With the high projected to hit 91 this afternoon, the Philadelphia area could find itself two-thirds of a way to the first heat wave of the season.
The forecasted high of 90 Wednesday would seal the deal.
But, the National Weather Service says, it will cool off Thursday with a high of 84 projected.
There is no rain in the forecast until Wednesday afternoon, when there is a 40 percent chance of showers and thunderstorms.
Officials in the meantime have issued an air quality alert for air pollution concentrations that could be unhealthy for children, asthma sufferers, those with heart or other lung diseases and the elderly.
Those potentially affected should avoid strenuous activity or exercise outdoors.
Looking ahead, the forecast calls for sunny, or mostly sunny skies Friday through Sunday with the highs in the low to mid 80s.
For more on the forecast, go to http://go.philly.com/weather.