Rain battered the Bay Area on Sunday, moving from the northern valleys toward the south, upending street events in San Francisco and triggering fears of blackouts or toppled trees.
For residents recovering from a record heat wave, in a season typically marked by grueling wildfires and protracted drought, the rain brought surprise and relief, along with a fair share of panic.
In downtown San Francisco, the showers came in bursts, picking up at about 9:45 a.m. and spraying down hard enough, within a few minutes, to rattle the tin chimney caps of apartment buildings. By 10 a.m. the dounpour had washed leaves down the drains and was threatening to clog them up and start building lakes in intersections.
A storm that generated weeks of tantalizing forecasts had finally arrived.
Then, by 10 :15 a.m., it was over, another illustration of California’s extreme vicissitudes in weather, where atmospheric rivers follow long parched periods. At 11 a.m. the wind began moaning and the rain started again. Organizers canceled the annual LeatherWalk in SoMa, the Stern Grove Festival and Sunday Streets in the Western Addition.
Inclement weather delayed Flower Piano, a popular performance festival in Golden Gate Park, by one hour, though the even proceeded at 11 a.m. with tents covering all twelve pianos.
“The crowd is less than if it were a sunny day but there is a phenomenal dedicated hearty group of pianists and listeners who are here and outfitted in their finest rain jackets, rain boots and umbrellas,” said Brendan Lange, spokesperson for Flower Piano.
Yasmin Bahl started her regular run up the Lyon Street steps in a pouring rain at Green Street. By the time she reached the top at Broadway, the rain had cleared enough to see a lone sailboat on the slate gray bay.
“We blinked. It’s over,” she said of the storm, before she started on her second of 10 sets on the steps.
Gentle rains that moistened pavement at about 8 a.m. in the East Bay turned fierce in the afternoon, when the rain came pouring down in sheets. By then the storm had doused much of the North Bay, where forecasters predict the mountains may see up to 3.5 inches of rainfall.
Powerful winds blasting southward along the coast prompted the National Weather Service to issue an advisory for Sunday and warn of likely outages.
“We’re definitely considering this a welcomed rain event that will bring moisture to some of our dry fuels,” said Brooke Bingaman, a National Weather Service meteorologist. Thouth the storm’s behavior may seem erratic, Bingaman and other weather scientists expect it will produce unseasonably high levels of precipitation.
Southerly winds gather over the Bay Area on Saturday night, with rain forecast for all area by Sunday night into early Monday.
National Weather Service Bay AreaIn downtown San Francisco, residents should anticipate about an inch, roughly four times what the city collects in an average September, Bingaman said.
Oakland should expect a half-inch to .75-inch of rain, possibly doubling its September rain average, she added. Forecasts expect similar totals in the San Jose area, which may also double its September average.
“But it’s another case where the North Bay is the winner for rain,” Bingaman said.
Sonoma and Napa counties should get 1-2 inches of widespread rain that began lashing the North Bay valleys Saturday night, pelting Mount Tamalpais and Mill Valley, the National Weather Service reported, as winds picked up in the East Bay hills and Santa Cruz mountains.
Commuters should expect a messy Monday morning drive, Bingaman said, as the first rain of the year mixes with oil residue on Bay Area roads, creating dangerous conditions, along with high winds blowing debris onto roadways.
The winds could also blow tree limbs onto power lines, causing outages. In addition, the first rains can turn accumulated dirt on power lines into mud, which conducts electricity and can damage the system and cause outages.
Fortunately, Bingaman said, the Bay Area’s wildfire burn scars are older, so she is not expecting mudslides in the area.
In the Lake Tahoe area, there’s the possibility of light snow in higher elevations, said National Weather Service meteorologist Zach Tolby. But the bigger impact should be cooler temperatures, breezier conditions and about an inch of rain in the area.
Tolby said residents are finally getting a reprieve from smoky air.
“The Mosquito Fire has been pouring in smoke,” he said. “So, we’re all quite excited to see some wind, rain and clouds.”
Matthias Gafni and Rachel Swan are San Francisco Chronicle staff writers. Email: matthias.gafni@sfchronicle.com, rswan@sfchronicle.com Twitter: @mgafni @rachelswan