An ARkStorm (for Atmospheric River 1,000 Storm) is a hypothetical but scientifically realistic "megastorm" scenario developed and published by the United States Geological Survey, Multi Hazards Demonstration Project (MHDP). It describes an extreme storm that might impact much of California causing up to $725 billion in losses (most caused by flooding), and affect a quarter of California's homes. The event would be similar to exceptionally intense California storms which occurred between December 1861 and January 1862. The storm would be a 1-in-1000-year event. The name "ARkStorm" means "Atmospheric River (AR) 1,000 (k)."
The hypothetical ARkStorm, if it occurred, would have the following effects:
- The Central Valley experiences hypothetical flooding 300 miles long and 20 or more miles wide.
- Serious flooding also occurs in Orange County, Los Angeles County, San Diego, the San Francisco Bay area, and other coastal communities.
- Windspeeds in some places reach 125 miles per hour (201 km/h), hurricane-force winds.
- Across wider areas of the state, winds reach 60 miles per hour (97 km/h).
- Hundreds of landslides damage roads, highways, and homes.
- Property damage exceeds $300 billion, most from flooding.
- Demand surge (an increase in labor rates and other repair costs after major natural disasters) could increase property losses by 20 percent.
- Agricultural losses and other costs to repair lifelines, dewater (drain) flooded islands, and repair damage from landslides, brings the total direct property loss to nearly $400 billion, of which $20 to $30 billion would be recoverable through public and commercial insurance.
- Power, water, sewer, and other lifelines experience damage that takes weeks or months to restore.
- Flooding evacuation could involve 1.5 million residents in the inland region and delta counties.
- Business interruption costs reach $325 billion in addition to the $400 billion required for property repair costs, meaning that an ARkStorm could cost a total of $725 billion, nearly three times the amount of damage predicted by the California ShakeOut authors for a severe Southern California earthquake, an event with roughly the same annual occurrence probability.
- USGS Multi-Hazards Demonstration Project: ARkStorm: West Coast Storm Scenario (including video)
- USGS Newsroom: ARkStorm: California’s other "Big One"
- Weather Underground - The ARkStorm: California's coming great deluge
- High Country News: The other Big One, Judith Lewis
- Water Education Foundation, Mar-Apr 2011: Plausible and Inevitable: The ARkStorm Scenario, by Gary Pitzer
- Megastorms Could Drown Massive Portions of California January 5, 2012 Scientific American
- Eric Zerkel (December 21, 2017). "California: The Flood That Could Change Everything". The Weather Company. Retrieved December 24, 2017.