Although Pennsylvania's months don't differ much on average precipitation, the autumn months of September, October and November usually are slightly drier than others.
But last year, September was incredibly wet in much of the state, with some counties recording rainfall amounts as much as 8 inches above normal for that month alone - approaching five times normal. The element of water, or lack of it, dictates our garden activities at any time of year, but in no other season is there such a marked transition from beginning to end.
The key to success is to adjust your watering habits, whether manual or automatic. Planting almost anything during the early fall is possible, but requires extra vigilance and an attentive eye toward the sky.
Reference: Accuweather; Berkeley Horticultural Nursery; Farm and Dairy