Heat advisory in effect for Philly Thursday, with strong storms possible

Like water ice and mosquitoes, heat advisories are staples of the meteorological summer in the Philly region, and another one is in effect Thursday for heat indexes that might flirt with 100.

Wednesday’s discomfort levels didn’t quite meet the National Weather Service’s “heat advisory” criteria, but the atmosphere has been brewing a potent variety of copious water vapor that has produced a combination of mugginess and volatility.

Strong thunderstorms are possible Thursday, said Nick Guzzo, a meteorologist with the weather service’s Mount Holly office, and an “isolated tornado” cannot be ruled out.

On the plus side, any storms would produce a respite from the mugginess. After a chilly interval that featured a run at a record, our bodies aren’t quite used to this.

And based on the longer-range forecasts for next week, this hot spell may be just a warm-up.

All thermal comfort being relative, the steaminess is going to take some getting used to.

This early in the season, the weather service has a lower threshold for triggering a “heat advisory” — a heat index of 95 degrees. That will go up to 100 July 1, said Guzzo’s colleague Joe DeSilva.

The index did not get above the low 90s on Wednesday.

But the index had not hit 80 earlier in the week, and the weekend was exceptionally cool. Sunday’s high, 65, just missed the 123-year-old record for the lowest maximum temperature for June 15.

Plus, overall, temperatures over the last several weeks have averaged very close to seasonal normals.

On Thursday, it is expected to be hotter, with highs likely to crack the 90s and heat indexes flirting with 100, and a better chance of afternoon thunderstorms.

Some of those could be severe, according to NOAA’s Storm Prediction Center, and that could have effects on Juneteenth activities.

The rest of the week should be more comfortable as the astronomical summer arrives Friday night; the meteorological community recognizes June 1 as the first day of summer. However, a more significant and prolonged hot spell looms starting Sunday.

“It’s going to be a pretty intense one,” said Dave Dombek, senior meteorologist with AccuWeather Inc.

While these things can change, forecasters are confident that a significant heat wave is all but a certainty next week, with an outside chance that Philly temperatures will approach 100 on Tuesday.

The duration of the heat is still a matter of conjecture, Dombek said, adding that June heat waves tend to be shorter than those of July and August.

However, computer guidance has it continuing for as many as seven days.

In the meantime, this week’s mugginess may hold some benefits for the sweating masses in that it will provide some measure of preparation.

“At least you have some teaser before the main course,” Dombek said.