Northern California rains raise flood concerns; warnings and watches in effect

SAN FRANCISCO -- Tourists streamed out of Yosemite National Park, San Francisco baseball fans had a game cancelled by rain for the first time in a dozen years, and authorities kept a close eye on swelling rivers and rising water at a damaged dam as a "Pineapple Express" storm drenched Northern California.

San Francisco had record rainfall on Friday as an "atmospheric river" of subtropical moisture streaming from Hawaii pounded the north while leaving Southern California high and dry. 

No major problems were reported, but flood warnings and watches remained in effect Saturday for the Sierra Nevada, the Santa Cruz Mountains south of San Francisco and other areas while authorities warned that flash floods, mudflows and rockslides were possible in heavy rain, especially in the wine country north of San Francisco where wildfires last October stripped the ground bare of soil-gripping plant life.

Runoff from melting snow could add to the chance of rapidly rising mountain streams and rivers in the Sierra, the National Weather Service warned. 

Yosemite National Park closed campgrounds and lodging in its busy Yosemite Valley because of flooding concerns, with the Merced River there expected to peak 5 feet above flood stage on Saturday. 

"Due to anticipated flooding, Yosemite Valley is closed," the National Park Service said on its website. "We will evaluate conditions on Sunday morning and open as soon as it is safe. All other areas of the park are open, conditions permitting." 

Downtown San Francisco saw nearly 2 inches of rain Friday, making it the fourth-wettest April day since records began.

San Francisco International Airport reported about 150 cancelled flights because of the weather and others were delayed an hour or more.

A cancelled flight stranded Santa Rosa native Lydia Smith who was trying to reach Oregon for a baby shower.

"I'm like on the verge of tears," she told KGO-TV.

The opener of the San Francisco Giants-Los Angeles Dodgers weekend series was rained out, the first at the Giants ballpark in 12 years. Saturday's game was also pushed back by two hours to 3:05 PDT. 

Sacramento broke its record for the day with well over an inch. Some areas got much more rain, however.

In Sonoma County on Friday, rescuers pulled two people and two dogs from a car that became swamped to the door handles on a flooded road. Bodega Bay in the county received nearly 6 inches of rain for the day -- more than the entire rainfall total for March, according to the weather service.

The big concern wasn't the amount of rain but how fast it might fall.

"When we start talking about half an inch of rain or more an hour, that's where we're more susceptible to mudslides and debris flow in and around our burn zones," said Paul Lowenthal of the Santa Rosa Fire Department.

The wine country city, which was one of the hardest-hit burn areas last fall, brought in extra firefighters and emergency personnel.

To the north, state officials warned this week that they may have to use the partially rebuilt spillway at Oroville Dam for the first time since repairs began on the badly damaged structure last summer.

Behind the dam, Lake Oroville has been filling up all winter, and more water was coming in than flowing out Friday. The water level Friday night had topped 793 feet. If it reaches about 830 feet, water managers said they may open the gates to the spillway.

In February 2017, a massive crater opened up in the 3,000-foot concrete chute that releases water from Lake Oroville, California's second-largest reservoir.

Crews shut down the spillway for inspections just as a major storm dumped a torrent of rain. The lake quickly filled, and water began flowing over an emergency spillway that had never been used.

The water eroded the barren hillside beneath the spillway, leading to fears it would collapse and release a wall of water that could swamp communities downstream. Authorities ordered nearly 200,000 people to flee, but the crisis was averted.

California officials say they hope to avoid using the main spillway but are confident it can safely function.

Researchers are eager to better understand Northern California's atmospheric river storm. In Bodega Bay, Scripps researchers are launching weather balloons to study this Atmospheric River storm. The data goes real-time to NOAA, CBS SF Bay Area reports.

Every three hours, researchers blow up weather balloons, take them to the launch site at the UC Davis Bodega Marine Lab and send them straight into the storm.

"We're looking at atmospheric rivers which are the source of half of California's annual precipitation," said Anna Wilson, field research manager at the Center for Western Weather and Water Extremes. "It's really important for the state. We're sampling this storm which is the most extreme storm we've seen this season."

The weather balloon and the attached GPS radiosonde, which is a battery-powered telemetry instrument, can measure all levels of the atmosphere up to 82,000 feet.

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