Drenching storm ends, but more rain on way

A powerful storm driven by an atmospheric river originating deep in the Pacific has dumped more than double the average rainfall for the entire month of April in San Francisco, forecasters said Saturday.

The storm began Thursday night and continued steadily through early Saturday morning, before tapering off by Saturday afternoon. The storm broke rainfall records Friday in San Francisco and several other surrounding cities.

As of early Saturday, San Francisco recorded 3.43 inches of rainfall from the storm, while Oakland received 3.00 inches. Areas in the North Bay got the most, with parts of Sonoma County drenched by more than 6 inches. The average rainfall for all of April in San Francisco is 1.46 inches.

Rainfall totals in both San Francisco and Oakland broke records for the calendar date on Friday.

Friday “was the second wettest day on record in April in San Francisco,” Roger Gass, a meteorologist with the National Weather Service in Monterey, said Saturday. The wettest — 3.45 inches — was recorded April 16, 1853.

Minor flooding was reported in Marin and elsewhere, causing traffic jams and some road closures throughout the day Friday. So far, no flooding has been reported in the burn scars left behind by the North Bay firestorm, Gass said.

Damage from the storm shut Muir Woods to visitors and vehicles Saturday, and Stinson Beach to vehicles Saturday and Sunday. It is unclear when both will reopen fully. “Stay tuned for reopening dates,” the Golden Gate National Park Service account tweeted.

Yosemite Valley lodging, campgrounds and roads are also closed because of potential flooding. “We’ll assess conditions on Sunday,” the Yosemite National Park account tweeted.

At the Oroville Dam, officials have been monitoring the storm closely, as it’s possible they may need to deploy their newly reconstructed flood control outlet spillway next week. Officials said they will need to use it if the reservoir’s elevation reaches 830 feet. On Saturday, the elevation reached 795 feet.

At San Francisco International Airport, 164 flights were canceled and 671 were delayed Friday. “That happens when we have weather,” said Dennis Zamaria, a spokesman for the airport. Weather delays at Oakland International Airport averaged about 20 minutes, a spokeswoman said, with 51 delays and three cancellations. No weather delays were reported out of San Jose International Airport.

Winter has been dry, causing concerns about the possibility of a return to drought conditions. But this storm delivered a significant boost to water totals.

On Saturday, San Francisco’s seasonal rainfall was at 78 percent of normal for this time of year — up from 63 percent Thursday. Santa Rosa’s numbers jumped 10 points to 73 percent of normal, Oakland leapt from 63 percent to 79 percent and San Jose’s tally rose from 56 percent to 62 percent, according to Steve Anderson, a weather service meteorologist.

“These rains we’ve gotten in late March and this system have helped in our overall deficit,” Gass said.

The storm did little to boost Sierra snowpack, because the precipitation turned to rain even at high elevations. There was less flooding than anticipated, however, so not much snow was washed away.

“If it were a more prolonged rain event, that would have been a big issue,” said Evan La-Guardia of the National Weather Service in Reno. “Then a cold front came through, and the storm moved through really fast.”

Both in the mountains and the Bay Area, another storm is on the way. Sunday and Monday are expected to be dry, but a new system will probably roll in Tuesday. “It’s anticipated to be rather weak and not anything close to this type of rainfall,” Gass said.

Sophie Haigney is a San Francisco Chronicle staff writer. Email: sophie.haigney@sfchronicle.com Twitter: @SophieHaigney