Heat advisories are in effect for both days, as temperatures are expected to climb as high as 96 degrees Thursday and 99 degrees on Friday, meteorologists said.
Residents are advised to stay hydrated with plenty of water, bring strenuous activities indoors or avoid them altogether, and wear lightweight, light-colored, loose-fitting clothes to beat the heat.
An NWS map shows excessive heat advisories in effect for all of Philadelphia and Delaware counties and parts of Bucks, Montgomery, and Chester counties.
A general heat advisory is in effect for upper Bucks, upper Montco, northwestern Chesco, as well as Lehigh, Northampton, Berks, Lebanon, Lancaster, Dauphin, York, Perry, Cumberland, Adams, and Franklin counties.
"Hot and humid conditions look to last through Saturday, but by Sunday, it appears we will see some relief," wrote NWS Mount Holly on Twitter.
Scattered rain showers and thunderstorms are expected to develop Thursday afternoon and into the evening with "damaging winds and localized flash flooding" possible, NWS warns.
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