A frontal system to our north is combining with this very warm and humid air mass to generate powerful thunderstorms across upstate Pennsylvania, northern New Jersey and southeastern New York. As a result, the Philadelphia and New York City metropolitan areas are under the threat for severe thunderstorms between about 4 and 9PM. The greatest threat for these strong storms in these two metro regions is across their northern suburbs. Any thunderstorm can contain very heavy rain, vivid lightning, strong winds of up to 40 mph, and possible small hail. "Blow-off" clouds well downstream of the storms will limit sunshine for the rest of the day. The storm threat diminishes late tonight, but could redevelop on Sunday.