© Art Frisch, San Francisco Chronicle
The storm began late Thursday and was forecast to continue into Saturday, dumping a total of 1 to 3 inches of rain in San Francisco and Oakland, according to the National Weather Service. In the North Bay, 3 to 6 inches were anticipated.
"This rainfall event itself could provide us with our normal rainfall for the entire month of April," said Roger Gass, a weather service meteorologist in Monterey.
Atmospheric rivers stretching across the Pacific Ocean inundate the West Coast with torrential rain and flooding, and buries the Sierra and Cascades and Pacific Northwest mountains with massive snowfall each year, so what causes them and how do they work?
While flooding concerns were modest in most areas, officials said urban areas with poor drainage could see deep pools form, and streams and creeks could overflow their banks.
Another concern was the scorched earth left behind by October's Wine Country wildfires. Officials were watching for mudslides in the affected areas.
"There will be the potential for locally heavy rainfall across the region and there will be your typical urban flooding concern in poorly drained areas or places where drains are clogged," Gass said. "But we are not anticipating a widespread flood event."
Still, the impending storm led the San Francisco Public Utilities Commission on Wednesday to deploy "temporary interlocking flood barriers" to protect properties in a flood-prone area around 17th and Folsom streets in the Mission District.
The commission was cleaning storm drains across the city, and the Department of Public Works was supplying sandbags at its operations yard, as it does every day of the week except Sunday.
"We're not doing anything beyond normal storm prep, but we are preparing and watching the storm," said Rachel Gordon, a spokeswoman for Public Works. "We have a crew out helping to clear debris out of storm drains, we have tree crews on call and we are continuing to give out sandbags."
She added, "We haven't had a rush on sandbags."
Marin County was providing sandbags to residents at fire stations in Point Reyes Station, Tomales and Woodacre.
Yosemite National Park was expecting "significant impacts" from the storm, park officials said.
Late Thursday, the National Park Service took additional precautions, preemptively announcing the closure of the entire Yosemite Valley beginning Friday at 5 p.m. All campgrounds and visitor lodging in the valley will close Friday afternoon, and reservations for camp spots and other accommodations Friday evening and Saturday evening have been canceled, the Park Service said.
At the Oroville Dam in the Sierra Nevada foothills, officials were monitoring the storm and waiting to see if they would need to release water through a recently rebuilt spillway.
The half-mile-long concrete chute partially collapsed in February 2017, and crews have been reconstructing it over the past year. The project isn't complete, but officials said the spillway should hold if it needs to be deployed.
"If you think about forecasts as kind of like a scale from driest to wettest, the wettest is still a possibility," said Erin Mellon, a spokeswoman for the California Department of Water Resources, which operates the reservoir. "There's a potential for use."
She added, "Forecasts can change pretty rapidly, so Sunday or Monday we should have a much clearer picture about what will need to happen."
The predicted deluge and potential for thunderstorms in San Francisco left the Giants-Dodgers game scheduled for 7:15 p.m. Friday hanging in the balance.
Since the opening of AT&T Park in 2000, just five Giants games have been rained out. The last such cancellation happened April 12, 2006, the team said, and the stadium has had the second-fewest rainouts among open-air baseball parks. The San Diego Padres are the dry-weather champions, with three home rainouts since Petco Park opened in 2004.
"We hope that by (Friday) around noon we'll be able to provide some type of update," said Shana Daum, a Giants spokeswoman. "We're definitely monitoring the weather."
Chronicle staff writer Dominic Fracassa contributed to this report.
Sophie Haigney is a San Francisco Chronicle staff writer. Email: sophie.haigney@sfchronicle.com Twitter: @SophieHaigney
Comment: Major flooding is on the rise and just one of many signs that there are great changes afoot on our planet: