As noted in Peter Mucha's online story, Philadelphia has a shot at hitting 90 officially today, meeting the technical requirements of a heat wave -- three days of 90 or better at the airport measuring station.
In the court of common sense, where the fans don't always work very well, any reasonable person who has spent a reasonable amount of time outside would agree that the last three days have been hot.
But that 90 threshold may be of some significance. The National Weather Service has put together a statistical analysis showing that if the first technical heat wave arrives before June 15, odds favor a warmer than normal summer.
The weather service analysis also takes into account April temperatures (above normal this year) and the date of the first 90-plus day (May 26).
Based on the criteria, if the temperature did reach 90 today, the weather service forecast would call for temperatures to average 2 degrees above normal for the June 1-Aug. 31 period.
A "heat wave" this early wouldn't be all that unusual, but it would be significantly earlier than the average. Our own analysis shows that the average starting date of the first heat wave is approximately June 27.
That's just an average. In the 137-year period of record, the dates are wildly variable -- from April 16, in 2002, to Aug. 30, in 1924.
For you polar bears out there who can't stand the heat, here's some comforting news: Philadelphia got through 16 summers without a single heat wave. (Those years weren't included in the starting-date calculation.)