Heat index values -- a measure of how sweltering conditions feel when humidity is considered -- could top 100 degrees from Wilmington to Trenton.
At work is a classic pattern, according to meteorologist Kristin Kline: "High pressure sitting off the East Coast, with the southerly to southwesterly flow around it bringing up the warm air from the south."
The Shore, though, should see highs in the 80s, while the Poconos won't get as sticky.
"The combination heat and humdity won't be quite as severe in those areas," Kline said.
The warning lasts into Friday morning, because the overnight low might only dip to around 80 in Philadelphia, while humidity remains high. Kline said. Temperatures could bottom out in the mid 70s in the suburbs.
A front moving through early Friday will bring some relief, as well as possible showers and thunderstorms. But while the humidity should lessen, temperatures could still climb into the low 90s, giving the region the year's first official heat wave.
Saturday, Sunday and Monday should see a return to highs in the 80s, with a 30 percent chance of showers Sunday.
For more on the forecast, see Philly.com's weather page.
Contact staff writer Peter Mucha at 215-854-4342 or firstname.lastname@example.org.