Philly hit 100 degrees Tuesday for the first time in 13 years, but a cooldown is coming

With the first official 100-degree reading in 13 years on Tuesday, Philadelphia broke a 100-year-old record for a daily high-temperature. And another might fall Wednesday, after what was a second consecutive historically warm morning.

By 9 a.m. it already had reached 90.

But this may well be a last hurrah for the first heat wave of 2025, and it’s possible that Friday’s high in Philly will be 25 degrees lower than Tuesday’s.

Long-duration heat waves are the most dangerous, say heat-health experts, but if the forecasts hold, this hot spell will have lasted just three days.

“We may get lucky with this one,” said Michelle Kelly, a nursing professor at Villanova University and children’s health specialist.

In the meantime, the National Weather Service’s “extreme heat warning” and the city’s heat emergency remain in effect until Wednesday evening.

The forecast high in the upper 90s would make a run at the all-time high for the date, 99 degrees, which occurred in 1952. Of perhaps of more consequence, the expected Wednesday morning low, around 80, similar to Tuesday’s, would be another June rarity.

In short, Wednesday will be the hottest day since Tuesday. The official thermometer at Philadelphia International Airport registered 100 at 1:55 p.m., said Alex Staarmann, meteorologist at the weather service office in Mount Holly. It was the first triple-digit reading in June since 1994, and the first at all since July 18, 2012.

That’s 100 in the shade, notes Craig Herrera, a Fox Weather meteorologist. It might feel like 120 degrees in the sun if you’re standing on asphalt, or 150 if you get into an unshaded vehicle.

And in all likelihood it was hotter in some city neighborhoods that are stranded on the so-called urban heat island than it is at the airport.

Herrera added that all this may feel more oppressive given that the last several weeks have generally been temperate around here and throughout the sun-baked Mid-Atlantic and Northeast.

“It’s almost like we flipped the switch and went right into it,” he said.

Philly’s high on Tuesday beat the 99-degree reading of June 24, 1923. Monday’s high of 99 was a record for the date, also.

The 100-less stretch was the longest since the one that ended in 1917, according to weather service records. For a variety of reasons it’s hard to hit 100 around here, given the proximity of bodies of water. (Note that Miami hasn’t hit 100 since 1942.) Plus, worldwide warming evidently is making things muggier.

Not that the average body could feel the difference, but the atmosphere was drier on Tuesday than it was Monday, and that was more favorable for nudging up the temperature, Staarmann said.

The more water vapor in the air, the more the sun diverts some of its ground-baking energy evaporating the moisture.

While the temperature was a shade higher than Monday’s, the heat index, a combination of the temperature and the relative humidity, was a few shades lower, peaking at 107, Staarmann said.

Some drier air from about 6,000 feet up was able to mix down toward the surface, said Dave Dombek, senior meteorologist with AccuWeather Inc.

The mercury bottomed out at 81 degrees Monday morning, setting a daily record for the highest low temperature, and would be the second-highest minimum temperature for any day in June in Philadelphia.

The low Wednesday morning, 80, another record for the date, beating the 76 degrees of 2002.

The reluctance of temperatures to drop after dark is a classic symptom of the mugginess. When conditions are clear and dry, daytime heat radiates into space. When the moisture level is high, it inhibits cooling.

Heat-health experts warn that nighttime heating is especially dangerous for people who live without air-conditioning, especially for older residents in rowhouse neighborhoods.

Without overnight cooling, those houses heat up rapidly when the sun returns.

Hot spells become progressively more dangerous the longer they endure as the health effects are cumulative.

Kelly, who is the co-director of the Mid Atlantic Center for Children’s Health and the Environment, said it is essential for seniors — and for children — to stay hydrated.

“They’re birds of a feather,” she said. “Their skin physiologically is similar to that of children.” She said it’s more of a challenge for the skin of the old and young to manage the loss of fluids.

It was a renegade cluster of “ring of fire” thunderstorms that developed well to the north of the region that kept the heat wave from starting on Sunday.

The high was 88 degrees, well shy of the forecast high in the mid- to upper-90s.

Now, a heat-wave-ending backdoor front — one that moves northeast to southwest and imports cooler ocean air — will be approaching and might make it through the region by Thursday morning, Dombek said.

It still will be humid, but Thursday’s heat indexes look to stay below 90, with highs in the upper 80s.

Philly has an outside chance at seeing showers as early as Wednesday afternoon, and they are likely on Thursday.

The maritime air will be more evident Friday, and highs might not get past the mid-70s.

Showers are possible during the weekend, with highs in the 80s, but no new hot spells are in the short-term outlooks. But the next two months would be July and August.