An excessive heat warning is in effect until 8 o'clock tonight as temperatures hit the 90-degree mark for the third day in a row, making this the first official heat wave of the season.

At 1 p.m., the National Weather Service issued a tornado watch, which is to remain in effect until 8 p.m., as well.

The heat and high humidity prompted the Philadelphia public and Catholic schools to close early at 1:30 p.m.

The NWS says the heat warning is primarily directed at the elderly, infirm, children and pets.

It urges people to drink plenty of fluids, stay in an air conditioned room, keep out of the sun, check up on relatives in neighbors and, under no circumstances, leave children or pets unattended in a car.

The warning, issued Tuesday, triggered the activation of the Philadelphia Corporation for Aging's Heatline, a service to help prevent heat-related deaths. Staff will be available until midnight Wednesday to speak with callers about detecting heat stress and medical problems related to the heat. The Heatline number is 215-765-9040.

PCA said the activation yesterday was the earliest in the Heatline's history.

Pet owners should bring their animals inside or at least insure they have shade and water, the NWS says.

Besides highs in the low to mid 90s, the forecast says there's a chance of showers and thunderstorms this afternoon and tonight.

It will cool off tonight and the highs Thursday will be in the lower 80s under sunny skies, the NWS says.

For Friday through Sunday, the highs will be around 80 under sunny or mostly sunny skies.

For more on the forecast, go to http://go.philly.com/weather.

Contact the Inquirer Online News Desk at online@phillynews.com or 215-854-2443.