A 2009 study analyzed several different sources of data, including government reports and medical records, and estimated annual lightning-related fatalities to be just over 100 a year in the United States. This includes an average about 3 to 4 in our area, although many more people are injured. In fact, the Philadelphia-Camden-Wilmington metropolitian region is the ninth most prevelant area in the nation for lightning fatalities. The reasons likely include the number of people we have living here, the number of outdoor activities enjoyed during warm weather, and the fact that our population is not as weary of severe weather, as compared to other regions like the Sun Belt, where thunderstorms more often produce tornadoes and greater weather awareness. An additional reason may be the so-called "urban heat island effect"; it's thought that rising heat from large urban centers like Philadelphia make the formation of thunderstorms in neighboring areas easier. In fact, over a period of about 50 years of record-keeping ending in 2006, a cluster of slightly higher lightning death occurences can be seen in parts of New Jersey to the northeast of the city, which may be an indication of this effect.
The most dangerous seasons for lightning fatalities are the spring and summer, with summer being number one. Oddly, most thunderstorms occur in the spring. So, why the summer surge in lightning deaths and injuries? It's almost certainly because of something I alluded to earlier. Most carnivals, county fairs and outdoor activities happen in the summer, after school's out, and when daylight lasts later into the evening. Basically, more people are killed and injured by lightning from mid June through August because these are the months when people are most likely to be outdoors.