Despite a boring weather pattern this week, high pressure building in overhead and clearing skies tonight will result in temperatures taking a bit of a dive overnight. It's possible that some locations drop again into the 30's in parts of Montgomery, Chester, and Bucks County overnight, along with points north of there.
Frost is also not out of the realm of possibility in the Poconos and in isolated parts of the Lehigh Valley.
In the city proper, it looks like 40's are likely again for a low. In fact, it could be the coolest night of the month so far if we get down to 46 or 47, which isn't out of the realm of possibility at the Airport. After not getting any nights in the 40's in September since 2006 in September, multiple lows in the 40's in September is a bit out of the recent norm. Not that anyone's complaining, of course!
The record low for Tuesday morning is 40 -- that isn't getting touched.
The last time we've had two September lows in the 40's? 2001. The following winter was among the warmest on record. We also had multiple nights in the 40's in 1997, 1999, and 2000 -- with '97 and '99 leading into warm winters, with 2000 leading into a cooler-than-average winter. September chill doesn't necessarily yield a winter result that goes one way or the other.