The hottest day since summer arrived with threats of unhealthful ozone levels and severe thunderstorms tomorrow.

Just before 2 p.m., the temperature hit 90 at Philadelphia International Airport - the city's high mark for the year. The previous warmest days - May 1 and 2, and April 7 - all topped out at 89.

Atlantic City and Trenton, though, recorded 91s on May 1, while Reading reached 90 the same day.

Today's predicted high of 92 wouldn't set a record, though. The 96 set in 1880 looks safe.

The Shore will be much cooler than the city today with highs in the mid to upper 70s from Cape May to Long Beach Island.

High ozone levels were expected today for Southeastern Pennsylvania, prompting an orange alert and the declaration of an Air Quality Action Day by the state Department of Environmental Protection.

People with asthma or other breathing problems should avoid strenuous activity, and so should small children and the elderly, according the Air Quality Partnership hotline (1-800-872-7261).

People are asked to avoid idling cars, mowing lawns and topping off gas tanks.

Hot weather often brings thunderstorms, and tomorrow afternoon and evening they could be "numerous" across Eastern Pennsylvania, New Jersey and Delaware, according to the National Weather Service.

"Some thunderstorms could become severe with damaging winds and large hail the main threats," according to a statement called a hazardous weather outlook.

After scattered showers and a high near 70 on Friday, the Memorial Day Weekend looks clear, with a high in the 70s on Saturday, in the 80s Sunday and Monday.

The Shore also faces thunderstorms tomorrow and showers on Friday, then looks clear for the three-day weekend, with highs around 70.

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

Contact staff writer Peter Mucha at 215-854-4342 or pmucha@phillynews.com.