California at risk of ARkStorm, a 1,000-year megastorm caused by a 30-day lashing

Scientists have issued a warning about a storm in California that could inundate residential areas with floodwaters as high as 20 feet and submerge everything between San Francisco and Los Angeles. This scenario has been named ARkStorm 2.0 and involves a chain of storms hitting the state one after the other, leading to a scene similar to the biblical flood, not only in name (ARkStorm - a direct play on Noah’s Ark from the biblical Genesis flood narrative) but also in the scale of devastation.

ARkStorm stands for "Atmospheric River 1,000-year Storm" and was developed by the US Geological Survey (USGS) and federal, state and academic experts in 2010. The purpose was to help prepare for an extreme weather disaster driven by atmospheric rivers. These long and narrow corridors flowing in the sky carry water vapour from the tropics towards higher latitudes. When they reach California, ARs slam into coastal mountains or the Sierra Nevada, with condensation leading to heavy rain or snow.

They are integral to the state's water supply and weather, but a glitch can cause them to sometimes arrive in quick succession. This would trigger a deluge, floods of astronomical levels, and landslides. In the ARkStorm 2.0 scenario, multiple atmospheric rivers will slam into the West Coast over nearly 30 days, dumping huge amounts of rain and snow, the Daily Mail reported. Floodwaters could reach 20 feet in some areas, more than 1.5 million people would have to be evacuated, and the losses from the destruction will exceed $1 trillion.

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California has been historically hit by several floods, including the Great Flood of 1861-62. Storms wreaked havoc across the state for nearly 45 days, and the Central Valley turned into a lake nearly 500 kilometres long. Everything submerged in the deluge, and the capital had to be shifted to San Francisco. ARkStorm 2.0 was modelled on this massive flood.

Historical records show that California has witnessed storms worse than the one in 1861. At least six storms have struck the state in the past 1,800 years, which proves another massive storm could be coming.

How climate change worsens the storm situation in California

With climate change a very real threat today, this scenario is becoming increasingly plausible. In 2022, scientists explored how this would happen in a warmer world. The "expanding atmospheric sponge effect" is one of the changes, which means for every 1.8 degrees Fahrenheit of warming, storms will have nearly seven per cent more water vapour in the atmosphere to play with. Weather events such as this that used to occur once every two centuries could happen three times per century by around 2060.

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Secondly, the frozen precipitation in the mountains of the Sierra Nevada would start falling as rainfall, with the warmer waters melting more snow, thus adding to the water levels. The runoff would rise by 200 to 400 per cent and cause rivers to be filled and spread through the Central Valley. The USGS has said that the existing flood defences, such as the levees, canals and pumping stations, are not built to withstand ARkStorm 2.0.

California is working to strengthen the systems in place to fight such a storm. It will spend an estimated $30 billion over 30 years for this. However, experts say the timeline needs to speed up, even though there is no clarity on when this mega storm will hit California.