and electric vehicles.

In Hill City, Kansas, a tiny high plains town 270 miles east of Denver, mail carrier Sabrina Hooper was struggling with the 100-plus-degree temperatures just one week after starting her job.

"It's completely debilitating," said Hooper, 34, of the heat's effect on her work, which entails walking up to 10 miles each day to deliver parcels. She said she gets some relief from lawn sprinklers: "It's so nice. You can take your hat off, get it wet, slap it back on your head."

Hill City was the nation's hottest spot for five consecutive days in 2012, when another record-breaking heatwave swept the region, pushing the town's heat index up to 108 degrees. The heat index measures how it feels when humidity is factored into the air temperature.

Dana Robles, who lives in Brownsville, Texas, a city just off the Gulf Coast at the U.S.-Mexico border, worried on Wednesday about the mounting costs of cooling her home as the heat index rose to 108 degrees. During peak temperatures, her family's monthly power bill can exceed $300, which is nearly one-third what they pay for rent.

Robles also fears blackouts due to the overtaxed power grid.

"I'm scared the electricity is going to go off all day and our food is going to get spoiled," she said.

In Chicago, high-school science teacher Michelle Klein, 57, had started preparing for the heat over the weekend. She filled her car with gas, did her weekly grocery shopping early, stocked the refrigerator with extra cold drinks and gave her plants a deep soak.

"The basil was being a diva and needed another drink of water this morning," Klein said on Tuesday evening after going on her usual evening walk despite the 103-degree heat index.

In the city's suburbs, property investor Amy Kaspar got an urgent call Monday night from a tenant whose air conditioner was only blowing out warm air. Kaspar discovered that the appliance was working fine – it simply could not keep up with cooling the tenant's unit, given the intense heat and humidity.

"Combined with the wind, it feels like standing behind the exhaust of a bus right now in Chicago," said Kaspar, 50.

COOLING CENTERS, CHECK-INS

Chicago's Office of Emergency Management and Communications urged residents on Wednesday to periodically check in with relatives, neighbors, seniors and other vulnerable populations. If contact cannot be made, the office said, Chicagoans can request a well-being check from the city by calling 311.

The scorching U.S. temperatures mirrored those in western Europe, which recently has been engulfed in its own record-breaking heatwave, an event scientists said would have been "virtually impossible" without human-caused climate change. Scientists have confirmed through years of studies that greenhouse gas emissions are making heatwaves around the world both more likely and intense.

The extreme heat only began creeping into New York City as of Wednesday morning, by which point the city had opened hundreds of cooling centers and deployed more than a dozen "cool vans" equipped with water, electrolytes, sunscreen and meals for New Yorkers in need of relief, Mayor Zohran Mamdani said at a press conference.

Air conditioning was on full blast at a senior center in Harlem on Wednesday, where a sign in 13 languages advertised the space as a "cooling center" for the public. The senior center's director, Richard Allman, said it would remain open beyond its usual hours over the July 4 weekend.

"We try to make this a comfortable place for people on an extra-hot day," he said.

Ahead of the heatwave, city leaders had asked operators of signs in the city's iconic Times Square to reduce the brightness of their billboards to lower energy consumption, and requested that businesses set thermostats no lower than 78 degrees. The city's energy provider, Con Edison, urged customers to limit energy use from 2 p.m. to 10 p.m.

The city has also extended public pool hours, opened additional cooling centers in libraries and municipal buildings, and expanded street outreach efforts.

(Reporting by Brad Brooks in Hill City, Kansas, Maria Alejandra Cardona in Brownsville, Texas, P.J. Huffstutter in Chicago, and Maria Tsvetkova and Joseph Ax in New York; Writing by Julia Harte; Editing by Mark Porter)

A woman sits in the shade and fans herself at the base of the Washington Monument due to excessive heat in Washington, D.C., U.S., July 1, 2026. REUTERS/Annabelle Gordon Annabelle Gordon Reuters

A man lies down under trees as excessive heat hits Washington, D.C., U.S., July 1, 2026. REUTERS/Annabelle Gordon Annabelle Gordon Reuters

Tourists use an umbrella to block the sun as excessive heat hits Washington, D.C., U.S., July 1, 2026. REUTERS/Annabelle Gordon Annabelle Gordon Reuters

An unauthorized fire hydrant opening offers temporary relief from extreme heat for children in Milwaukee on Monday. Angela Peterson USA Today Network via REUTERS

Copyright Reuters or USA Today Network via Reuters Connect.

This story was originally published July 1, 2026 at 11:07 PM.

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