An ARkStorm is a hypothetical but scientifically realistic megastorm that could devastate much of California due to flooding, landslides, and strong winds.

The name “ARkStorm” means atmospheric river (AR) 1000 (k) as the storm is projected as a 1-in-1000 year event. Similar scenarios have occured 10 times. Scientists have concluded that these events have a mean return period of around 150-290 years.

In December 1861/January 1862, a similarly intense storm occured, dumping 10 feet of rain in 43 days.

If the scenario were to occur today, it is expected that there would be a lake in the Central Valley 300 miles long and 20 miles wide, wind speeds would reach 125 mph, hundreds of landslides would occur, property damage would exceed $300 billion, agricultural losses of $400 billion, and 1.5 million residents would need to evacuate.