A powerful storm barreling toward California from the tropical Pacific threatens to cause widespread river flooding across the state as warm rains melt a record-breaking accumulation of snow and send runoff downhill 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 has described the combination of warm rain, epic snow cover and wet soils as “ bad news”.
“We expect rain in 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 for the California Governor’s Office of Emergency Services. “So what’s going to fall into the rivers is not just rain falling from the sky, but it’s going to unlock some of the precipitation that fell as snow, so it could be big.”
The National Weather Service is already warning residents that several rivers could rise beyond their flood stage, inundating nearby roads and properties. Also, some reservoir managers have already begun to release water in anticipation of heavy inflows over the weekend.
At the Oroville Dam, which suffered a near-catastrophic failure amid a series of atmospheric rivers in 2017, state operators said they could start releasing water from the dam’s rebuilt main spillway on Friday.
Although Southern California can expect to see some fallout from the warm “Pineapple Express,” the brunt of the storm system is expected to hit northern and central California the hardest. Areas below 4,000 feet, where snowpack has been unusually deep this year, are anticipated to see the most runoff.
According to the weather service, rivers that could flood include the Russian River in Hopland; the Salinas River at Bradley and Spreckels; the Merced River in Stevinson; the Tuolumne River in Modesto; the Cosumnes River in Michigan Bar; the Mokelumne River at Benson’s Ferry; and Bear Creek at McKee Road.
Many of those rivers flooded in January, when nine consecutive atmospheric rivers battered 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 rising waters. .
During those storms, the Salinas River at Spreckels reached one of its highest flood levels on record, while the Cosumnes River recorded its second-highest river flow. Bear Creek rose to 26.18 feet, surpassing a previous record set in 2006.
Dan Harty, a meteorologist with the National Weather Service in Hanford, said it’s possible that some locations, including those along the Merced River, could experience flooding of a similar magnitude during the coming storm.
“It’s kind of a low-lying area that’s prone to flooding with high rainfall rates, so that’s definitely a concern again,” Harty said. The Hanford office has issued a flood watch for almost its entire coverage areastretching from Grapevine, in Kern County, north to Yosemite Valley.
Reservoir operators are watching the storm closely.
“The good news is that it appears that many of our reservoirs still have enough space to absorb a lot of water in the short term,” said Tyler Stalker, a spokesman for the US Army Corps of Engineers in Sacramento. “Obviously with the heavy snowpack though, there’s still a lot of potential water to flow down there.”
Stalker said the Army Corps is already doing some discharges from its reservoirs to accommodate incoming flows, including Schafer Dam on the Kern River, Pine Flat Dam on the Kings River and New Hogan Dam on the Calaveras River. The agency has also made communications from Lake Mendocino, according to Nick Malasavage, the San Francisco division’s chief of operations and readiness.
“Our reservoirs are going to do what they are designed to do, which is capture the rain that falls from the sky and the snow that melts because of that rain that falls from the sky,” Malasavage said. But while January storms helped fill reservoirs severely depleted by the state’s three driest years on record, circumstances are a little more precarious now, she said.
“Where we didn’t have a lot of concern in January, there is a little more concern now because we have to be more precise in 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.
“There is a decent amount of flood risk that is mitigated, or prevented from becoming too severe, because of the existence of the lakes, but there is always that threat to us and our communities when there is just one aggressive storm,” he said. saying.
While individual river weather events aren’t always a cause for concern, they can quickly become a threat when they interact with other factors, such as snow pack and wet soils, according to Hayden, the former emergency services official, who he is now a professor of practice in public and ecosystem health at Cornell University.
“What was perhaps a little unusual about the January event was that it was one after another, and now this forecast event for March is yet another, while the ground is still, in many places, saturated, so it can exacerbate flooding. Hayden said.
In fact, the National Weather Service predicts that nearly two dozen additional rivers across the state may exceed their “control level,” meaning they could overflow their banks and cause minor flooding in low-lying areas. That includes multiple locations along the Sacramento and San Joaquin rivers.
officials in ash tree and Wood Counties have issued evacuation warnings for residents in their areas, warning of potentially severe flooding from the oncoming storm.
“The threat level is high enough that an Evacuation Warning is necessary and prudent due to the upcoming weather event,” the Madera County Sheriff’s Office said in its alert.
Fresno County Sheriff’s Office spokesman Tony Botti said officials are particularly concerned about flooding around Pine Flat, as well as Lake Millerton, which empties into the San Joaquin River and flows west into Mendota, Tranquility and other communities. He said the department is stationing rescue personnel and equipment, including boats and snowplows, at various locations around the county.
The atmospheric river, which is packing warm subtropical moisture from Hawaii, will fall on some of the deepest snowpack California has ever recorded. The Department of Water Resources’ third snow survey of the season, conducted Friday, found snowpack across the state to be 190% of normal, just below the record set in the winter of 1982-83.
But while recent storms have been cooler, the heat from the incoming system means there will be “more precipitation in the form of rain rather than snowfall,” said Jeremy Hill, manager of DWR’s Flood and Hydrology Operations Branch.
“We expect more runoff into rivers and streams, and several places reaching levels that require some monitoring of conditions and potentially some localized impacts as well,” he said.
In 2017, heavy rains and erosion from flooding damaged spillways at Oroville Dam, one of California’s largest reservoirs, forcing more than 100,000 people to flee a potential surge of overflowing water. Hill said there is no danger of such an event in the incoming storm as “Oroville has room to absorb these flows that are forecast at this time.”
“Luckily, they have room in the reservoir, as does Shasta even more,” he said, referring to California’s other largest reservoir. Lake Oroville and Lake Shasta are currently at 75% and 61% capacity, respectively. according to state data.
“There are a lot of preparations underway,” Hill said. “Reservoir operators are of course aware of storms so they are doing releases as needed and of course they will also be monitoring storm conditions throughout the event.”
Hayden said he hoped the January flooding would help officials identify potential areas of weakness before the next storm. But he also said that climate change is making many weather patterns harder to predict.
“Previous events are a race to future events, but each one will play out differently,” he said. “So I don’t know how well we know, even if an area was flooded last time, what it will be like this time.”
He noted that while atmospheric rivers can cause some damage, they also account for a large amount of the state’s moisture in any given year. This year’s wet winter has already helped ease California’s drought conditions considerably, with more than half of the state no longer under drought conditions, according to federal government estimates.
However, it is still important that people take precautions. Several officials said residents should heed all evacuation orders and never drive through floodwaters, even if they appear shallow.
Officials are also concerned that additional storms could create more problems in the wake of this one. Although the clouds are expected to clear by the weekend, another atmospheric river system is forecast to arrive Monday night.