ARLINGTON, Va. − A swath of the East saw temperatures linger near 100 degrees or more Sunday as cities opened cooling centers and labored to protect residents without air conditioning from the relentless heat gripping the region.

Baltimore declared a "code red extreme heat alert" through Monday. In New York, Mayor Eric Adams said city cooling centers were open Sunday and that some schools across the city have opened their doors as well.

"Heat is the deadliest weather we have in New York City," Adams warned on social media. "If you don’t have an (air conditioning), don't sit at home in the heat − it can be deadly."

Arlington County, a suburb of Washington, declared a heat advisory Sunday with a warning that the heat index could reach 107 degrees. The actual temperature at Reagan National Airport reached 99, breaking the previous record for the date of 98 reached twice − in 1988 and 1874.

Washington Mayor Muriel Bowser extended the city's heat emergency and urged residents without air conditioning to seek relief in air-conditioned buildings and the city's cooling centers. The city was bracing for the hottest day since Aug. 15, 2016, and could approach the all-time June record of 104 degrees, AccuWeather said.

Bowser warned that extreme heat can have impacts on a person’s mental health, including increased irritability, aggression, depression, drug and alcohol use and even suicide. Some psychiatric medications can affect body temperature regulation, increasing the risk of heat exhaustion and heat stroke.

"Stick to your treatment regimen and talk with your doctor about any concerns," Bowser said.

A modicum of relief was in sight. AccuWeather meteorologists say that a change in the weather pattern will cause temperatures across the Northeast to retreat from record territory within days, although afternoon temperatures will still reach in the 90s.

Developments:

∎ More than 100 million Americans were under heat advisories or warnings from the National Weather Service. Not all the big heat was in the East: Triple-digit high temperatures were possible in central and southern California, Arizona, and Utah, the weather service said.

∎ The heat wave was global: The toll of Muslims who died while taking part in the annual Hajj pilgrimage to Mecca in Saudi Arabia, where temperatures climbed as high as 120 degrees, has risen to more than 1,300, officials said Sunday. Most of the dead were Egyptians. The BBC said more than 200 Indonesians and almost 100 Indians had died. Other nations also reported dozens of deaths.

Record-breaking temperatures

Temperatures broke records across the region, according to the National Weather Service's offices in the area.

The weather service office in Mount Holly said Sunday that as of 2 p.m., record highs were set in New Jersey and Pennsylvania. The Atlantic City International Airport, Philadelphia, and Reading all reached 98 degrees, surpassing record highs in 1988, 1888, and 1908, respectively.

Reading also hit triple-digit temperatures on Saturday — a high of 101 degrees — making it the first time since July 2012 that the city has experienced triple-digit temperatures, according to the weather service.

For two consecutive days, the Washington, D.C., area had record-breaking daily temperatures, the weather service in Baltimore-Washington said.

Baltimore saw a high of 101 degrees on Saturday, breaking the record of 100 set in 1988, according to the weather service. On Sunday, the area reached 98 degrees, which surpasses the previous daily high-temperature record of 97 degrees set in 2010.

On Saturday, the temperature at Dulles International Airport hit 100 degrees, breaking the previous daily high temperature record of 99 degrees set in 1988.

"This is also the first time it's hit 100º F in June since 2012," the weather service said, later adding that the area hit another record at 98 degrees on Sunday.

In Arlington, Virginia, the temperature reached 99 degrees on Sunday, breaking the previous daily record in 1988.

Flooding in Iowa, South Dakota

In Iowa, Gov. Kim Reynolds declared a flooding emergency in 21 counties and requested an expedited presidential disaster declaration. She flew over parts of northwest Iowa where towns were awash in water, "rivers cresting above 1993 record levels and the devastation is widespread," she said.

In South Dakota, Gov. Kristi Noem declared an emergency after storms that blasted parts of the state with up to 18 inches of rain over the last few days. The rain was slowing down, but "we need to keep vigilant. The worst of the flooding along our rivers will be Monday and Tuesday," she said.

Sioux Falls Mayor Paul TenHaken asked residents to temporarily avoid washing laundry, running dishwashers or doing anything else that would contribute unnecessary wastewater to the system.

"If you can avoid a shower today, that will be great," TenHaken said.

FBI offers reward for information on NM fires

The FBI is offering a reward of up to $10,000 for information leading to the conviction of those responsible for igniting wildfires near Ruidoso, New Mexico, that killed two people and destroyed hundreds of homes.

The South Fork Fire and Salt Fire were discovered last Monday, forcing thousands of residents from their homes.  Ruidoso Mayor Lynn Crawford said residents will be allowed back into Ruidoso this Monday, warning that people with asthma or other breathing issues may not be able to tolerate the ash, smoke and soot in the air. And many homes will have no gas, electric or water service, Crawford said.

"Please bring at least a week's worth of food and drinking water as grocery stores are not operating at full capacity," Crawford warned.

Most heat wave deaths take place in homes

Almost half the 3,142 people who died from heat-related ailments in the past 20 years died at home, according to an analysis of National Weather Service data by the Cincinnati Enquirer, part of the USA TODAY Network.

Many people who die in heat waves lack air conditioning or have health problems that can turn deadly in extreme heat. Social isolation may also play a role in heat-related deaths because those living alone may not have anyone checking on them. A study in Rome found a program aimed at decreasing social isolation among people over age 80 was linked to a lower number of deaths during the summer.  

“Maybe they use a walker or a cane or a wheelchair, and maybe their caregivers are not available," said Tasha Turner-Bicknell, a nurse and associate professor at the University of Cincinnati College of Nursing. "They might be unable to get out of their house without assistance.” Read more here.

− Elizabeth B. Kim

Mosts heat wave deaths occur at home:Here's why

7 essentials to help beat the heat

The summer season was made for all-day fun in the sun, but being unprepared in the scorching heat can quickly ruin a good time. We've compiled a list of 7 essential items that you need to keep on hand to beat the summer heat and enjoy the season to the fullest. From portable strollers and spray fans to an easy DIY misting spray and quick-dry towels, our comprehensive list of summer must-haves covers all the bases so you can stay cool and comfortable wherever your summer days take you. Read more here.

− Maryal Miller Carter

Contributing: Thao Nguyen, USA TODAY; Dominik Dausch, Sioux Falls Argus Leader; Reuters