An ARkStorm (for Atmospheric River 1,000 Storm) is a hypothetical but scientifically realistic "megastorm" scenario developed and published by the Multi Hazards Demonstration Project (MHDP) of the United States Geological Survey, based on historical occurrences. It describes an extreme storm that could devastate much of California, causing up to $725 billion in losses (mostly caused by flooding), and affect a quarter of California's homes. The event would be similar to exceptionally intense California storms that occurred between December 1861 and January 1862, which dumped nearly 10 feet of rain in parts of California, over a period of 43 days. The name "ARkStorm" means "Atmospheric River (AR) 1,000 (k)" as the storm was originally projected as a 1-in-1000-year event. However, more recent geologic data suggests that the actual frequency of the event is likely in the 100- to 200-year range.
The USGS sediment research in the San Francisco Bay Area, Santa Barbara basin, Sacramento Valley, and Klamath Mountain region have indicated that these "megastorms" have occurred in the following years A.D.: 212, 440, 603, 1029, 1418, c. 1300, 1605, 1750, 1810, and December 1861–January 1862 (the latest occurrence). Based on the intervals of known occurrences, ranging from 51 to 426 years, scientists have concluded that ARkStorms have a mean return period of around 150–200 years. Geologic data also indicates that at least several of the previous ARkStorm events were more intense than the one in 1861–62, particularly the events in 440, 1418, 1605, and 1750, each of which deposited a layer of silt more than one inch thick. The largest ARkStorm was the one in 1605, which left behind a layer of silt two inches thick at the Santa Barbara basin; data indicates that this ARkStorm was at least 50% more powerful than any of the others recorded. The ARkStorms more intense than the 1861–62 event appear to occur at intervals of 200 years or so.
If another ARkStorm occurred, similar to those in hypothetical models, it 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.
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- Extreme weather
- Lists of floods in the United States
- Hurricane Harvey (2017) – Category 4 hurricane that caused catastrophic flooding in the Houston area and Southeast Texas, becoming the wettest tropical cyclone recorded in the U.S. The rainfall totals from Harvey were similar to those unleashed by previous ARkStorms in California.
- Dettinger, M. D.; Ingram, B. L. (January 2013). "The Coming Megafloods" (PDF). American Scientific. 169: 64–71.
- Null, J.; Hulbert, J. (2007). "California Washed Away: The Great Flood of 1862". Weatherwise. 60 (1): 26–30. doi:10.3200/wewi.60.1.26-30.
- Porter, Keith; et al. (2011). "Overview of the ARkStorm scenario". USGS Open-File Report 2010-1312.
- Ingram, B. Lynn (January 1, 2013). "California Megaflood: Lessons from a Forgotten Catastrophe". Scientific American. Retrieved 22 February 2019.
- 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.