An incoming low-pressure system is expected to bring the Bay Area its most significant rain since February, with thunder and lightning expected to occur early Friday morning. The National Weather Service has stated that a second "reinforcing shot" is now appearing more forceful than it was originally. This could result in at least three-quarters of an inch of rain in the low-elevation interiors of the region, and possibly 1½-to-2 inches in the coastal mountains. The weather service also noted that temperatures in the upper level of the atmosphere are expected to mix with temperatures closer to Earth's surface that remain relatively warm. The best chance for thunder and thunderstorms is Friday afternoon.