Forecast: Hottest week of the year

The summer's first heat wave has started, if the forecast is correct, and with it comes an air quality alert for parts of the region.

Three straight days of 90 degrees or more qualifies as a heat wave, and the area is predicted to get at least five.

Yesterday, the high was 91. Today and tomorrow could hit the mid-90s, followed by highs in the low 90s Wednesday and Thursday.

Friday could extend the streak, if it nudges past the expected 88.

Today could even challenge the record of 96, set in 1995. Tomorrow's record high is 97, followed by 94 on Wednesday.

So far this year, the only heat wave was April 25 to 27. The only other period with back-to-back days in the 90s was July 28 and 29.

The air quality warning - that ozone and particulates will be high - applies to all of New Jersey and a band of Eastern Pennsylvania counties beyond the Philadelphia suburbs: Lancaster, Berks, Lehigh, York, Lebanon, Northampton, Cumberland and Dauphin.

That's another reason to avoid strenuous work outside in the heat, officials say.

Thunderstorms are in the forecast from tomorrow night through Saturday, although a 40 percent chance is the highest probability.

For more on the forecast, go to http://go.philly.com/weather.