A tour guide fans herself while working in Times Square as temperatures rise Thursday in New York. Nearly 60% of the U.S. population is under a heat advisory or flood warning or watch as the high temperatures spread and new areas are told to expect severe storms. John Minchillo/Associated Press

Nearly 200 million people in the United States, or 60% of the U.S. population, are under a heat advisory or flood warning or watch as high temperatures spread and new areas are told to expect severe storms.

The National Weather Service said a “dangerous” heat wave began to scorch the Northeast and mid-Atlantic on Thursday and will continue into the weekend. Severe thunderstorms and flash floods are possible for parts of the Northeast and South, New England, and South Florida. Meanwhile, the string of record-breaking temperatures will persist for the Southwest and Midwest.

“It’s (hitting) all the big cities,” said Bob Oravec, lead forecaster with the National Weather Service’s Weather Prediction Center. “That’s why the population (affected) is so high.”

Scientists have long warned that climate change, driven by the burning of fossil fuels, will lead to more prolonged bouts of extreme weather.

The prediction for continued excessive heat comes a day after the World Meteorological Organization and the European Union’s Copernicus Climate Change Service declared July 2023 the hottest month on record.

On Thursday, heat and humidity in major cities along the East Coast, including Washington, D.C., Philadelphia, and New York City, created a real feel above 100 degrees Fahrenheit. Forecasters expect several records may break Friday with temperatures 10 to 15 degrees above average.

In New England, communities are bracing for the “dual threats,” as Oravec called them, of extreme heat and flash floods.

“You could have really bad heat for a good part of the day and then get a strong thunderstorm that produces heavy rains and then can produce flooding,” he said.

The Southwest and southern Plains continue to experience record-breaking heat. There, the oppressive temperatures have been blanketing the region for weeks. One meteorologist based in New Mexico called the prolonged period of temperatures over 100 degrees unprecedented.

“They probably aren’t going to have a lot of sympathy for the rest of the country,” Oravec said.

Due to the extreme heat, two of the nation’s largest power grids are under stress, which could affect Americans’ ability to cool off.

The country’s largest power grid, PJM Interconnection, declared a level one energy emergency alert for its 13-state grid on Wednesday, meaning the company is concerned about its ability to provide enough electricity.

“PJM currently has enough generation to meet forecast demand, but operators continue to monitor the grid conditions for any changes,” said Jeffrey Shields, a spokesperson for the company.

PJM isn’t the only electrical grid to issue such an alert. The Midcontinent Independent System Operator, which mostly covers states in the Midwest and Northern Plains, issued a similar alert on Thursday.

The California Independent System Operator also issued an energy emergency alert for the evening on Wednesday, in part due to excess heat in Southern California, but it expired the same day. Anne Gonzales, a CAISO spokesperson, said they expect to be able to meet demand in the next few days.

And a spokesperson for the Electric Reliability Council of Texas, which covers most of Texas, said they expect their grid will operate per usual during this latest blast of extreme weather across the country.