The following article, “Mysterious Atmospheric River Soaks California, Where Megaflood May Be Overdue” is by Mark Fischett, Scientific American – Fri, Nov 30, 2012:
Northern California is experiencing the first days of what weather forecasters are warning will be a long series of torrential rainstorms that could cause serious flooding across the northern one-third of the state. The relentless storms are being driven by a feature in the atmosphere you have probably never heard of: an atmospheric river.
Oh, and another atmospheric river created the worst flooding since the 1960s in western England and Wales this past week, where more than 1,000 homes had to be evacuated.
An atmospheric river is a narrow conveyor belt of vapor about a mile high that extends thousands of miles from out at sea and can carry as much water as 15 Mississippi Rivers. It strikes as a series of storms that arrive for days or weeks on end. Each storm can dump inches of rain or feet of snow. For more details, see this feature story that Scientific American has just published, written by two experts on these storms. Scientists discovered atmospheric rivers in 1998 and have only recently characterized them fully enough to allow forecasters to warn of their arrival. They can strike the west coasts of most continents, but California seems to be a prime target. As many as nine small atmospheric rivers reach the state each year, each lasting two to three days, including the famous “pineapple express” storms that come straight from the Hawaii region of the Pacific Ocean. Ironically, although the storms are dangerous, they are also vital; they supply 30 to 50 percent of California’s rain and snow–in the span of about 10 days a year.
The real scare, however, is that truly massive atmospheric rivers that cause catastrophic flooding seem to hit the state about once every 200 years, according to evidence recently pieced together (and described in the article noted above). The last megaflood was in 1861; rains arrived for 43 days, obliterating Sacramento and bankrupting the state. The disaster is largely forgotten, but the same region is now home to more than six million people. Simulations of a 23-day storm there indicate that more than $400 billion of damage and losses would occur, far surpassing the $60 billion estimates for Hurricane Sandy’s effects. New research also shows that climate change may make these storms more likely to occur.
You may begin to hear the term “atmospheric river” more often. The Weather Channel is using it, in quotation marks, in warnings for northern California, as well as the coasts of Oregon and Washington. And some popular media are beginning to adopt the verbiage as well.
Editor’s note: The USGS simulation alluded to above is part of the U.S. Geological Survey, Multi Hazards Demonstration Project (MHDP) which uses hazards science to improve resiliency of communities to natural disasters including earthquakes, tsunamis, wildfires, landslides, floods, and coastal erosion. The project engages emergency planners, businesses, universities, government agencies, and others in preparing for major natural disasters. The project also helps to set research goals and provides decision-making information for loss reduction and improved resiliency. The first public product of the MHDP was the ShakeOut Earthquake Scenario published in May 2008. This detailed depiction of a hypothetical magnitude 7.8 earthquake on the San Andreas Fault in southern California served as the centerpiece of the largest earthquake drill in United States history, involving over 5,000 emergency responders and the participation of over 5.5 million citizens.
This document summarizes the next major public project for MHDP, a winter storm scenario called ARkStorm (for Atmospheric River 1,000). Experts have designed a large, scientifically realistic meteorological event followed by an examination of the secondary hazards (for example, landslides and flooding), physical damages to the built environment, and social and economic consequences. The hypothetical storm depicted here would strike the U.S. West Coast and be similar to the intense California winter storms of 1861 and 1862 that left the central valley of California impassible. The storm is estimated to produce precipitation that in many places exceeds levels only experienced on average once every 500 to 1,000 years.
Extensive flooding results. In many cases flooding overwhelms the state’s flood-protection system, which is typically designed to resist 100- to 200-year runoffs. 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, hurricane-force winds. Across wider areas of the state, winds reach 60 miles per hour. 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 property repair costs, meaning that an ARkStorm could cost on the order of $725 billion, which is nearly 3 times the loss deemed to be realistic by the ShakeOut authors for a severe southern California earthquake, an event with roughly the same annual occurrence probability (source). (Reference: Porter, Keith, Wein, Anne, Alpers, Charles, Baez, Allan, Barnard, Patrick, Carter, James, Corsi, Alessandra, Costner, James, Cox, Dale, Das, Tapash, Dettinger, Michael, Done, James, Eadie, Charles, Eymann, Marcia, Ferris, Justin, Gunturi, Prasad, Hughes, Mimi, Jarrett, Robert, Johnson, Laurie, Dam Le-Griffin, Hanh, Mitchell, David, Morman, Suzette, Neiman, Paul, Olsen, Anna, Perry, Suzanne, Plumlee, Geoffrey, Ralph, Martin, Reynolds, David, Rose, Adam, Schaefer, Kathleen, Serakos, Julie, Siembieda, William, Stock, Jonathan, Strong, David, Sue Wing, Ian, Tang, Alex, Thomas, Pete, Topping, Ken, and Wills, Chris; Jones, Lucile, Chief Scientist, Cox, Dale, Project Manager, 2011, Overview of the ARkStorm scenario: U.S. Geological Survey Open-File Report 2010-1312, 183 p. and appendixes.)
Addenda: “Little-known fact, but Goleta Slough used to be called Goleta Bay because it was a fully-navigable body of water – Juan Rodriguez Cabrillo sailed around it in the 1542 – and after heavy grazing of cattle on the plains adjacent to 1861, the rains of 1861/62 washed so much sediment into the Bay that it changed it forever. The military completed the fill-in using much of Mescalitan Island during WWII. Remnants of the earthen landing of the old whaling station that lasted until 1870 can still be seen as you drive up the 217 ramp to campus, on your right-hand side” (Dylan Parenti, private correspondence).
