The coolest air of Fall so far is moving in today -- you can already feel it with the chill in the air compared to yesterday. This crisp, cool feeling holds in place through the balance of the week before moderating temperatures take over early next week. This airmass will provide the first legitimate shot at frost for most everyone outside of Philadelphia, with Thursday night probably setting up as the coldest night of the upcoming airmass.
Modeling suggests may locations get into the 30's -- Philadelphia included. There's some uncertainty about whether the city pulls it off tonight but I think Thursday night affords a much better shot. While the city probably avoids getting down to a frosty level, I do think many suburbs will see their first frost of the season no later than tomorrow night as lows get into the lower and middle 30's as close as the Blue Route, Pennsylvania Turnpike boundary (west of 476, north of 276) and east of 295 in South Jersey. Some frost is also possible tonight in the outlying areas north and west -- frost advisories are up in Upper Bucks, Upper Montgomery, and Upper Chester Counties, but tomorrow night probably brings the coldest air of the week to everyone and if frost doesn't happen tonight north and west of the city, it is more likely tomorrow.
While Friday night will also be chilled, it probably does not approach the levels Thursday night does in many locations...we probably will be a degree or two "warmer" for lows. Still chilled, still a good bit below average, but nothing entirely unusual for late October.
Record lows in Philadelphia on Thursday, Friday, and Saturday are 25, 25, and 29. Those will not fall...nor come close to falling. In Allentown, those records are 22, 26, and 24. Those are probably safe as well.
That said, welcome to the coolest air of Fall so far!