"http://news.yahoo.com/blogs/lookout/scientists-warn-california-could-struck-winter-superstorm-20110117-064937-458.html

"A group of more than 100 scientists and experts say in a new report that California faces the risk of a massive "superstorm" that could flood a quarter of the state's homes and cause $300 billion to $400 billion in damage. Researchers point out that the potential scale of destruction in this storm scenario is four or five times the amount of damage that could be wrought by a major earthquake."   

"California's geological history shows such "superstorms" have happened in the past  --  212, 440, 603, 1029, 1418, and 1605."  

"The most recent superstorm occurred in the winter of 1861-1862".  

"The risk is gathering momentum now, scientists say, due to rising temperatures in the atmosphere, which has generally made weather patterns more volatile."    

From the U.S. Geological Survey, Multi Hazards Demonstration Project (MHDP)  

http://urbanearth.gps.caltech.edu/winter-storm-2 /

"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.

"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 Shake Out 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 Shake Out 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.