California averages about six tornados a year, although most of those touch down near the Los Angeles Basin or in the Central Valley, according to a National Weather Service meteorologist.

Today marked a day of unusual weather, however, as the Bay Area had several brushes with cyclones of various forms, including a funnel cloud that flattened a shed in Santa Rosa and a waterspout the twisted over the waters near San Francisco’s Ocean Beach.

“It’s not completely rare,” meteorologist Chris Stumpf said. “They do occur occasionally.”

Stumpf said the last time a tornado touched down in the Bay Area was March 21, 2005, when it struck South San Francisco. That vortex had wind speeds between 73 and 112 mph, which were strong enough to lift roofs and awnings off houses and down power lines.

Although the weather service issued a tornado warning earlier today for San Mateo County, the warning expired at 11:30 a.m. with no report of damage.

“The Doppler radar indicated some rotation with the San Mateo-Half Moon Bay storm,” which prompted this morning’s pre-emptive measure, Stumpf said.

Farther north, a San Francisco resident in the Outer Sunset today had a digital camera handy when a funnel of water extended from the line of gray storm clouds hovering over Ocean Beach.

In the YouTube video, the waterspout can be seen edging closer to the shoreline before losing its momentum and dying out before posing a hazard to coastal homes.

A shed in Sonoma County was not as fortunate. At about 9:45 a.m., a funnel cloud descended on Santa Rosa near 1330 King St., where the fierce rotating winds tossed debris onto nearby power lines and properties three houses away.

Cold, unstable air mixed with strong winds helped to form those twisters, Stumpf said; “It really helped to spin those things up.”

Storms expected to pass through the Bay Area on Sunday will bring another chance for thunderstorms, Stumpf said, with “some really strong winds” and more cold, unstable air.

As for the likelihood of a twister touching down this weekend?

Don’t count on it, Stumpf said.

“There could be a brief spin-up. It won’t be quite as unstable as this one was.”

Small hail accompanying heavy rain could be likely and wind gusts near 50 mph are also expected to affect “pretty much the whole Bay Area,” he said.

Patricia Decker, Bay City News

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