March 1, 2024

California floods: how atmospheric rivers led to a state of emergency | Extreme weather

California’s rainy season is officially underway. The state is facing another round of severe storms this week and the dangerous potential for flash flooding, mudslides and furious winds that are expected to come with them.

Just like last year, when record rainfall hit the state, the storms are blamed on atmospheric rivers (ARs), systems that have long played a role in California’s precipitation levels — both for better and for worse.

Here’s what you need to know about these weather patterns and the effects they could have on the western US.

What is an atmospheric river?

The National Oceanic and Atmospheric Administration refers to these systems as “rivers in the sky” for good reason. Characterized by long streams of moisture in the atmosphere that extend between 250 and 375 miles wide on average, ARs affecting the western U.S. are overwhelmed by water vapor that evaporates in the Pacific Ocean and is transported by other weather systems from the tropics or the subtropics.

The average atmospheric river carries an amount of water vapor that rivals the flow at the mouth of the mighty Mississippi River – and the strongest can contain more than 15 times that amount. This moisture is released as rain or snow when ARs make landfall, and depending on the size, timing, and intensity, the resulting storms can be highly destructive or extremely beneficial.

ARs are important contributors to California’s water supply, providing up to half of the rain and snow the dry state depends on throughout the year. But big ones can also overwhelm rivers and reservoirs, causing damaging floods. The systems also tend to come equipped with strong winds that knock down trees and power lines, increasing their destructive tendencies.

What is a Pineapple Expresso?

The storms that hit California this week are part of a “Pineapple Express” system, which is just one type of atmospheric river. Named for their origin in the Hawaiian Islands, they are often strong systems and are known to release torrents of precipitation when they reach the west coast of the US and Canada. According to the National Ocean Service, they can dump up to 5 inches of rain on California in a single day.

Streets flooded in Santa Barbara during the powerful storm, the second to hit California in less than a week. Photography: Mario Tama/Getty Images

Bomb cyclone

These low-pressure storm systems help create ARs, pushing them from the Pacific toward the coast. A bomb cyclone is a storm whose pressure is falling rapidly but its strength is increasing. Unlike hurricanes or other storms where the center is strongest, bomb cyclones can generate the worst weather around their edges.

Bomb cyclones are born from “bombogenesis,” a term meteorologists use to measure pressure drops (which correlate with strengthening) at different latitudes. Bombogenesis occurs when hot air and cold air collide. These so-called “extratropical cyclones” can form atmospheric rivers, but they can also be driven by them.

Emergency declaration

Governor Gavin Newsom declared a state of emergency for eight counties in Southern California in response to the recent storms, covering Los Angeles, Orange, Riverside, San Bernardino, San Diego, San Luis Obispo, Santa Barbara and Ventura counties. The proclamation helps mobilize resources to impacted areas and authorizes the deployment of the California National Guard as needed. It also streamlines the process for out-of-state utility providers and contractors to make repairs during and after the storm to ensure a smoother recovery.

Why are people so worried?

This week’s storms have left officials and residents on edge, in part because of the timing. Back-to-back systems have a stronger impact and have greater potential to cause damage when systems are already flooded and soils are saturated.

The second storm in the set is also distressing, and forecasters have urged residents across the state to prepare for potentially destructive and life-threatening weather. Equipped with overlapping hazards – strong winds, heavy rain, strong waves along the coast and whiteout conditions in the mountains – the massive system is poised to wreak havoc across the state.

California saw a similar pattern last year, when a series of brutal atmospheric rivers drenched the state, triggering historic levels of rain that caused urban flooding, mudslides and road closures. The dangerous conditions caused 22 confirmed deaths and researchers at Stanford University recorded more than $3 billion in losses.

Although this winter has not seen this level of total precipitation, there have been cases of severe flooding before this weekend, and these areas are still recovering. Ventura County was hit hard by a deluge late last year that led to emergency water rescues and left streets underwater. In January, another furious storm hit San Diego, causing its aging stormwater systems to quickly exceed capacity. Rising waters invaded homes and businesses while streets turned into rivers. The event was recorded as one of the rainiest days in the city’s history.

A farm flooded after a series of atmospheric river systems on March 10, 2023 near Strathmore, California. Photography: David McNew/Getty Images

While these storms have been incredibly destructive and dangerous, and in some cases have set new records, scientists have warned that they are just a taste of what is to come. Fueled by the climate crisis, the risks of a catastrophic megaflood – which could displace millions of people and generate more than $1 billion in losses – have doubled in California.

Before this week’s storms, unsubstantiated social media posts raised anxiety that the weather was about to get much worse, but researchers and meteorologists, including climatologist Daniel Swain, who co-authored a paper on the worrying scenario, quickly dispelled the idea. disinformation. Still, he warned, California has a long way to go to prepare for such an event — and time is running out.

Could storms have any positive impact?

Despite their potential for harm, ARs are extremely important in California and are welcomed by water authorities still facing the emergence of a prolonged drought. Even with the wet weather, California’s snowpack is lagging this year and there are hopes that heavy dusting could bring it closer to historical averages by the end of the rainy season in April.

Snowpack is a critical part of California’s water supply, acting as a sort of water savings account for next year as it slowly melts and flows into streams, soils and reservoirs during the driest periods of the year. Snowpack provides just under a third of California’s water needs for the year.

The storms are forecast to dump a new layer of snow on the mountains, a sign of hope after the February snow survey showed snow in the Sierra Nevada range at just 52% of average. It was a big jump compared to the first measurement, taken in January, which measured just 28% of the average.

Meanwhile, rainfall across the state was 82% of the average, water resources department officials said.

What is the role of the climate crisis?

California’s climate has long fluctuated dramatically from wet to dry, but models show these changes will occur with increasing intensity as the world warms. ARs are also increasingly likely to arrive in clusters, which can cause up to four times more economic damage than each would have caused individually, according to a study published in Science Advances.

Burdened by more moisture from the Pacific as ocean temperatures rise, scientists expect ARs could also become more severe, adding more flood risk to California and the West. As temperatures rise, precipitation is more likely to fall as rain rather than snow, which could pose problems for the state’s water supply.

Reuters contributed reports

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