The humidity and heat of the past three days is finally, slowly, pushing out of the region. Drier air is moving on in and we'll see a very comfortable weekend (and in fact, a several day stretch of comfortable weather) throughout the region.
Today is the transition as humidity will gradually lower. This sets the stage for a comfortably cool night throughout the Delaware Valley as low temperatures drop into the 40's (north and northwest of the city), with low and mid 50's common around Philadelphia and points south. We'll probably avoid record lows in the Delaware Valley and outside of it as the record in Allentown is 40 and in 44 in Philadelphia on Saturday morning. Those won't be touched but it will be a cool start to your Saturday.
Of the two weekend mornings, tomorrow morning (above) probably sets up as the slightly cooler of the two as Sunday morning should yield temperatures a degree or two warmer at the onset as some higher level clouds may move over during the course of Saturday night and early Sunday, providing just enough of a blanket to prevent temperatures from really tanking. Also, as high pressure builds overhead, we will lose the push of northerly winds that will draw cooler air in. This push of cool will help us get a notch or two cooler overnight tonight.
It won't be a dramatic difference -- we're talking lower and middle 50's both nights this weekend in the city itself, which is below average for mid September and definitely a more autumnal feel!
Regarding Saturday, our highs around or just above 70 would provide us the coolest day in the region since May 28th's 66 degree high, provided we don't hit 72 (that occurred on June 7th). Sign of the gradual, inevitable, transition of seasons at play!