Atmospheric river could raise flood risk in California, has dam operators on alert

Computer models are showing a growing likelihood of an atmospheric river storm hitting California late next week, raising concerns that if a warm “Pineapple Express” barrels in with enough force, it could melt parts of the state’s big Sierra Nevada snow pack and increase flood risk.

Atmospheric scientists and meteorologists say more will be known in a few days. The storm could still fizzle the way hurricanes that develop far out in the Atlantic Ocean sometimes fail to materialize or make landfall.

But for now, they say, a large wet storm seems to be coming together and is most likely to hit Central or Southern California. Some models are showing a shift toward the north.

“It looks a batten-down-the-hatches kind of storm right now,” said Marty Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego. “But it is still pretty far out, so there’s some uncertainty.”

Ralph, one of the nation’s experts on atmospheric river storms, said his team will be working with the federal government to send two C-130 planes over the Pacific Ocean from Hawaii on Sunday and San Diego on Tuesday to measure conditions.

Steve Anderson, a forecaster with the National Weather Service in Monterey, said Friday that for now, it looks like the storm seems to be headed roughly for the Santa Barbara area. Depending on the size of the storm, in that location it could cause rain in the Bay Area or, if it moved further north, rougher conditions.

“If it shifts just 50 or 100 miles north — which is almost nothing on the global scale — we could get heavy rain here, or if it shifts further south we could get nothing,” Anderson said. “We’re keeping an eye on it.”

Memories of the flooding in 2017, when atmospheric river storms in February wrecked the spillway at Oroville Dam in Butte County, are still fresh. In 1997, warm rain storms on a large Sierra snowpack caused significant flooding in Yosemite National Park and other parts of the state.

“The main problem if it does come to pass is that there is the potential for major flooding issues,” said Daniel Swain, a climate scientist at UCLA who closely monitors atmospheric rivers. “It’s a big if, but this is one of the few times recently we have had an above-average snow pack. This storm could cause some really big problems. But it is a big if.”

On Friday, the statewide Sierra Nevada snowpack was 123 percent of its historical average, and 135 percent in the Southern Sierra. Storms this weekend are expected to bring up to 3 feet of new snow.

Many of the big reservoirs around the state are already at high levels, due to soaking storms over the past month. Lake McClure, in the Sierra Nevada foothills 40 miles east of Modesto, is 66 percent full, or 135 percent of its historical average. New Melones lake, in the Sierra Foothills near Angels Camp, is 133 percent of its historical average, or 79 percent full. And Shasta Lake, the largest reservoir in California, is at 98 percent of its historical average, or 67 percent full.

Swain said weather experts will know more by Monday.

“In the context of this winter, this would probably be the biggest storm by a significant margin,” he said. “If it’s cold it will be manageable.”

But he added: “A very warm rain on a mountain snow pack will be very problematic.”

Dam operators around the state said Friday they are watching closely.

Todd Plain, a spokesman for the federal Bureau of Reclamation in Sacramento, said his agency has begun increasing water releases from Folsom Reservoir near Sacramento and from New Melones reservoir this week. But that’s to free up space to handle water coming in from storms that are forecast this weekend and from storms a few days ago that brought water that is still pouring off saturated soils into reservoirs.

Early next week, when more is known about the potential atmospheric river storm, the agency could further increase releases, or begin increased releases from other big dams it operates, like Shasta, Plain said. Typically, dam operators want to save as much water as they can in reservoirs for summer months. But if they let reservoirs fill to the top, and are hit by a series of soaking storms, that can cause flooding to homes and communities downstream, as the water pours down dam spillways into rivers without the ability for dam operators to limit the amount.

“We will adjust accordingly to current conditions,” Plain said. “It’s a balancing act.”

State Water Project officials said Friday they have not yet begun increased releases to free up space in state-owned reservoirs, and they will make a decision of whether to do so by Monday. At Oroville, they kept the lake level low during construction to rebuild the spillway, said Molly White, chief of water operations for the State Water Project. On Friday, Oroville was 43 percent full or 65 percent of its historic average. By some estimates, it could go to two-thirds full in the next 10 days.

“We still have quite a bit of room,” she said.

Already this week, the Santa Clara Valley Water District announced it will draw down six of its 10 reservoirs. At the largest reservoir, Anderson, near Morgan Hill, state seismic safety rules prohibit it from going above 58 percent full until a modernization project is finished in the coming years. Friday, it was 35 percent full. Some other district reservoirs have similar limits, and some are being lowered due to recent storms and the forecast.

The district, which came under fire after flooding in 2017 from Anderson caused $100 million in damage to downtown San Jose, doesn’t anticipate water shortages this summer, said spokesman Marty Grimes, because its groundwater supplies are robust and there are still two more months of the winter rainy season to go.

“We want to make more room so there is less chance that the reservoirs will spill,” he said. “That has the effect of reducing flood risk. We can do that without jeopardizing our water supply.”

Mike Anderson, state climatologist with the Department of Water Resources, said the storm, if it materializes, could also bring a lot more Sierra snow. How much tropical moisture it carries and how long it lingers over land will be key, he said.

“Pay attention to the forecast,” Anderson said. “Each day we’ll know a little more.”