Archive · 11 Aug 2026
California Warned of ARkStorm: 1,000-Year Megastorm Could Bring Massive Flooding
Scientists are worried about a very big storm that might happen in California one day.
They call it ARkStorm, a name that makes people think of Noah's Ark and a giant flood.
The storm would happen when many 'rivers in the sky' bring rain and snow one after another for almost 30 days.
That much water could cover houses and entire cities with floodwaters as high as 20 feet.
So many people would have to leave their homes that more than one and a half million would need to evacuate.
Fixing all the damage could cost more than one trillion dollars.
Scientists studied a giant flood in 1861 that turned part of California into a huge lake.
Because the Earth is getting warmer, big storms like this may happen more often in the future.
That is why California wants to spend about 30 billion dollars to build better walls, canals and pumps to protect people.
Scientists warn that ARkStorm 2.0 could bring floodwaters up to 20 feet high and submerge areas between San Francisco and Los Angeles.
The scenario involves multiple atmospheric rivers hitting California's coast in quick succession over nearly 30 days.
ARkStorm stands for 'Atmospheric River 1,000-year Storm' and was developed by the US Geological Survey and experts in 2010.
More than 1.5 million people would need to be evacuated and economic losses could exceed $1 trillion.
Climate change makes the scenario more plausible, with events that once occurred every 200 years possible three times per century by around 2060.
- Who
- Scientists from the US Geological Survey (USGS) along with federal, state and academic experts who developed the ARkStorm 2.0 scenario, warning the people of California.
- What
- A warning that California faces the risk of ARkStorm 2.0, a 1,000-year megastorm that could bring floodwaters up to 20 feet high, overwhelm flood defenses and cause over $1 trillion in losses.
- When
- The scenario was developed in 2010 and modelled on the Great Flood of 1861-62; scientists project that such extreme events could occur three times per century by around 2060.
- Where
- California, with flooding potentially submerging areas between San Francisco and Los Angeles, including the Central Valley and the Sierra Nevada.
- Why
- Climate change is increasing the plausibility of the scenario because warmer air holds more water vapor and rising snow levels in the Sierra Nevada could increase runoff by 200 to 400 percent.
- Scenario name
- ARkStorm 2.0 (Atmospheric River 1,000-year Storm)
- Developed by
- US Geological Survey and federal, state and academic experts in 2010
- Storm duration
- Multiple atmospheric rivers hitting the West Coast over nearly 30 days
- Expected flood depth
- Up to 20 feet in some areas
- Evacuation estimate
- More than 1.5 million people
- Estimated losses
- Over $1 trillion
- Historical model
- Great Flood of 1861-62, which lasted nearly 45 days
- Flood defense investment
- About $30 billion over 30 years planned by California