Atmospheric rivers are inundating California – but what are they? | California

Atmospheric rivers, the severe storms that have doused California during its extremely wet winter, have dumped 30tn gallons of water across the state, buried mountain towns in snow and caused widespread flooding – and they aren’t over yet, with more expected to continue into spring.

But what exactly are they and why are they causing so much damage? Here’s what you need to know.

What is an atmospheric river? The basics

It’s all in the name here – ARs are exactly what they sound like. These long streams of overhead moisture – or as the National Oceanic and Atmospheric Administration (Noaa) simply says, the “rivers in the sky” – have delivered both destructive and drought-reducing downpours across the state with alarming intensity.

A tree, blown over by 50 mph winds, crushes a parked car in El Camino Real in Burlingame, California.Pin
A tree, blown over by 50 mph winds, crushes a parked car in El Camino Real in Burlingame, California. Photograph: Terry Schmitt/UPI/REX/Shutterstock

The storms are supercharged by warm water vapor that evaporates off the Pacific Ocean, loading them with enough water to rival the average flow at the mouth of the mighty Mississippi River, with up to 15 times its volume. Moving with weather systems, they appear as a trail of wispy clouds that can stretch for hundreds of miles, ready to unleash deluges wherever they make landfall.

The storms are both a destructive force and a welcome relief for a state still grappling with drought. California was desperately dry just months ago but the storms – which began in earnest in late December – have refilled reservoirs and supplied a strong snowpack that will provide essential moisture through the hotter months to come. ARs have always played an important role, providing for roughly half of the state’s annual precipitation, but their power and quick succession this year has increased the dangers. On top of huge amounts of water, which can overload rivers and reservoirs, dangers can include strong gusty winds, and are responsible for an overwhelming majority of the flood damage across the western US in recent decades.

“Whether there are too many or too few determines whether parts of California are above or below normal in precipitation,” said Dr Marty Ralph, director of the Center for Western Weather and Water Extremes and researcher at Scripps Institution of Oceanography. “When you get a couple of them back to back, especially if the watersheds are already very wet, we start to see the pile-on effects.”

What impact will the climate crisis have on them?

California’s climate has long vacillated dramatically from wet to dry, but models show these shifts will occur with increasing intensity. The destructive set of storms falls in line with climate models’ predictions, according to Ralph.

Supercharged by more moisture coming off the Pacific as ocean temperatures rise, scientists expect that ARs will only grow more severe as the world warms, adding more risks for floods across California and the US west. The storms will also drop more rain than snow due to hotter weather on land.

Along with amplifying risks from rain-related floods, warmer downpours on the snowpack also spark rapid runoff concerns. It also could eat into what California considers a water savings account of sorts, by rapidly melting the snow that sits high atop mountain ranges and typically trickles more slowly through systems during spring and summer.

A person walks past a snowbank in front of a restaurant in Mammoth Lakes, California.Pin
A person walks past a snowbank in front of a restaurant in Mammoth Lakes, California. Photograph: Mario Tama/Getty Images

What’s coming next?

Recent atmospheric river storms have forced entire towns to evacuate and unleashed hurricane-force winds and knocked out power to hundreds of thousands of people. And there’s more on the way in the coming weeks.

“The potential for another potent storm system to impact California early next week is increasing, with heavy precipitation and flooding possible,” the National Weather Service warned on Wednesday, adding that “any additional precipitation on top of a melting snowpack, saturated soils, and already swollen streams will exacerbate flooding concerns”.

State officials foresee the possibility of at least two more storms in March alone, raising the potential for a “worst-case scenario” of back-to-back atmospheric rivers between 21 to 23 March, according to Michael Anderson, a state climatologist.

Ultimately, residents across the state are bracing for more severe weather. It’s a taste of what’s to come as the world warms, and Californians will have to come to grips with more dramatic swings from wet and dry as the climate intensifies. “This pattern is consistent,” Ralph said, “where we go from a very deep drought to a flood situation.”

Reuters contributed reporting

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