Schools are closing early on Tuesday in Philadelphia, where temperatures are predicted to top 90 for the second straight day.
Suburban Bucks and Montgomery counties are also under heat warnings. The National Weather Service says people should stay hydrated and try to avoid over-exerting themselves.
A record high temperature that had stood since 1908 was broken Monday in Mount Pocono. Temperatures reached 86 degrees, passing the previous record of 85.
But in western Pennsylvania, early afternoon thunderstorms brought cooler weather and even a hail warning. The region remained under a severe thunderstorm watch until 7 p.m., with wind gusts up to 60 mph and temperatures in the mid-70s.
In parts of central Pennsylvania, heavy downpours have closed flooded roads in Columbia and Schuylkill counties.
A flash flood watch was in effect for eastern Pennsylvania, mainly between 5 p.m. and midnight. Severe thunderstorms are possible.
With rain expected to sweep through much of the state, slightly cooler temperatures are predicted for the next few days.