Not much is left of what once was Tropical Storm Barry, but its remains are a major player in the Philadelphia region — they have set off flood and severe thunderstorm watches, along with flight delays, and are adding steam to the first serious heat wave of the season by swelling the air with water vapor.

The National Weather Service’s “excessive heat warning” is in effect through Sunday, with the year’s highest daytime temperatures and a sequence of hazardously sultry nights expected. The heat index hit 104 in Philadelphia at mid-afternoon Wednesday, the 95-degree reading at 4 p.m. made it the hottest day of 2019 so far.

The declaration sets off a variety of actions by the city, including the opening of cooling centers and activation of the Heatline, operated by the Philadelphia Corporation for Aging. (The number is 215-765-9040.)

Juiced in part by Barry’s remnants, the air will be engorged with water vapor Wednesday afternoon and Thursday, forecasters said. In addition to threat of flash flooding and up to 4 inches or in isolated downpours, the atmospheric moisture help drive heat indexes to 100 or higher.

Incoming flights to Philadelphia International Airport Wednesday evening were being delayed about an hour, and departing flights up to 45 minutes as a result of storms in parts of the Northeast, according to

» READ MORE: What’s a heat index? From warnings to watches, here are the weather terms you need to know

Episodic showers might keep temperatures from reaching 90 on Thursday, but the atmospheric soup will create what the weather service calls that “gross, sticky feeling.”

Then, as Barry’s remnants ooze away, the atmosphere will turn up the burners, as temperatures make a run at 100 on Saturday in Philadelphia for the first time since 2012, with heat indexes as high as 110. Any Barry-related rains would help make the atmosphere more oppressive by supplying more water for evaporation, O’Hara said.

“It’s going to get hot; actually, dangerously hot,” said Barry Rossio, a meteorologist with AccuWeather Inc.

» READ MORE: Where is temperature measured in Philly, and why?

The government sees a high risk of hazardous heat along the East Coast from Jacksonville, Fla., to New York City; in a broad area of the Midwest, from Lincoln, Neb., to Chicago; and along the Mississippi Valley as far south as the Arkansas-Louisiana border.

High pressure, or heavier air, in the upper atmosphere will impede the progress of any refreshing fronts, says the government’s Weather Prediction Center. Meanwhile, steamy subtropical air will continue to be lured northward.

Perhaps the most insidious and dangerous aspect of all that air moisture is its effect on nighttime temperatures. Water vapor keeps daytime heat from radiating into space.

Without overnight cooling, houses without air-conditioning are primed for rapid heating after the sun comes up.

Various studies, including The Inquirer’s, have documented an increase in nighttime temperatures in recent years, especially in cities.

In Philadelphia, summertime average minimum temperatures in the 21st century, 68.3 degrees, have been 2.6 degrees higher than those in the 20th century. The average daytime high for this century, 85.6, is 2.1 degrees higher.

The imbalance likely is related to worldwide warming — theoretically, a warmer atmosphere can hold more water vapor — and urbanization. Buildings and paved surfaces absorb solar energy by day and are reluctant to release it at night.

Plus, winds tend to die down once the sun retires for the day. Said Patrick O’Hara, a weather service meteorologist at the Mount Holly office. “The air doesn’t blow around very much.”

Fortunately, not a single heat-related death has been reported in Philadelphia in what has been a generally benign summer in terms of temperature.

Unfortunately, that could change during the next several days.

» READ MORE: We asked for your best Shore hacks. Here’s what you said.

» READ MORE: How to stay hydrated this summer