Thunderstorms form because the air is fairly warm and humid. Warm air and humidity are two important ingredients for thunderstorms. Aside from the thunderstorms, July is a mostly sunny month for the Northeast. The coldest recorded temperature is -52 degrees Fahrenheit in Old Forge, New York on February 18, 1979. For more climate data, look at the Weather Research Center site.
Climate data is listed for many cities, including temperature and precipitation information.