Forecast Discussion:
Very good air quality today with gusty northwest winds. Winds will diminish this evening and there will be a period of calm before light southwest winds fill in on Saturday. Humidity will rise steadily through the day on Saturday and that, coupled with light winds, will allow particles to accumulate. Starting at very clean levels in the morning, we will end the day still in the good range.

Extended Forecast:
Warmer, sunny and more humid on Sunday as high pressure stalls over the southeastern US. Light winds, limited mixing and higher PM concentrations moving in from the west-southwest will allow local PM to reach the moderate range for the first time in two weeks. More of the same on Monday with moderate air quality although increasing afternoon winds will limit the rise in PM concentrations. To follow air quality in PHL between forecasts: "aqforecast" at www.twitter.com -Ryan