The National Weather Service issued a rare tornado warning in the South Bay. The NSW issued an emergency declaration just after 2 p.m. for Santa Cruz and Santa Clara Counties that expired by 2:30 p.m.
People in the Bay Area Weather Service office who issued the warning said it was the tornado warning they have ever sent out.
A dangerous band of weather was over Santa Cruz County at 2:15 p.m. headed NE at 35 mph.
The system was strong enough to produce a tornado. It could also produce moth ball size hail and wind gusts up to 50 mph.
The system quickly weakended. A "significant weather alert" remained in effect for Santa Clara, Santa Cruz and Alameda Counties until 3:15 p.m.
Wednesday's weather was the latest in a series of storm to hit the Bay Area. It brought thunderstorms, wind and the heaviest rain of the week.
The storm included "quite a bit of instability," National Weather Service meteorologist Brian Tentinger said. "That's what's firing off these thunderstorms that are embedded in the rainfall."
Thousands of PG&E customers were without power by early afternoon and most of them were in the South Bay, where 31,000 customers were affected, PG&E spokeswoman Katie Romans said
One more note for drivers: Highway 101 at Alum Rock flooded Wednesday and caused a real traffic mess. All southbound lanes were closed for a time. Even after two lanes reopened, traffic back up for miles.