Tomorrow's frontal boundary will mark the beginnings of a drop off of the temperature cliff as we switch from a relatively nice regime in temperatures to one that includes the prospects for chilled rain within an eighteen hour window.
Saturday's not going to be a bad day overall and, in fact, model guidance is nudging the possibility of another warm day around most of the region as the front approaches from the west through the day. Clouds will increase during the day but with southwest breezes the prospect of another 80 degree high is pretty reasonable for Philly and points south and east. In fact, it wouldn't surprise me if mid 80's popped up in Central and Southern Delaware tomorrow afternoon. If you're heading to the Shore, it's going to be nice but breezy as winds could nudge towards 20 mph ahead of the front.
That front will cross through Philadelphia during the middle to latter afternoon hours -- the NAM from earlier this morning suggests a 3-5 PM frontal passage. It may be marked with a line of showers or sprinkles but it should not be a robust round of precipitation. It will be a marked wind shift from southwest to northwest, however, and that will mark the end of the warmth around here and the beginning of a temperature free fall.
The combination of northwest breezes and cooler air seeping in from the north will send temperatures down into the 50's quickly after sunset to the northwest of the city, with 40's lurking not long after Midnight. The Sunday system will approach towards daybreak, with rounds of rain moving in on what will shape up as a raw and chilled Sunday.
In fact, Sunday's high might be reached in the pre-dawn hours ahead of the rain...with the daytime temperature much closer to 50 degrees while rain actually falls.
If you want to enjoy any portion of this weekend tomorrow is the day to do so. For most of the region, it should be a rather nice day...and at the Shore, outside of the wind, a very nice Fall Saturday.