Today is expected to be the first day of what could become the fourth heat wave of the summer and the National Weather Service has issued an alert for much of the Philadelphia region for heat indexes in the 100 plus range.

The forecast calls for a high of 94 today today in Philadelphia, but with dew points above 70, it will be downright steamy.

The weather service has issued a heat advisory beginning at 11 a.m. that extends until 7 p.m. Wednesday for most of the region except for the New Jersey counties bordering Delaware Bay and the Atlantic Ocean south of Long Beach Island.

Philadelphia public schools announced they would close at noon Wednesday because of the heat. All after-school activities, including athletic programs and professional development sessions, are canceled. Universal Schools will also dismiss classes noon on Wednesday.

The projected highs for Wednesday and Thursday are 92 and 91, just enough to qualify for a heat wave. Overnight lows will be in the mid-70s, enough to hold the heat in.

A cold front with the potential to generate thunderstorms should break the heat Thursday night. The high for Friday will be about 82, according to the latest forecast.

There's a 40 percent chance of showers and thunderstorms both Saturday and Sunday.