A tornado warning was first issued for San Francisco County early Saturday morning as thunderstorms and wind gusts of up to 60 miles per hour swept across the region, officials said.

“Now seek shelter in a basement or an interior room on the lowest floor of a sturdy building,” warned a National Weather Service advisory on social media. Parts of San Mateo County were included in the brief warning.

More than a million people were affected by the brief warning, which sounded at 5:51 a.m. and was lifted at 6:15 a.m., said Lamont Bain, a meteorologist with the National Weather Service Bay Area.

The notice marked the first time the agency had ever issued a tornado warning for San Francisco County, he said. A tornado warning was last issued for San Mateo County on March 18, 2011.

“It’s certainly unusual. “We don’t see that the ingredients for tornadoes always come together,” Bain said.

In addition to moisture, “you need a combination of lift, some instability and some wind shear. We saw all of those things come together for a short time this morning.”

He said the tornado alert was “a good reminder for people to always have multiple options” for weather and warning information. This includes signing up for text message alerts, reading news reports and listening to weather radio.

Rain showers with wind gusts of 30 to 35 mph are forecast in San Francisco for the rest of the day until at least midday. The rain is expected to end this evening and dry conditions will return on Sunday before further showers on Monday.