Over 75 million people in the United States were under extreme heat alerts on Monday as a heat wave moved eastward, and the mid-Atlantic and New England are likely to see highs in the 90s as the week progresses. Excessive humidity will make it feel even more oppressive.

The US 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.

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 112F (44C) on Saturday. Weather service forecasters say the first two weeks of June in Phoenix have been an average of 5.6 degrees 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 10am and 6pm, staying hydrated and wearing light, looser fitting clothing. More than 100 cooling centres were open in the city and surrounding county, including two new overnight ones.

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

A woman stretches up her hand while laying in a patch of shade with books at Yards Park, Sunday, June 16, 2024, as temperatures begin to steam up in Washington. Photo / AP

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 recognise extreme heat and wildfire smoke as major disasters.

The agency did not immediately issue a specific response to the petition. A Fema spokesperson for the western US states said there was nothing that would preclude an emergency declaration for extreme heat, but noted that there would need to be an immediate threat to life and safety that local authorities could not respond to.

While much of the US 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 (51cm) 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.