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. 

“Maricopa County has brought in 10 refrigerated coolers to increase capacity at our Medical Examiner’s Office.  We have not yet had to utilize them,” Jessie Caraveo, a spokesperson for Maricopa County, told the Arizona Republic, part of the USA TODAY Network.

According to Caraveo, a surge in deaths is normally seen during this time of year. But with a historic heatwave scorching, this year may be worse than normal. On Wednesday, a weekly heat report from Maricopa County confirmed seven new heat-associated deaths since last week; increasing the number of such deaths to 25 so far this year, with hundreds of other deaths under investigation.

-Fernando Cervantes Jr., Arizona Republic

Where are Americans under heat warnings?

Over 41 million Americans were under excessive heat warnings as of Thursday morning, including most of Illinois and other parts of the Midwest. Phoenix residents and parts of Southern California were also under warnings.

Much of the Midwest, from the Twin Cities to the whole state of Ohio, were under a heat advisory that covered nearly 129 million people. Also impacted: parts of Texas, Oklahoma and Tennessee.

Heat advisories were in place in the Northeast as well, stretching from parts of Virginia up through Massachusetts. The New York City region and parts of northeastern New Jersey will see heat indices from 103-106 degrees Thursday and Friday, the weather service said.

The weather service in Twin Cities, Minnesota, said heat indices were expected in near or hit above 100 with serious humidity. People should wear light, loose-fitting clothing, stay hydrated, avoid strenuous activity outdoors and try to stay in air-conditioned areas, the weather service said.

‘GLOBAL BOILING HAS ARRIVED’:July temps to smash records, maybe 120,000-year-old ones

Heat isn’t done with the Southwest yet

Record-breaking heat will remain a concern across the Southwestern and Central U.S. through the end of the week, the National Weather Service said.

“Widespread heat advisories are in effect through Friday evening as dangerous heat indices of 105 F to 110+ F are expected” in areas of the Central Plains, Upper Mississippi Valley, Ohio Valley, and Midwest, the weather service said.

In Phoenix on Wednesday night, the temperature fell below 90 degrees at Sky Harbor airport for the first time since the morning of July 9, the weather service there said. Still, dangerous conditions were predicted to last through at least Saturday with highs from 110-116.

El Paso reached 100 for the 41st consecutive day on Wednesday, 18 days more than the previous record, the weather service there said.

Biden announces actions to protect workers in extreme heat

President Joe Biden will meet with mayors of cities experiencing some of the most extreme heat this summer – Phoenix and San Antonio – to discuss how heat is impacting their cities, the White House announced Thursday.

The White House also said Biden has instructed the Department of Labor to issue a “hazard alert,” which will “reaffirm that workers have heat-related protections under federal law.”

The Department of Labor will give employers information on how to protect workers during inclement heat and inform workers of their rights, as well as enhance enforcement for any heat-safety violations, the White House said.

New York City opens over 500 cooling centers

Officials in New York City announced residents could beat the heat by visiting one of hundreds of cooling centers being opened around the five boroughs. The city’s website lists over 500 air-conditioned public facilities, including some designated for older residents and some that will accept pets.

The city activates the network of cooling centers whenever it is under a heat advisory by the National Weather Service with a heat index of at least 95 degrees for two consecutive days or 100 for any period, according to the city’s heat plan.

“Heat kills more New Yorkers every year than any other kind of extreme weather event,” Mayor Eric Adams said at a news conference Thursday.

Adams said the heat index will hit 105 on Thursday and be between 105 and 108 in the city on Friday, and New Yorkers should call 311 to find out where to go for a respite from the heat.

What is a heat wave?

“It’s not the heat, it’s the humidity.” That’s a partly valid phrase you may have heard in the summer, but it’s actually both, the weather service said. The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature, according to the weather service.

A heat wave is a period of unusually hot weather that typically lasts two or more days, according to the National Oceanic and Atmospheric Administration. To be considered a heat wave, the temperatures have to be outside the historical averages for a given area.

A couple of 95 degree summer days in Maine, for example, might be considered a heat wave, but a couple of 95 degree summer days in Death Valley would be pretty unremarkable, the weather service said.

EVERYTHING YOU NEED TO KNOW ABOUT HEAT:From the heat index to a heat dome to an excessive heat warning

− Doyle Rice, USA TODAY

When will this heat wave end?

The question on many Americans’ minds as they slog through day after day of extreme heat is: When will this be over? It depends on where you are, according to Porter, the AccuWeather chief meteorologist.

The same weather conditions that have caused the heat dome to stretch on in the South and Southwest, an area of high pressure in the atmosphere that produces sinking air currents, is expanding into the Midwest and Northeast, but the prognosis differs by location.

It will be “brief but impactful” in Midwest cities such as Chicago and those in the Northeast, Porter said. That’s because the jet stream – narrow bands of wind high in the atmosphere sometimes called “rivers of air” – will dip as we go into the weekend, bringing thunderstorms and scaling back the heat. The peak of the wave in the majority of those regions will be Friday, Porter said.

As the heat wave has expanded to the east, it’s loosened its grip on some parts of the Southwest, but only slightly. Phoenix has seen a slight reprieve. Thunderstorms have helped drop temperatures a bit, which will last into the weekend.

In Houston, Porter said the heat is expected to build once again and a heat wave will likely start Sunday and last through about Aug. 8. Temperatures will hover around 100 and there will be plenty of humidity leading to a “dangerous heat situation once again.”

How to stay safe in a heat wave

Heat is the cause of the most weather-related deaths in the United States every year. Porter calls it a “silent killer.”

The most at-risk populations are the very young, the elderly and people with certain underlying conditions or who take medications that make them less able to regulate body temperature, Porter said. People experiencing homelessness are also at extreme risk. He said kids playing sports or people who work outdoors are at special risk if they don’t realize the dangers.

“As a heat wave goes on longer and longer, that accumulates greater heat stress on people’s bodies and increases the risk for heat-related illnesses and heat-related stroke,” he said.

The National Weather Service in Los Angeles gave residents in different environments tips for staying safe:

• Working outside: Stay hydrated and take plenty of breaks in the shade.
• In the car:Do not leave children or pets unattended in cars and check to make sure they aren’t left behind.
• Inside: Check up on people who are elderly, sick or don’t have air conditioning
• Outdoors: Reduce strenuous outdoor activity, drink water and find shade.