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Are atmospheric rivers the reason for Northern California's extreme weather?

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It’s been a tough few years for Northern California, disaster-wise. The north state has been hammered in quick succession by catastrophic drought, intense flooding and rampaging wildfires. While it’s reassuring to know such extreme events are historically uncommon, they’re also not simply a result of bad luck.

Scientists have only recently begun to understand that they share a common natural link.

They’re all tied to a strange and powerful event that occurs unpredictably in the atmosphere thousands of miles away. In 2017, the American Meteorological Society gave them an official name: atmospheric rivers. Driven by high winds, these are tight ribbons of dense moisture stretching more than a thousand miles long from the tropical Pacific Ocean.

Atmospheric rivers, or ARs, can transport tremendous amounts of moisture — as much as 15 times the average flow of water at the mouth of the Mississippi River. And after forming above the tropical ocean near Hawaii, these atmospheric jets aim directly at the coast of California.

Sasha Gershunov, of the Scripps Institution of Oceanography at UC San Diego, is one of the nation’s experts on atmospheric rivers. ARs are one of the planet’s most extreme weather events, he said, and their impact on the state is both good and bad. In normal years, Gershunov said, ARs are essential. They’re a critical source of water for the Golden State’s bountiful agriculture, thick forests and ecosystems, snowpack and drinking supplies, dropping 50% to 60% of the entire state’s annual precipitation.

But when they fail to arrive for several years, reservoirs fail to fill, and the state sinks into severe drought. Such dry periods can kill forests and intensify the effects of fire, which can run hotter and wild across a bone-dry landscape.

When ARs come frequently, and stall overhead, they can drop inches of rain in a few hours and feet over days, creating floods. All that rain can also produce extra growth in brush and wild vegetation, which can be fuel for future fires.

As a result, in California, ARs are responsible for tens of billions of dollars in damage.

Because of their far-ranging importance, researchers like Gershunov are now working to find ways to predict, track and measure them. They hope, with the right tools, they’ll be able to provide better advance warning and give agencies here the chance to plan ahead for the impact ARs may cause.

This year, a team of WC-130 Hercules four- engine propeller aircraft, flown by the Air Force’s 53rd Weather Reconnaissance Squadron — the legendary Hurricane Hunters — will be diving deeply into any ARs that develop off the California coast. The mission of the five-person crew is to drive through the storms, dropping instrument-laden devices, to help scientists uncover how they’re composed and how they work.

Such studies are beginning to provide some insights. Gershunov and other members of his team recently published a study on what the most advanced models suggest for the future. Based on 16 global climate models, their data predicts that atmospheric rivers will be less frequent over California, he says. But when they come, they’ll be more intense.

“In Northern California — a region of exceptionally strong AR activity — they are projected to greatly increase the frequency of heavy and extreme precipitation events,” he said.

Extreme precipitation in the Russian River basin, Gershunov said, “will become more than twice as likely in the second half of the 21st century.”

To help planners, government agencies, businesses and the public prepare for future ARs, a new scale was developed by F. Martin Ralph, director of the Center for Western Water and Weather Extremes (CW3E) at Scripps, in collaboration with Jonathan Rutz from the National Weather Service and several other experts. Their system ranks atmospheric rivers from 1 to 5, ranging from “weak” to “moderate,” “strong,” “extreme” and “exceptional.”

The category 1 and 2 storms are mainly beneficial, with modest to moderate rainfall, amounts that help build snowpacks and refill reservoirs. Category 3 can be both beneficial and hazardous. For example, a Category 3 AR in 2016 lasted 36 hours and produced nearly 10 inches of rain, but pushed some rivers to near flood stage.

The California AR in January 2017 was a Cat. 4, dropping 14 inches of rain in the Sierra Nevada, flooding a dozen rivers. The exceptional Category 5 AR that stalled for 100 hours over the Central Coast after Christmas in 1996 left more than $1 billion in damage.

According to the U.S. Geological Survey, between 80% and 90% of the major river flood events in the western United States happen as a result of landfalling atmospheric rivers.

Where and when the next set of atmospheric rivers will arrive can’t be predicted. But experts throughout the state will be watching, and as agencies learn more, we’ll be better prepared to deal with their effects.

Stephen Nett is a Bodega Bay-based Certified California Naturalist, writer and speaker. Contact him at snett@californiasparks.com.

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