It’s going to be deadly hot Monday, with a heat index that will make it feel as if it’s nearly 110 degrees.

The National Weather Service issued an excessive heat warning Sunday, when temperatures were in the mid-90s, with a heat index of about 100.

Monday will be the hottest day of the stretch of hot, humid temperatures that began Saturday in the region, with temperatures in the high 90s, Jonathan O’Brien, a meteorologist with the National Weather Service in Mount Holly, said Sunday. The record for that date, July 20, is 99 and was set in 1930.

“Heat is the deadliest of all natural hazards,” O’Brien said. “It’s the number-one weather-related killer.”

The weather service is urging people to take precautions and stay indoors and avoid strenuous outdoor activities.

“Drink plenty of water, check on the elderly and those who are without access to air-conditioning,” O’Brien said.

In Philadelphia, Thomas Farley, the city’s health commissioner, issued a heat health emergency beginning at 8:30 a.m. Monday and ending at midnight.

The city had closed its Free Library branches due to the COVID-19 pandemic, but will reopen some of them on Monday to serve as cooling centers. Everyone will be required to wear masks and practice social distancing, Farley said in a statement Sunday.

These libraries will be open between 1 p.m. and 5 p.m. on Monday, July 20.

  • Haddington Library at 446 N. 65th St.
  • Lillian Marrero Library at 601 W. Lehigh Ave.
  • Logan Library 1333 Wagner Ave.

“While the city has opened a number of sites to help our most vulnerable stay cool during the heat health emergency, the best thing that we can do to help our family members, neighbors, and loved ones is to use this time to check on them.,” Farley said.

He said relatives can check on seniors by telephone or video. They can also wear a mask and knock on their doors, and ask whether they need anything, but stay six feet away.

The heat health emergency declaration activates the city’s emergency heat programs, which include the Philadelphia Corporation for Aging’s Heatline, cooling centers, home visits by special field teams, enhanced daytime outreach for people experiencing homelessness, and a reminder to the public to safely check on older friends, relatives, and neighbors from a distance.

The Philadelphia Corporation for Aging’s Heatline number is 215-765-9040. It will be available between 8:30 a.m. and midnight Monday.

People should call if they have questions about to stay safe in the heat or how to detect signs of heat stress. Health Department nurses will be available to speak with callers about heat-related medical problems.

On Facebook and Twitter, the Weather Service sent out posts with hashtags: #LookBeforeYouLock, #petsafety, #heatsafety, and #heat wave to remind people of the dangers of leaving children and pets inside cars, especially during the summer.

Things won’t really cool down until the end of the week, because it will remain in the 90s Tuesday, Wednesday and Thursday, O’Brien said.

A slight chance of thunderstorms on Wednesday and Thursday may cool things down by Friday.

It won’t be that much cooler at the New Jersey Shore on Monday, either.

Although there was a slight sea breeze that brought relief on Sunday, there won’t be much of a breeze Monday, and the highs at the Shore will be in the low- to mid-90s.