Extended forecasts, moving targets

The blazing sun of a heat wave keeps many people indoors - and their fans or air-conditioners running. (Bill Uhrich / Reading Eagle)

For the first time in seven years, June will have run its course before Philadelphia experiences its first official heat wave – as in three consecutive days of 90-plus temperatures.

Based on the extended forecasts posted Tuesday on the website of  AccuWeather -- which supplies the daily weather package for The Inquirer  -- summer was going to turn serious in July.

Blistering heat was due to bake the region for nine days starting on Tuesday, with a forecast high of 95, followed by 90-pluses for seven of the next eight days. This got our attention.

We considered posting on the coming heat wave, but on Wednesday we were glad we waited. The outlook for the first 10 days posted on that day had quite a different look.

The onset of the serious heat was pushed back to Sunday, July 6, and the 90-pluses were reduced to five days.

The Thursday and Friday outlooks wrought yet more changes, and taken literally, erased any heat wave.

The day-to-day volatility in the daily forecast highs was quite notable. On Tuesday, the forecast high for Wednesday, July 9, was 94. On subsequent days it rose to 97, dipped to 89, and then went back up to 98.

This is not to pick on AccuWeather – try the same exercise with any weather service – but just to point out that the art of the extended forecast still is countless brush strokes away from perfection.

As we’ve noted, wonderful as they are, those computer models that generate long-term forecasts have something in common with humans in that they are fallible, and forecast accuracy decreases with time.

For now we should savor our generally good fortune so far this summer, and regardless of the extended forecasts -- no heat wave before its time.