The USGS Multi Hazards Demonstration Project (MHDP)’s second full scenario, called ARkStorm, addresses massive U.S. West Coast storms analogous to those that devastated California in 1861–62. Storms of this magnitude are projected to become more frequent and intense as a result of climate change.
The MHDP assembled experts 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, snowlines, wind, and pressure data the modelers will characterize the resulting floods, landslides, and coastal erosion and inundation. These hazards will then be translated into the 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.
Background on Massive Winter Storms
Beginning on Christmas Eve, 1861, and continuing into early 1862, an extreme series of storms lasting 45 days struck California. The storms caused severe flooding, turning the Sacramento Valley into an inland sea, forcing the State Capital to be moved from Sacramento to San Francisco for a time, and requiring Governor Leland Stanford to take a rowboat to his inauguration. William Brewer, author of “Up and down California,” wrote on January 19, 1862, “The great central valley of the state is under water—the Sacramento and San Joaquin valleys—a region 250 to 300 miles long and an average of at least twenty miles wide, or probably three to three and a half millions of acres!” In southern California lakes were formed in the Mojave Desert and the Los Angeles Basin. The Santa Ana River tripled its highest-ever estimated discharge, cutting arroyos into the southern California landscape and obliterating the ironically named Agua Mansa (Smooth Water), then the largest community between New Mexico and Los Angeles. The storms wiped out nearly a third of the taxable land in California, leaving the State bankrupt.
The 1861-62 series of storms were probably the largest and longest California storms on record. However, geological evidence suggests that earlier, prehistoric floods were likely even bigger. There is no scientific evidence to suggest that such extreme storms could not happen again. However, despite the historical and prehistorical evidence for extreme winter storms on the West Coast, the potential for these extreme events has not attracted public concern, as have hurricanes. The storms of 1861-62 happened long before living memory, and the hazards associated with such extreme winter storms have not tested modern infrastructure nor the preparedness of the emergency management community.
Building the ARkStorm Scenario
Atmospheric Rivers: Wind, Rain, and Waves
The nontechnical term “Pineapple Express” is popularly used to describe the meteorological phenomenon that causes moisture to be drawn from the Pacific Ocean near the equator and transported to the U.S. West Coast with firehose-like ferocity. Atmospheric rivers are embedded within much broader atmospheric storms referred to technically as “extratropical cyclones” (ECs). ECs are the winter-time analogue to hurricanes, but have much different structure. Also, they gain their energy largely from the pole-to-equator temperature contrast, unlike hurricanes, which draw their energy from ocean surface heat content. ARs are the business end of ECs because where the AR hits the mountains it can create extreme precipitation, flooding and high winds. In terms of impacts, an AR is to the broader EC it is embedded within, as the hurricane eyewall is to the broader hurricane of which it is a part. The importance and structure of ARs has become recognized recently through new satellite data and field experiments.
Because there is not yet a suitable scale for atmospheric rivers, the storm scenario is named “ARkStorm” to represent an atmospheric river (AR) with a value of 1,000 on a scale of atmospheric rivers to be determined by atmospheric scientists. The scenario storm then will be an “AR 1,000,” and other U.S. West Coast storms could be scaled in comparison. One of the outcomes of this project currently being is the furthering of a common nomenclature for West Coast Winter Storms. For more information on Atmospheric Rivers, visit NOAA’s Physical Science Divisions page on the topic.
Though the ARkStorm Scenario report is online, below are some of the key findings of the project:
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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. (USGS/URBAN EARTH)
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