San Francisco residents were woken up by a tornado warning early Saturday morning during a powerful storm system that battered the region with heavy rain and wind.

It was the first-ever tornado warning for San Francisco, but not the first tornado, cited CBS News Bay Area meteorologist Paul Heggen.

The National Weather Service issued the warning for northern San Mateo County and parts of downtown San Francisco, just before 6 a.m. until 6:15 a.m.

According to the NWS, there was a cyclonic signature on the radar that had produced enough rotation signature to prompt the warning. 

“The radar scan at 5:52am clearly shows a “hook echo” offshore — that’s the signature we look for to identify a developing tornado. This feature was embedded within a broad area of very heavy rain, but it still stands out,” Heggen explained. 

The NWS Bay Area said the warning was over for San Francisco at 6:07 a.m. And let the tornado warning expire at 6:15 a.m.

The warning led to a flurry of social media posts from residents who were woken up by the alert on their cell phones. It was being reported as the first-ever tornado warning issued for San Francisco.

There was also a special alert issued for a possible water spout on San Francisco Bay in the area of the Bay Bridge that expired ten minutes after the tornado warning.

While the strongest part of the system has passed, isolated severe thunderstorms remain possible for parts of the Bay Area through the morning, the National Weather Service said. 

The risk of a severe storm is marginal, and the NWS said it will only last through Saturday morning.  According to PG&E, the storm conditions caused power outages for thousands of people in the Bay Area.