The coming hot spell may be Philly’s longest September heat wave since 1931

Heat waves in September in Philly aren’t all that unusual, but this back-to-school special due to start Sunday could last through the week and become the longest of the year — with an outside shot at becoming the longest since 1931.

At least one daily temperature record likely will fall, on Labor Day with a forecast high of 95, but that would be more or less a climatological quirk, and the heat won’t prompt any National Weather Service warnings.

The duration, however, would be notable, with forecasts calling for 90-plus temperatures into Friday, with caveats, of course: A wild card might be the remnants of Idalia.

Showing its trust in science, the Philadelphia School District already has announced that more than 70 schools that don’t have air-conditioning will be closing early this week. The district is acting “wisely,” in the opinion of Ruth McDermott-Levy, codirector of the Mid-Atlantic Center for Children’s Health and the Environment at Villanova University.

Heat, she said, “reduces learning. It also makes children and their teachers irritable, so this could lead to classroom disruptions and exasperated teachers. Not a great start to a school year.”

While the heat indexes will still be below the triple-digit danger levels, said Sarah Johnson, the local weather service’s warning coordination meteorologist, “it’s still going to feel like a shock after the last couple of days.”

And by the time the week is over, it appears as though September will have had more 90-plus days than August, which had a grand total of three.

Heat waves — using the generally accepted criterion of three successive days of 90-plus temperatures — historically have occurred in Philly every five to six years in September. One even occurred in October in 1941. Even the pumpkins were sweating.

Official readings in Philly have reached 90 or better after Aug. 31 about 250 times, according to weather service records dating to 1874.

In this instance, an area of rain-repelling high pressure is due to move across the region, generating warm and mostly dry winds from the the southwest, although “the humidity is going to head up during the week,” said John Feerick, a senior meteorologist with AccuWeather Inc.

Monday’s forecast high, 95, would best the record of 93, set in 2018. However, among the first 13 days of the month, Sept. 4 just happens to have the lowest record for a maximum temperature.

Whether the heat will make it to Friday is questionable, he said. Some computer guidance brings the remnants of Idalia north, and then west toward New England. He said that might generate clouds and compromise the heat-producing high pressure, at least along the I-95 corridor.

That said, with the sun drying out the ground, Feerick said it’s not out of the question that the temperature reaches the upper 90s at some point during the week, or even 100. It’s been known to happen.

It hit 100 on Sept. 2 and 3, 1953, and 102 on Sept. 7, 1881, the latest 100-degree reading in Philly on record, during a blistering eastern heat wave when the nation was in crisis.

It also was brutal in Washington, peaking at 104 on the 7th. President James A. Garfield, battling for his life after he was shot by an assassin in July, insisted on being moved to Long Branch, N.J., hoping that sea breezes would provide relief.

They may well have, but Garfield would end up dying there on Sept. 19.

September heat waves three years later, and again in 1898, would last for six days, as would one in 1931. According to an Inquirer analysis, they were the longest September hot spells in those records dating to 1874.

It’s uncertain what effect heat had on Philly schools in the 19th century. Compulsory education didn’t begin in Pennsylvania until 1895.

But the weather is likely to be a factor this week, and not just because of the heat, said Levy-McDermott. Chances are that ground-level ozone levels will be building this week, a trigger for asthma, a condition shared by 1 in 5 Philadelphia children. She said that school nurses should be prepared for increases in asthma attacks.

The heat is expected to back off by the weekend, with the arrival of a cool front; however, the updated Climate Prediction Center outlook issued Thursday sees the odds favoring above-normal temperatures for September in much of the nation.

Staff writer Kristen A. Graham contributed to this article.