It's going to get hot, hot, hot, beginning this afternoon, and it will be that way through the weekend and beyond.

How hot? Hot enough to crank up the air conditioner. Hot enough that the city's Health Department has issued an excessive-heat watch. Hot enough to become the season's first heat wave.

Today's high is expected to be about 84, and then the heat machine goes on overdrive. Tomorrow's high is expected to reach 97, just a degree shy of the historic heat wave of 1925. Sunday's high could challenge the record 95 set in 1999. Monday's could reach the mid-90s. And highs Tuesday and Wednesday are expected to be in the low 90s.

Humidity will be a problem, too, with dew points expected to be at or above 65 - the number to remember for major mugginess - during that time.

The Health Department warned yesterday that older people, those with chronic medical conditions such as heart disease, pregnant women, small children, and others are particularly vulnerable during a heat wave, especially the first one of the season. For more forecast information, go to http://go.philly.com/weather.

- Peter Mucha

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