Tens of millions of Americans were under heat notices Thursday − and major metropolitan areas were set to hit temperatures not seen in years − as the crushing heat felt in the West and South began to engulf parts of the Midwest and Northeast.

A “dangerous” heat wave will settle over cities including Washington, D.C., Philadelphia and Boston from Thursday through at least Saturday, the National Weather Service said. Temperatures could creep up toward triple digits, and the “oppressive humidity” will make it feel more like 105 degrees. Nighttime lows will be 10 to 15 degrees hotter than usual.

The situation was bleak in Phoenix, where the Maricopa County medical examiner’s office was over capacity Thursday amid a growing number of heat-related deaths, forcing officials to bring in coolers for the first time since an early pandemic wave.

The widespread coverage of intense heat this week, from Phoenix and Houston to cities such as Chicago to the East Coast, makes it the “most widespread heat” this summer, according to AccuWeather chief meteorologist Jonathan Porter.

“This week seems to us to be probably the hottest week of the summer if you spatially average that across the country,” Porter told USA TODAY on Thursday.

The air was thick and sticky with humidity in the Washington, D.C., region Thursday morning. The nation’s capital could see 100 degrees for the first time in seven years, and the city declared a hot weather emergency. Baltimore, too, is poised to hit 100 for the first time in years, AccuWeather reported.

Arizona county brings in coolers to store bodies amid heat deaths

A relentless heat wave pummeling Arizona has brought with it a wave of heat-related deaths. The Maricopa County Medical Examiner’s Office was at 106% capacity Thursday, prompted the county to bring in coolers for the first time since the peak of the COVID-19 pandemic in 2020.