Philadelphia is poised to hit its fourth heat wave of the summer this week, with an excessive heat warning and possibly the hottest day of the year so far in store for Thursday.

The National Weather Service says to expect a high temperature in the low-to-mid 90s on Wednesday, marking the region's second consecutive day of 90-degree weather.

The heat is expected to intensify Thursday, when the heat wave becomes official and forecasters are calling for the mercury to reach the mid-to-high 90s. Heat-index values, or the "real feel" temperatures, are expected to surpass 100 degrees, prompting the weather service to issue an "excessive heat warning."

The warning is in place from 11 a.m. to 8 p.m. Thursday.

"The combination of hot temperatures and high humidity will combine to create a dangerous situation in which heat illnesses are likely," the warning reads. People are advised to drink plenty of water, stay out of the sun, use air-conditioning, avoid strenuous activities outdoors, take frequent rest or shade breaks if spending time outside, and wear light or loose-fitting clothing.

So far, fortunately the hot spells have been short-lived, and Philadelphia hasn't had a single reported heat-related death, the Health Department said Wednesday.

The warmest day of summer so far in Philadelphia was June 13, when the mercury hit 96 degrees, setting a record for the date.

The current record for July 13 stands at 98 degrees, set in 1994, and Thursday's high temperature could come close to that mark.

Other heat waves in 2017 were May 17 to 19, June 11 to 13, and July 2 to 4.

There's a chance of scattered thunderstorms both Wednesday and Thursday, forecasters said. The temperature is forecast to drop a bit Friday, reaching the high 80s, but there is a higher chance of showers and storms then, the weather service said.