New storm could bring more peril to state’s rivers

LOS ANGELES — A powerful storm barreling toward California from the tropical Pacific threatens to trigger widespread river flooding throughout the state as warm rain melts a record accumulation of snowpack and sends runoff surging down mountains and into streams and reservoirs.

Although state officials insist they are prepared to manage runoff from what is now the 10th atmospheric river of a deadly rainy season, at least one expert described the combination of warm rain, epic snowpack and moist soils as “bad news.”

“We’re expecting rain in the areas where there was snow, and the rain is warm, and it will melt the snow that is already there,” said Alistair Hayden, a former division chief at the California Governor’s Office of Emergency Services. “So what’s going to run into the rivers is not just the rain that’s falling from the sky — it’s going to unlock some of the precipitation that fell as snow, so it could be big.”

Already, the National Weather Service is warning residents that a number of rivers could surge beyond their flood stage, inundating nearby roads and properties. Likewise, some reservoir managers have already begun releasing water in anticipation of heavy inflows through the weekend.

At Oroville Dam — which suffered a near catastrophic failure amid a series of atmospheric rivers in 2017 — state operators said they could begin releasing water down the dam’s re-built main spillway as early as Friday.

Although Southern California can expect to see some fallout from the warm “Pineapple Express” the brunt of the storm system is expected to strike Northern and Central California the hardest. Areas below 4,000 feet elevation, where snowpack has been unusually deep this year, are anticipated to see the heaviest runoff.

According to the weather service, rivers that could see flooding include the Russian River at Hopland; the Salinas River at Bradley and Spreckels; the Merced River at Stevinson; the Tuolumne River at Modesto; the Cosumnes River at Michigan Bar; the Mokelumne River at Benson’s Ferry; and Bear Creek at Mckee Road.

Many of those rivers flooded in January, when nine back-toback atmospheric rivers hammered the state, causing nearly a dozen levee breaches along the Cosumnes River near Sacramento and contributing to at least 22 deaths, including people killed by falling trees and surging waters.

Dan Harty, a meteorologist with the National Weather Service in Hanford, said it’s possible that some places, including locations along the Merced River, could see flooding of similar to the flooding of 2006 during the incoming storm.

“It’s kind of a low-lying area that is prone to flooding with high rain rates, so that’s definitely a concern again,” Harty said. The Hanford office has issued a flood watch for almost its entire coverage area, which runs from the Grapevine, in Kern County, north to Yosemite Valley.

Reservoir operators are eyeing the storm closely.

“The good thing is it looks like a lot of our reservoirs still have adequate space to take on a lot of water in the near term,” said Tyler Stalker, a spokesman for the U.S. Army Corps of Engineers in Sacramento. “Obviously with the big snowpack, though, there’s a lot of potential water to still flow in there.”

Stalker said the Army Corps is already making some releases from its reservoirs to make room for incoming flows, including the Schafer Dam on the Kern River, the Pine Flat Dam on the Kings River and the New Hogan Dam on the Calaveras River. The agency has also made releases from Lake Mendocino, according to Nick Malasavage, chief of operations and readiness with the San Francisco division.

“Our reservoirs are going to do what they’re designed to do — which is to capture the rain that falls from the sky, and the snow that melts because of that rain that’s falling from the sky,” Malasavage said. But while January’s storms helped fill reservoirs severely depleted by the state’s driest three years on record, circumstances are now a bit more precarious, he said.

“Where we had not a whole lot of concern in January, there’s a little bit more concern now because we have to be more precise about our decision-making,” he said. Along the Russian River, for example, only about 20% of the watershed is behind Army Corps dams and the rest is unregulated, he said.

The atmospheric river, which is gathering warm subtropical moisture coming up from Hawaii, will fall atop some of the deepest snowpack California has ever recorded. The Department of Water Resource’s third snow survey of the season, conducted Friday, found that statewide snowpack was 190% of normal — hovering just below the record set in the winter of 1982-83.

But while recent storms have been colder, the warmth of the incoming system means there will be “more precipitation as rainfall instead of snowfall,” said Jeremy Hill, manager of the Hydrology and Flood Operations Branch at DWR.

“We are expecting more runoff in the rivers and streams, and several locations to reach levels that require some monitoring of the conditions, and potentially some localized impacts as well,” he said

Officials are also concerned that additional storms could create more problems in the wake of this one. Though clouds are expected to clear by the weekend, another atmospheric river system is forecast to arrive as early as Monday night.