With heat indexes forecast to climb past 100 once again, Philadelphia’s heat emergency remains in effect through 8 p.m. Wednesday.

As of 2 p.m., the index had reached 99 officially at Philadelphia International Airport. While the temperature, 91, was several degrees lower than the highs the previous two days, that likely had to do with the sun’s having to evaporate some of water vapor filling the air, which could incite trouble later on when strong storms are possible.

While the National Weather Service hasn’t issued a heat warning for Wednesday — only an advisory — the city is erring on the side of caution, given the potential cumulative effects of a heat wave that began Friday and the fact that the nights have been oppressive.

The low Monday morning, 81, tied a record for a highest minimum temperature for the date in Philadelphia; Tuesday it got no lower than 76, and Wednesday, 77.

“The threat is definitely still there, and we want to offer as many supports as we can,” said James Garrow, spokesperson for the Public Health Department. As part of the emergency response, the Philadelphia Corporation for Aging will again staff its heat “helpline” (215-765-9040); the city is keeping some cooling shelter open; and in an unprecedented move, is enlisting SEPTA’s help.

Several of SEPTA’s large accordion buses will keep their motors idling and air-conditioners running until 5 p.m to serve as makeshift cooling centers. It might not be as refreshing as a dip in a city swimming pool, all or which are closed, but it is cool.

“This is a first for SEPTA,” said spokesman John Golden, adding that it was the brainchild of the city’s Office of Emergency Management. “We’ve never offered our buses as cooling stations before.”

Health experts heat waves become more hazardous the longer they last, and this one is in Day 6.

Protracted heat makes it harder for houses to cool off, especially when nights are hot. That buildup is especially dangerous for the elderly who live alone, and the Corporation for Aging estimates that more than 100,000, or better than 35%, of Philadelphians 60 and older live alone.

And it has been quite hot this week. Philadelphia officially got up to 94 by 3 p.m. Tuesday, and that was considered a respite as relative humidity decreased.

That was a mere retreat, however, as water-vapor levels have returned. Any storms could be drenchers. The most likely period for storms would be in the early evenings Wednesday and Thursday.

The “precipitable water” values will be quite high, and given that the atmosphere is in a state of summery torpor, “slow storm motion looks to be an additional issue.”

The government’s Storm Prediction Center in Norman, Okla., has the region under a “slight risk” for severe storms, primarily for wind. The criterion is gusts of about 60 mph or better. The chance of severe weather is higher on Thursday, it said.

Thursday might also be the technical end of the heat wave, as temperatures might fall just short of 90. The weekend will still be warm, with highs in the 90 neighborhood, forecasters say, but it should be comfortable, at least relatively.

In the meantime, although the coronavirus has presented unique challenges, Health Commissioner Thomas Farley advised family members and neighbors to look in on those who live alone.

Here are the cooling options. Those using them are asked to wear masks and maintain social distancing.

Libraries

  • Haddington Library, 446 N. 65th St.
  • Lillian Marrero Library, 601 W. Lehigh Ave.
  • Logan Library, 1333 Wagner Ave.

Schools

  • West Philadelphia High School, 49th and Chestnut Streets.
  • Kensington High School for Creative and Performing Arts, 1901 N. Front St.

Buses

  • Germantown and Hunting Park Avenues.
  • Wyoming and Rising Sun Avenues.
  • Frankford and Allegheny Avenues.
  • 52nd Street and Larchwood Avenue.
  • 29th and York Streets

In addition, all the city’s “spraygrounds” will be open.

Here is a complete map of the city’s cooling options.