After our last heat wave ended on August 5th, Philadelphia has had just two 90 degree days over the past two plus weeks. It's been a nice break from a rather relentless stretch of heat that resulted in tying a record for most 90 degree days in a month in July (21). Simply because August's temperatures have been much more mundane does not mean the furnace of heat is shut off completely around here for the summer. 90 degree temperatures are theoretically possible into October, with the latest 90 on record occurring on October 10th.
In past years, the date of the season's last 90 had varied quite a bit. Last year featured the earliest "last 90" since 1984 (when we had our last 90 in July). You can thank nearly thirty inches of rain for shutting off the furnace last year...it came with a price of sorts. Other years, we've dealt with 90 degree temperatures into mid or even late September (2010). There isn't much bearing between the season's last 90 degree day and the following winter.
Could last Friday have been the last 90 degree day of the season? Odds lean against it. Long range computer modeling and some of the modeling MJO trends do suggest a potential run back towards 90 for Labor Day weekend or just after that...the timing waffles back and forth a bit. I don't think summer is completely over...yet...but time is ticking as daylight slowly begins to wane.