A bomb cyclone looms over California and Pacific north-west – but what is it? | California

A set of powerful storms that could rank among the strongest in decades will slam British Columbia, thepacific north-west, and northern California this week, with torrents of rain, inches of snow in high altitudes and damaging winds. The region is bracing for widespread power outages and flash flooding, with extreme rainfall that could linger until the weekend.

After a relatively mild autumn, these rains are part of a familiar pattern caused by atmospheric rivers – strong storm systems that can bring both relief and ruin.

This week’s weather event is predicted to be the strongest atmospheric river the regions have seen this season. Heavy rainfall started 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).

A low-pressure system, which has helped spur the storm and whip up winds, intensified so quickly that it is considered a “bomb cyclone”, explained Richard Bann, a meteorologist with the National Weather Service weather prediction center

What exactly are atmospheric rivers and bomb cyclones, 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 Noaa simply says, the “rivers in the sky” – have delivered both destructive and drought-reducing downpours 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.

ARs have always played an important role, providing for roughly half of California’s annual precipitation, but their power and quick succession in recent years 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 about bomb cyclones?

These low-pressure storm systems help create ARs, pushing them from the Pacific to the coast. A “bomb cyclone is a storm whose pressure is really falling rapidly and in that case it is rapidly strengthening”, said Alex Lamers, the warning coordination meteorologist for the National Weather Service Weather Prediction Center. But unlike hurricanes or other storms where the center is the strongest, bomb cyclones can generate the worst weather at their edges.

Bomb cyclones are borne out of “bombogenesis”, a term meteorologists use to measure drops in pressure (that correlate with strengthening) at different latitudes. Bombogenesis occurs when warm air and cold air collide. These so-called “extra-tropical cyclones” can form atmospheric rivers, but they can also be boosted by them.

“It is like a feedback loop,” said Dr Marty Ralph, director of the Center for Western Weather and Water Extremes and a researcher at Scripps Institution of Oceanography. “When you have an AR already present, the AR can supercharge the new cyclone, which can then strengthen the AR.”

What impact will the climate crisis have on them?

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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. They 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, 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 3,500ft (1,066 meters), where 15in (28cm) of snow was possible over two days. Wind gusts could top 75mph (120km/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 coastal west are bracing for more severe weather. It’s a taste of what’s to come as the world warms, and residents of the region 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