PHOENIX — Nearly 73 million people in the United States were under extreme heat alerts Monday as a heat wave moved eastward, and the mid-Atlantic and New England were likely to see highs in the 90s as the week progresses. Excessive humidity will make it feel even more oppressive.

The U.S. last year saw the most heat waves, consisting of abnormally hot weather lasting more than two days, since 1936. Officials again warned residents to take precautions.

Much of the Midwest and Northeast were under heat warnings or watches.

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Record Heat-Related Deaths in Phoenix

The heat has been especially dangerous in recent years in Phoenix, where 645 people died from heat-related causes in 2023, which was a record. Temperatures there hit 112 degrees Fahrenheit (44 Celsius) on Saturday. Weather service forecasters say the first two weeks of June in Phoenix have been an average of 5.6 degrees Fahrenheit hotter than normal — the hottest start to June on record there.

A meteorologist with the National Weather Service in Phoenix, Ted Whittock, advised reducing time outdoors between 10 a.m. and 6 p.m., staying hydrated and wearing light, looser fitting clothing. More than 100 cooling centers were open in the city and surrounding county, including two new overnight ones.

In neighboring New Mexico, the high in Roswell was expected to hit 107 F (42 C) on Monday, while temperatures in southern Colorado were expected to surpass 100 degrees (38 C).

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Wildfires and Extreme Heat: A Growing Concern

In Southern California, firefighters increased their containment of a large wildfire in mountains north of Los Angeles on Monday after a weekend of explosive, wind-driven growth along Interstate 5.

The warming temperatures come amid growing concern about the effects of extreme heat and wildfire smoke. The nonprofit Center for Biological Diversity on Monday sent a petition to the Federal Emergency Management Agency asking it to recognize extreme heat and wildfire smoke as major disasters.

A spokesperson for the agency overseeing western U.S. states said she was unfamiliar with the petition but would look into it.

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While much of the U.S. swelters, late-season snow was forecast for the northern Rockies on Monday and Tuesday. Parts of Montana and north-central Idaho were under a winter storm warning. As much as 20 inches (51 centimeters) was predicted for higher elevations around Glacier National Park.

Meanwhile, a fresh batch of tropical moisture was bringing an increasing threat of heavy rain and flash flooding to the central Gulf Coast.

Hurricane season this year is forecast to be among the most active in recent memory.