published on April 6, 2018 - 1:28 PM
Written by Edward Smith

(AP) — A fierce Northern California storm is shutting down Yosemite National Park, threatening mudslides in wildfire-ravaged wine country and possibly presenting the first test of a partially repaired offshoot of the nation’s tallest dam that nearly collapsed last year.

Heavy rainfall in the past few months has led to problems for a state recovering from devastating wildfires, forcing people to flee their homes repeatedly for fear of debris flows tearing down hillsides stripped bare by flames. But recent downpours also have provided relief as parts of California plunged back into drought less than a year after a historic dry stretch.

The spring storm is closing popular tourist sites this weekend. Yosemite National Park has banned all visitors as it expects flooding in the heavily visited valley. Visitors will no longer be able to enter Yosemite Valley starting at noon Friday and visitors who were already there will be asked to leave at 5 p.m.

A couple hundred miles northwest in wine country scorched during October wildfires, the National Weather Service predicted 4 to 6 inches (10 to 15 centimeters) of rainfall through Sunday.

The city of Santa Rosa, one of the hardest-hit burn areas, brought in extra firefighters and emergency personnel, fire department spokesman Paul Lowenthal said.

“We’re preparing not only for issues within burn areas but also for issues around the entire city,” he said.

It’s not the amount of rainfall worrying city officials but the rate at which it falls, he said. Workers have been monitoring hundreds of storm drains, especially those protecting the neighborhoods destroyed by the fires.

“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,” Lowenthal said.

The weather service issued several flood warnings throughout Northern California ahead of the expected “atmospheric river,” a long plume of subtropical moisture stretching to areas near Lake Tahoe.

Some places in the Santa Cruz Mountains south of San Francisco could see up to 8 inches (20 centimeters) of rain over a three-day period, leading forecasters to warn of possible flooding, mudslides and rockslides.

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, but more water was being released through smaller outlets than was flowing in as of Thursday. The water level was last at 793 feet (242 meters) and dropping. If it reaches about 830 feet (253 meters), water managers say they will open the gates to the spillway.

In February 2017, a massive crater opened up in the 3,000-foot (914-meter) 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 to capacity, 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 if needed.

“It’s a big storm, it’s a warm storm,” said Erin Mellon, a spokeswoman for the state Department of Water Resources. “Obviously, we have plenty of flood capacity within the lake, considering it’s still 110 feet (34 meters) below the top of the reservoir.”

Construction crews last year fully rebuilt about a third of the spillway with reinforced concrete that’s anchored into bedrock. The other two-thirds were left with temporary concrete until construction resumes.

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