As people in northern California begin to assess damage from the high winds and heavy rain of October 13 and 14, they may wonder what hit them. The answer, according to Dave Reynolds, meteorologist-in-charge at NOAA's National Weather Service forecast office in Monterey, was an "atmospheric river," the meteorological phenomenon that draws water vapor from the Pacific Ocean near the equator and transports it to the U.S. West Coast with firehose-like ferocity.
Atmospheric Rivers, or “ARs,” are technically associated with cyclones,” says Dr. Marty Ralph, Research Meteorologist at NOAA's Earth System Research Laboratory in Boulder. "Although these cyclones are not of the iconic circular pattern commonly associated with hurricanes, they do include hurricane-strength winds and can yield rainfall comparable to that of hurricanes. The ARs in these cyclones are regions where low-altitude winds transport huge amounts of water vapor up against the mountains, which wrings out extreme rainfall." The AR this week produced a remarkable 21 inches of rain in just one day in the mountains near Big Sur, and winds that exceeded hurricane force.
While the atmospheric river that hit central California on October 13 and 14 was large, breaking some daily rainfall records and streamflow records for this time of year on several California creeks and rivers, including the Salinas River, the Russian River, and the Merced River, according to USGS Research Hydrologist, Dr. Michael Dettinger, in many cases, the results can be much larger, especially later in the winter. Nonetheless, this AR would have been notably long and strong any time of year.
For weather experts, storms this large always bring to mind the historically massive storms that impacted both northern and southern California in 1861 and 1862, flooding the Central Valley of California, obliterating at least one community in southern California, and causing the state capital to me moved from Sacramento to San Francisco. According to scientists, storms of this magnitude will eventually happen again.
To address storms of this magnitude and help prepare emergency responders and resource managers, the USGS Multi-Hazards Demonstration Project, creators of the ShakeOut Earthquake Scenario, are working with researchers like Ralph, Dettinger and Reynolds to construct a massive, though scientifically plausible, hypothetical storm scenario, called "ARkStorm."
The ArkStorm scenario has adapted data and information from recent U.S. West Coast storms to simulate the type of storms that impacted northern and southern California in 1861 and 1862. With the atmospheric scenario completed, experts are now examining in detail the probability, cost, and consequences of floods, landslides, coastal erosion and inundation; debris flows; environmental effects; agricultural loss; and possible physical damage such as bridge scour, road closures, dam failure, property loss, and levee-system collapse. The ARkStorm scenario is scheduled to be the basis of statewide emergency response drill in 2011.
"There is no specific scale for categorizing atmospheric rivers,” said Dale Cox, Project Manager of the USGS Multi-Hazards Demonstration Project. “We tend to call West Coast storms by the year they occurred, like 1969, 1986 and 1997. ARkStorm represent an atmospheric river (AR) with a value of 1,000 on a scale of atmospheric rivers yet to be determined by atmospheric scientists."
In addition to the ARkStorm scenario, the USGS Multi-Hazards Demonstration Project led the creation of the ShakeOut Earthquake Scenario that became the basis of the Great ShakeOut earthquake preparedness drill last fall. They are also responsible for the multi-hazards, multi-agency, and multi-scientific discipline approach behind the recent debris flow assessment and warning system being used at the Station Fire in southern California.
"There was a unnerving convergence of coincidence, danger and irony as this atmospheric river roiled into California, threatening to cause massive debris flows, on the day of the annual Great California ShakeOut, " said Lucy Jones, Chief Scientist of the USGS Multi-Hazards Demonstration Project, who's home is directly below the Station Fire burn perimeter. "We just dodged a bullet."