July Fourth Heat Wave Forecast: Latest Updates

Philadelphia on the morning of July 4, 1776, was a pleasant 68 degrees, according to a diary kept by Thomas Jefferson, who was not only a founding father but also the founding meteorologist of the United States.

The temperature in the city climbed to 76 by the early afternoon — a little cooler than the few days that had preceded it — as the Second Continental Congress met to officially sever the colonies’ ties with Britain. On his way home, where he would record that the temperature at 9 p.m. had dipped to 73.5 degrees, Jefferson stopped to buy a new thermometer.

The people of Philadelphia in 2026 are facing far less pleasant weather on Saturday, as dangerously high temperatures that have parked over much of the United States this week will drag on for another day. The city’s high on Saturday could reach 100 degrees, with a heat index making it feel more like 105 to 110.

“It’s going to feel like you’re in an oven,” said Ray Martin, a meteorologist at the National Weather Service office that covers Philadelphia. “It will actually be slightly less oppressive and oven-y compared to Thursday and Friday, but still very, very, very hot.”

Similarly high temperatures are expected up and down the East Coast.

In Middlesex County, Mass., home to Lexington and Concord, where the Revolution’s first major skirmishes took place, Saturday’s high could reach about 96, which will feel a tad cooler than Friday’s triple digits, but still hot.

“We are at least fortunate to have access to air-conditioners,” said Bryce Williams, a meteorologist with the Weather Service office in Norton, Mass., which covers Boston. “Back in the day, the heat would have been more taxing.”

In Boston, closer to the coast, Saturday’s temperature was likely to peak at 92.

In Washington, where a full array of celebrations including military flyovers, concerts, a fair on the National Mall and fireworks have already been disrupted by the brutal heat this week, the temperature could hit 102 between noon and 3 p.m. With the sticky air it will feel more like 109.

Some temporary relief could arrive Saturday with a chance for rain and thunderstorms, particularly over the Mid-Atlantic. But the rain is likely to be brief and the temperatures could quickly rise again when the sun comes back out.

Temperature records have fallen across the Midwest and Eastern United States this week. Central Park hit 100 degrees on Thursday, for the first time in 14 years. On Friday, Atlantic City, N.J., reached 105 as of 4 p.m., breaking a record for the date and tying with its second-hottest day ever. The all-time record for the city is 106, set in 1969.

When will the heat wave end?

“The end is in sight,” said Frank Pereira, a meteorologist with the Weather Prediction Center.

Temperatures will cool some on Sunday, with a return to typical warm summer weather with widespread afternoon highs in the 80s from the Great Lakes into the Northeast by Monday.

New York City was projected to hit 81 and Philadelphia 83 on Monday.

But air-conditioners may remain cranked up across the Southeast into early next week.

“There are some areas in the Southeast that will hang on longer, such as the Carolinas,” Mr. Pereira said.