The drastic change in the character of June is already under way, even though the temperature hasn't made it past the low 80s this afternoon.
At 5, the humidity was 62 percent at Philadelphia International Airport, and the all-important dewpoint was up to 67, heading toward the discomfort level.
The National Weather Service has hoisted an "excessive heat warning" for tomorrow afternoon until daybreak Friday, and chances are it will be extended through Friday afternoon.
On Friday, the weather service says it likely will hit 90 before lunch, and top out at 95. Forecasts highs for tomorrow and Thursday are 95 and 99, respectively.
Fortunately, the first heat wave of the year is unlikely to rank among the deadlier ones in the period of record.
Typically, a heat wave has to last four or more days before heat-wave mortality spikes up significantly.
This one should fizz out with a round of shower late Friday, and the the weekend again is looking comfortable, if not splendid.