Over the last several days computer models have ever-so-slowly turned up the heat in this week's forecast.
What looked last week to be a modest re-surgence is turning into an all-out bake-a-thon, with temperatures on Saturday now projected to make a run at 100.
The National Weather Service has posted an "excessive heat watch" in effect for tomorrow afternoon on into the weekend.
The intensity of the heat is ominous in and of itself, but perhaps a bigger concern is duration.
This will be day 5 of the current heat wave, and day 5 with official temperatures failing to get below 70 in PHiladelphia. Expect at least 5 more hot days and warm nights.
Heat has a cumulative effect, particularly in the black-roof, brick rowhouse neighborhoods where the elderly live-alone populations tend to be concentrated.
With minimal overnight cooling, the houses heat up in a hurry after the sun comes up. Duration of heat is the critical factor in heat-related mortality.
This afternoon won't be that bad; not quite as comfortable as yesterday, but the humidity will remain manageable.
Tomorrow, the air will be swollen with water vapor, and that's going to erode the comfort. That's also going to enhance the chances of picnic-spoiling thunderstorms during the day.
The rest of the week will remain steamy, witht heat peaking on Saturday, when heat indexes could approach 105.
On the plus side for those becoming anxious over the looming air-conditioning bills, all indications are that the heat will break after Sunday.
Next week could turn out to be quite pleasant, with perhaps as much as a 10-day break in the heat.
Of course, we are learning anew this week that in summer, the atmosphere doesn't take extended outlooks too seriously; nor in winter, for that matter.