ARkStorm Scenario (2018)
ARkStorm Scenario | U.S. Geological Survey
ARkStorm was the second full scenario after ShakeOut developed by the SAFRR team.  ARkStorm is named to acknowledge the meteorological phenomena behind most large storms on the U.S. West Coast, namely Atmospheric Rivers (ARs), and to provide some future scale to compare past and futureevents. The hypothetical scenario would be an Atmospheric River, AR with a value of 1,000 (k), or an ARkStorm. The scenario was based on repeated historical events that were documented in geologic and historical records. In particular, ARkStorm addresses massive U.S. West Coast storms analogous to those that devastated California in 1861-62 as the magnitude of this event is projected to become more frequent and intense as a result of climate change.
Experts were assembled from the National Oceanic and Atmospheric Administration (NOAA), USGS, Scripps Institute of Oceanography, the State of California, California Geological Survey, the University of Colorado, Federal Emergency Management Agency (FEMA), the National Center for Atmospheric Research (NCAR), California Department of Water Resources, California Emergency Management Agency (CalEMA) and other organizations to design the large, but scientifically plausible, hypothetical storm scenario that would provide emergency responders, resource managers, and the public a realistic assessment of what is historically possible.
The ARkStorm storm is patterned after the 1861-62 historical events but uses modern modeling methods and data from large storms in 1969 and 1986. The ARkStorm draws heat and moisture from the tropical Pacific, forming a series of Atmospheric Rivers (ARs) that approach the ferocity of hurricanes and then slam into the U.S. West Coast over several weeks. Atmospheric Rivers are relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics.
Using sophisticated weather models and expert analysis, precipitation, snow lines, wind, and pressure data, the modelers characterize the resulting floods, landslides, and coastal erosion and inundation that translate into infrastructural, environmental, agricultural, social, and economic impacts. Consideration was given to catastrophic disruptions to water supplies resulting from impacts on groundwater pumping, seawater intrusion, water supply degradation, and land subsidence.
In contrast to the recent U.S. East and Gulf Coast hurricanes, only recently have scientific and technological advances documented the ferocity and strength of possible future West Coast storms. ARkStorm is intended to elevate the visibility of the very real threats to human life, property, and ecosystems posed by extreme storms on the U.S. West Coast. This enhanced visibility will help increase the preparedness of the emergency management community and the public to such storms.
The key findings from the full ARkStorm Scenario report are summarized below:

Megastorms are California's other Big One. A severe California winter storm could realistically flood thousands of square miles of urban and agricultural land, result in thousands of landslides, disrupt lifelines throughout the state for days or weeks, and cost on the order of \$725 billion. This figure is more than three times that estimated for the ShakeOut scenario earthquake, that has roughly the same annual occurrence probability as an ARkStorm-like event. The \$725 billion figure comprises approximately \$400 billion in property damage and \$325 billion in business-interruption losses. An event like the ARkStorm could require the evacuation of 1,500,000 people. Because the flood depths in some areas could realistically be on the order of 10-20 ft, without effective evacuation there could be substantial loss of life.
An ARkStorm would be a statewide disaster. Extensive flooding is deemed realistic in the California Central Valley, San Francisco Bayshore, San Diego, Los Angeles and Orange Counties, several coastal communities, and various riverine communities around the state. Both because of its large geographic size and the state's economic interdependencies, an ARkStorm would affect all California counties and all economic sectors.
An ARkStorm could produce an economic catastrophe. 25% of buildings in the state could experience some degree of flooding in a single severe storm. Only perhaps 12% of California property is insured, so millions of building owners may have limited or no ability to pay for repairs. That degree of damage would threaten California with a long-term reduction in economic activity, and raise insurance rates statewide - perhaps nationwide or more - afterwards.
An ARkStorm is plausible, perhaps inevitable. Such storms have happened in California's historic record (1861-62), but 1861-62 is not a freak event, not the last time the state will experience such a severe storm, and not the worst case. The geologic record shows 6 megastorms more severe than 1861-1862 in California in the last 1800 years, and there is no reason to believe similar events won't occur again.
The ARkStorm is to some extent predictable. Unlike for earthquakes, we have the capability to partially predict key aspects of the geophysical phenomena that would create damages in the days before an ARkStorm strikes. Enhancing the accuracy, lead time, and the particular measures that these systems can estimate is a great challenge scientifically and practically.
Californian flood protection is not designed for an ARkStorm-like event. Much has been done to protect the state from future flooding, but the state's flood-protection system is not perfect. The existing systems are designed among other things to protect major urban areas from fairly rare, extreme flooding. The level of protection varies: some places are protected from flooding that only occurs on average once every 75 years; others, on average every 200 years. But the levees are not intended to prevent all flooding, such as the 500-year streamflows that are deemed realistic throughout much of the state in ARkStorm.
Planning for ARkStorm would complement planning for earthquakes. The ShakeOut exercise has become an annual activity in California, with more than 7 million people participating each year. Many of the same emergency preparations are useful for a severe winter storm: laying in emergency food and water, shelter preparations, exercising emergency corporate communications, testing mutual aid agreements, and so on.

