That's what the National Weather Service is warning as a dangerous heat wave reaches its peak this weekend, with more than two-thirds of the US feeling temperatures push into the triple digits.

So far, one death has been attributed to the high temperatures. Former NFL player Mitch Petrus , 32, fell ill while working outside Thursday during a heat advisory in Arkansas. He died of heat stroke, officials said.

Cities in Virginia and North Carolina will see some of the hottest air of the season . New York, meanwhile, has opened almost 500 cooling centers. And officials in Massachusetts are urging residents to wear loose clothing and be alert for signs of a heat stroke.

About 157 million people are under heat warnings and heat advisories Saturday as daytime temperatures will climb into the mid to upper 90s --- and feel like more than 100 degrees --- from the Great Plains to the East Coast.

Unfortunately, it won't get any better at night. Overnight temperatures over the weekend will not drop below 80 degrees in many cities across the East Coast, the National Weather Service's Weather Prediction Center said.

9 PM EDT Friday: Heat index values are mainly in the lower/mid 90s. Probably won't get below 80 tonight, and we'll be back up around 105 for the third straight day on Saturday. #kywx #BeatTheHeat pic.twitter.com/mGPd5Zq96S — NWS Louisville (@NWSLouisville) July 20, 2019

Across the upper East Coast on Saturday, the weather service said, the most dangerous time will be mid-afternoon into early evening, when temperatures and humidity will peak.

"Take extra precautions if you work or spend time outside," the service said. "When possible, reschedule strenuous activities to the early morning the late evening."

Heat index forecasts for Saturday, as of late morning.

In Philadelphia and New Jersey, the weather service warned the heat may cause heat stress or heat stroke. The elderly and people with pre-existing conditions are most at risk.

In Detroit, where residents will see a heat index of up to 105 degrees Saturday, more than 200,000 people did not have power in the early morning hours after thunderstorms caused trees and branches to take down power lines.

By Saturday afternoon, power had been restored to all but about 70,000, officials said.

But relief is coming. A cold front will drop into the central US and portions of the Midwest late Saturday, which will bring cool air in the Midwest Sunday, CNN meteorologist Ivan Cabrera said. The East Coast will begin cooling down Monday, Cabrera said.

Until then, city officials are taking precautions against the sweltering heat and preparing for what could be a deadly weekend.

Heat wave in the Big Apple

In New York -- which has declared an emergency -- many events have been canceled or postponed after concerns of participants' health.

Saturday's card of horse racing at New York's Saratoga Race Course was canceled. So were all races Saturday and Sunday at the Maryland Jockey Club in Laurel.

"The health and safety of our horses and jockeys is our highest priority," Maryland Jockey Club President and General Manager Sal Sinatra said.

The New York City Triathlon also was canceled.

"After exhausting all options to mitigate athlete, volunteer, spectator and staff exposure alike, we are unable to provide either a safe event experience or an alternate race weekend," organizers said.

In an executive order that lasts until Sunday night, New York Mayor Bill de Blasio ordered office buildings 100 feet or taller to raise thermostats to 78 degrees and encouraged residents to set their thermostats higher to reduce energy use.

The city is opening about 500 air-conditioned "cooling centers" in public facilities. Public pools were to be open an extra hour (until 8 p.m.) from Friday through Sunday, And the city will set up portable drinking fountains at busy pedestrian areas through Sunday.

We're currently projecting this heat emergency will be over by end of day Sunday. We'll have more updates throughout the weekend. Keep an eye on @NotifyNYC and again, keep cool, drink plenty of water and keep safe, New York City. — Mayor Bill de Blasio (@NYCMayor) July 19, 2019

The Brooklyn Defender Services called on the mayor and the Department of Corrections Commissioner Cynthia Brann to protect inmates from the heat.

"Most incarcerated people are without air conditioning and the limited number of fans are only in the day rooms, leaving people to swelter, particularly while in their cells," the organization said. "DOC is not providing appropriate summer clothes to many of our clients. People with medical needs have reported feeling nauseous and dizzy."

Peter Thorne, DOC deputy commissioner of public information, said in a statement that the health and safety of inmates was "top priority."

"During these warm weather conditions, as always, we remain in close contact with Correctional Health Services, the department's health care provider," Thorne said in a statement.

Mayor de Blasio tweeted Saturday that Brann "has been on Rikers Island to monitor response to the heatwave."

For any housing unit without A/C, DOC has fans, ice, water, and access to multiple cool showers. DOC senior leadership have been on site to monitor the response and protect health and safety of everyone in the facility. — Mayor Bill de Blasio (@NYCMayor) July 20, 2019

"Summer clothes have been distributed, clinics are open 24/7, and medical personnel are on site for emergencies," the mayor tweeted. "DOC senior leadership have been on site to monitor the response and protect health and safety of everyone in the facility."

Protect your animals

In many locations across the country , city officials and emergency departments are warning against the danger that pets will face.

"A closed car can rise to 125 degrees in just 8 minutes," the Massachusetts Department of Public Health wrote on Twitter.

Animals, according to the weather service, can die of heat stroke within 15 minutes. Cracking car windows won't help. Keep your pets at home during hot weather, the service said, and if you see an animal inside a car, don't leave without solving the problem.

As New York City heats up, #NYPD Mounted cools down. To ensure the safety of our 4 legged partners the Mounted Unit remains indoors during heat advisories. Please remember to check on anyone including pets that may be vulnerable to the heat. Stay hydrated and keep cool. #heatwave pic.twitter.com/8Yi3X7aCht — NYPD Special Ops (@NYPDSpecialops) July 19, 2019

The American Society for the Prevention of Cruelty to Animals recommends lots of fresh, clean water.

said. "Know the symptoms of overheating in pets, which include excessive panting or difficulty breathing, increased heart and respiratory rate, drooling, mild weakness, stupor or even collapse," the organization

You know that burning feeling you get when you go barefoot onto asphalt/concrete etc.. you're dogs feel it too. With the #heatwave the ground temps will be in excess of 120*F.

Keeps your dogs on the grass when you take them out for their walk! pic.twitter.com/xqSFZ6OVmP — Upper Darby Police (@UDPolice) July 18, 2019

How climate change has played a role

Each year, summer heat kills more Americans on average than any other natural disaster, CNN meteorologist Cabrera said.

The threat of heat waves will become more serious across the globe and more widespread as the climate crisis continues, says Francesco Rocca, president of the International Federation of Red Cross and Red Crescent Societies.

A new report by the Red Cross Red Crescent Climate Centre says 17 of the 18 warmest years on record have occurred since 2001.

"Climate projections indicate that if greenhouse gas emissions continue on their current pathway, by the year 2100 three out of four people on Earth could be subject to at least 20 days per year of potentially deadly heat and humidity levels," the report says.