July 4th weekend in Philly will be hot and muggy. Will it rain on the fireworks?

Along with other far-more pleasurable Philadelphia July Fourth outdoor traditions, get ready for another staple of the season — muggy air sticky enough to glue sweat to the skin.

Temperatures aren’t expected to approach any records, but moist air will be building into the region on Thursday through the weekend, forecasters say, as Philadelphia may well experience its second heat wave of the season, interspersed with shower chances.

Heat indexes might reach triple digits during the weekend in the immediate Philly area with highs around 90 degrees, and don’t expect to be doing 70 mph on the eastbound Atlantic City Expressway. If you’re heading to the Shore, sources indicate that other people have the same idea.

While the heat may fall short of any warning criteria, it is always a concern in the city, where so many elderly and vulnerable people live alone in rowhouse neighborhoods, especially at a time when neighbors might be out of town.

“Those rowhouses literally become brick ovens,” Lawrence Robinson, a former Health Department official who was an architect of the city’s widely praised heat-response system in the 1990s, said last week.

But overall the heat should be bearable and the sheer mathematical odds would suggest that showers should spare most outdoor events.

To paraphrase Prince Hamlet, that is often the question for July Fourth. And the short answer is, that may be a fireworks-time decision.

The National Weather Service and private forecasters see a chance of lottery-ball showers and thunderstorms. The timing is uncertain, of course, but the “afternoon and evening” would be the likely periods, the weather service says.

» READ MORE: Your Philly Fourth of July guide

Dave Dombek, senior meteorologist at AccuWeather Inc., said Independence Day forecasts tend to have a certain Groundhog Day component: “It’s pretty much a guarantee: Partly sunny, hot, humid with scattered thunderstorms.”

In fact, in eight of the last 10 years, the weather service forecast on July 3 called for a chance of showers on Independence Day. Measurable rain did fall in Philly on six of those holidays.

Sweating was not on The Inquirer’s “best things to do” list for the holiday weekend, but it will be a common activity, if not a popular one.

Thursday will mark the dawn of a same old air mass, forecasters are saying, with dew points, an absolute measure of moisture in the air, kicking upward.

“It’s going to juice up again and stay that way for a few days or more,” Dombek said.

The forecasts are promising plentiful sun in Philly and at the Shore, but shower chances pop up Thursday, Friday, and Saturday and on all three nights. Sunday should be dry.

One important difference at the Shore: The daytime temperatures are going to be in the 80s, forecasters say.

And one thing to watch: Steady south winds are forecast through the weekend, and that may cause surf temperatures to drop again late in the weekend, said Jim Eberwine, a meteorologist who is the former marine forecaster at the National Weather Service.

Winds can blow off the warmer top layer of the surf, allowing the deeper cooler water to rise to the surface, a process known as “upwelling.” Strong winds Sunday resulted in surf temperatures falling into the mid-50s Monday night, according to NOAA data, but they were back in the mid-60s Tuesday afternoon.

Climatologically, we’ve only just begun.

July Fourth marks the beginning of the hottest stretch of the year in Philly, with average daytime highs of 88 degrees through July 30.

Fittingly, Wednesday marks the unofficial beginning of the unofficial “Dog Days” that continue through Aug. 21. The 40-day period is centered on the rise of Sirius, “the Dog Star.”

This time of year (any time, actually) skepticism about extended forecasts is merited, but for now, the weather service has highs in the 90s through Monday.

Those who view summer as a punitive experience should avoid looking at the Climate Prediction Center’s longer-term outlooks. The government forecasters have odds strongly favoring above normal temperatures through July 16.

In their updated monthly outlook issued Sunday, they also see above-average readings through the 31st.

Forecasters say it won’t be as hot as it was during that seven-day stretch in June, during which it reached 98 and set a daily temperature record.

And that hot spell might even end up being a blessing, said Robinson, now in private practice.

» READ MORE: Philly summers are getting — and staying — hotter

Early heat, he said, can be a form of inoculation against extreme hot spells later in the summer by providing a measure of acclimatization.

And Dombek noted that this next ration of steamy weather shouldn’t come as a shock to those who have spent time in Philly this time of year.

“It comes with the territory,” he said. “It’s July. It’s nothing really crazy.”