An air-quality alert has been issued for the Philadelphia region as temperatures are expected to soar into the 90s for three more days.

The Pennsylvania Department of Environmental Protection says the "code orange" air-quality alert means pollution concentrations are unhealthy for sensitive groups, such as the elderly, young children and people with respiratory problems like asthma, emphysema and bronchitis.

Those people should limit outdoor activities today, the department says. Forecasters say the air quality should be similar Friday and another alert is expected to be in effect.

The department's alert covers much of Pennsylvania, including Philadelphia, the Lehigh Valley, the Susquehanna Valley, Pittsburgh and Liberty-Clarion.

"A variety of conditions on Thursday will be conducive for high ozone levels" in the Philadelphia area, says Air Now, the Environmental Protection Agency's air-quality website.

An Appalachian lee trough -- a low-pressure region that forms east of the Appalachian Mountains -- is expected to form along the Interstate 95 corridor, "which will act as a focus for convergence of pollutants and ozone precursors," according to the EPA. Light winds will also allow pollutants to accumulate, and humid conditions will aid particle formation, the agency says.

The air-quality alert comes as forecasters say temperatures should reach the 90s for three more days.

The high temperature in Philadelphia reached 90 on Wednesday -- 12 degrees above average, and the hottest day of the year thus far.

The National Weather Service says a high of 92 degrees is expected today, with highs of 93 and 91 predicted for Friday and Saturday. A slightly lower high of 88 degrees is expected Sunday.

The region should brace for a "multi-day hot and somewhat muggy airmass," a weather service forecast discussion says.

Today, Friday and Saturday should be mostly sunny, forecasters say. There's a chance of thunderstorms on Sunday.

Some relief from the blistering heat will come Monday, when the weather service says a cold front will pass through the area. Thunderstorms are likely Monday, and forecasters are calling for a high temperature of just 79 degrees.

Until then, the entire East Coast will be sweating.

According to AccuWeather, "even areas that received accumulating snow in upstate New York and New England this past weekend will feel the heat." Across New England, high temperatures are slated to reach 92 degrees in Bangor, Maine, 88 in Burlington, Vt., 94 in Hartford, Conn., and 92 in Boston.

Contact Emily Babay at 215-854-2153 or Follow @emilybabay on Twitter.
Contact the Breaking News Desk at 215-854-2443; Follow @phillynews on Twitter.