California and Pacific north-west brace for an atmospheric river – but what is it? | California

Northern California and the Pacific north-west are bracing for what is expected to be a powerful storm that will bring enough heavy rain and winds to potentially cause power outages and flash flooding.

This week’s weather event is predicted to be the strongest atmospheric river the regions have seen this season. Heavy rainfall is expected to begin Tuesday evening with the worst of the event unfolding throughout the rest of the week, causing “life threatening” conditions, according to the weather prediction center for the National Oceanic and Atmospheric Association (Noaa).

The storm system has intensified so quickly that it is considered a “bomb cyclone”, explained Richard Bann, a meteorologist with the National Weather Service weather prediction center

But what exactly is an atmospheric river and why will it cause 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 Noaa simply says, the “rivers in the sky” – have delivered both destructive and drought-reducing downpours across the state with alarming intensity.

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 have 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.

What’s coming next?

Previous atmospheric river storms have forced entire towns to evacuate and unleashed hurricane-force winds and knocked out power to hundreds of thousands of people.

“Be aware of the risk of flash flooding at lower elevations and winter storms at higher elevations. This is going to be an impactful event,” said Bann, of the NWS.

A winter storm watch was issued for the northern Sierra Nevada above 3,500 ft (1,066 meters), where 15 in (28 cm) of snow was possible over two days. Wind gusts could top 75 mph (120 km/h) in mountain areas, forecasters said.

“Numerous flash floods, hazardous travel, power outages and tree damage can be expected as the storm reaches max intensity” on Wednesday, the Weather Prediction Center warned.

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