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Atmospheric Rivers Can Be Too Much of a Good Thing

Atmospheric rivers are generally beneficial, but with the intensity of storms increasing and a couple of close calls, California is ripe to get hit with a storm that could be devastating to communities and the economy.

Atmospheric rivers are long, narrow bands of moisture that descend from the tropics to higher latitudes like from Hawaii to California. They used to be referred to mostly as a pineapple express.

They are mostly good, replenishing the water supply and putting out fires at the end of the wildfire season. But there can be too much of a good thing. These rivers in the sky can transport 10 times the volume of the Mississippi River in water vapor. When that vapor rises over coastal mountains, it drops rain and snow and can cause devastating flooding.

A study by researchers at the Scripps Institution of Oceanography suggests that one of these events could cause catastrophic damage to California and its economy and thus the nation’s economy. The state has had a couple of near misses, including the 1997 floods and the Oroville Dam spillway incident in 2017. The study suggests that because forecast models predict more intense atmospheric rivers in decades to come, the state should be ready and take steps to mitigate possibly devastating effects.

The areas around Sacramento and Sonoma counties are the most likely to suffer catastrophic consequences, according to the study. Research shows that the biggest hit so far by one of these storms has caused about $3 billion in damages. That’s not a huge number given the state’s economy, but the damages were concentrated.

A previous study suggested that a one-in-100-year storm could cost California $860 billion in damages, according to Thomas Corringham, professor at the University of California, San Diego, who participated in the Scripps study.

Corringham said parts of California have been lucky to avoid catastrophe so far. “Sacramento had a close call in 1997 and also there was the Oroville incident with the spillway in 2017 when they had to evacuate 180,000 people,” he said. “If that dam had failed and a significant amount of water had gone over, the damages would have been catastrophic.”

The research is straightforward. All the models show increased intensity for these types of storms. “If you put more carbon into the atmosphere, you retain more of the heat, and as you increase the heat you get more evaporation and more moisture in the air so all storm systems — hurricanes, atmospheric rivers — they all become more intense,” Corringham said.

The Center for Western Weather and Water Extremes at Scripps has begun an atmospheric river reconnaissance program where they fly jets into the storms to collect data. It’s like what’s done with hurricanes. The hope is that the data will provide information to expand forecasts. “As we develop forecasting capabilities, we should be able to give people a week to evaluate, move their cars to higher elevation or move the contents of their homes up in the attic,” Corringham said.

Longer-term interventions are needed as well, such as discouraging new development in flood plains, providing financial incentive for people to harden their homes and, and in some communities, elevating them.

“And then probably the biggest one, after a big flood event instead of providing assistance for people to rebuild in place, we should be providing assistance for people to move to safer communities,” Corringham said.

He acknowledged that it’s politically difficult to do. “Especially for the higher-value properties, like coastal properties. It’s a large source of tax revenue for the municipality, so the idea of moving people comes with the loss of revenue.”

Another issue is paying someone market value for a home that they may have lived in for 20 or 30 years and been a part of the community is not enough. “But if we provide just compensation, I think the payoff on the long-term would be significant rather than having people rebuild decade after decade, just do a one-time payment and put people in a safer area.”