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Categorizing West Coast Storms


People in Sacramento navigate K Street in rowboats during the California flood of 1861-62, which historians say obliterated up to 25 percent of the assessed property value in the state. Photograph by Charles L. Weed.

While popular culture often portrays California as sunny, California residents are familiar with the rains that often batter their state in winter and spring. Measured in total rainfall over a three-day period, these West Coast storms are as big and as frequent as any in the United States, according to USGS research hydrologist Mike Dettinger, who studies the giant atmospheric rivers of water vapor flowing from west to east over the Pacific Ocean that trigger the biggest West Coast storms.

Because West and East coast storms have different characteristics and causes, they are difficult to compare using existing metrics, such as the Safir-Simpson scale used to classify hurricanes. Dettinger is co-leading the development of a simple classification system with Marty Ralph of the National Oceanic and Atmospheric Administration (NOAA) to help scientists and public officials compare East and West Coast storms, and to provide accurate and easily grasped warnings to West Coast residents when a big storm is headed their way.

Dettinger, a research hydrologist with the USGS National Research Program, studies the giant atmospheric rivers — found in the lowest 2 kilometers above Earth’s surface and stretching 400km wide on average, each carrying the equivalent of 10 to 20 Mississippi Rivers of water in the form of vapor — that are responsible for the biggest West Coast storms. When these atmospheric rivers encounter high mountains such as California’s Sierra Nevada, they are forced upward, cooling as they go, and their vapor condenses and falls as rain or snow. Precipitation from atmospheric rivers is double-edged for Californians, says Dettinger: It can manifest in destructive storms, but it also provides much of California’s water supply.

To quantify West Coast storms, Dettinger and Ralph propose a simple ranking called R-Cats, or Rainfall Categories. An “R-Cat-1” storm brings between 200 and 300 millimeters (approximately 8-12 inches) of rain in a three-day period. The scale rises in 100mm intervals to an R-Cat-4 storm, which unleashes more than 500mm (just under 20 inches) of rain in a three-day period. The R-Cat system is meant to be simple enough to facilitate communication in public and technical arenas. As a communication and research tool, it is meant to provide a clear, objective perspective on the severity of precipitation in West Coast storms, and potentially of storms elsewhere.

R-Cats is just part of USGS’ ongoing work to inform the public and management officials about the potential of extreme storms. In 2011, USGS and partners unveiled the ARkStorm Scenario, a hypothetical storm similar in magnitude to the intense West Coast winter storms of 1861 and 1862 that destroyed up to a quarter of California’s taxable property and rendered the state’s 480-kilometer- (300-mile) long Central Valley impassible. Today, the economic loss from such an event would total billions. To help emergency managers, universities, businesses, public agencies and others in planning for such natural disasters, ARkStorm (for “Atmospheric River 1,000 Storm”) modeled a storm that the West Coast might expect, on average, once every 500 to 1,000 years.

One implication recognized by ARkStorm’s creators was the need to develop consistent and easily understandable terms to communicate the risk and magnitude of West Coast storms to the public. Dettinger and Ralph, who were part of the more than 120 scientists on the ARkStorm team, and co-led the development of the meteorological part of the scenario, see the R-Cats system as a response to that need.

The Russian River at Guerneville, Calif., on Jan. 20, 2010, in the midst of a series of storms. Photograph by Stumptown Brewery.

The idea for a scaling system came to Dettinger and Ralph as part of their collaboration exploring the causes and impacts of extreme precipitation on the U.S. West Coast.  They were bad, locals asserted, but how bad were they in comparison, for example, to storms in Texas? They compared 30 years of daily precipitation reports from more then 5,800 weather stations across the United States and grouped them into R-Cat levels.  They found that nearly all the R-Cat-3 and -4 events occurred in California, Texas or the Southeastern states. Seventeen storms west of the 115th parallel during the 30-year reporting period were R-Cat-2 or higher – that is, they dropped more than 300mm (11.81 inches) of rain in three days – and all were associated with atmospheric rivers. Further, only in California did any stations report multiple R-Cat-3 or -4 episodes during those 30 years.

“Three-day precipitation extremes associated with landfalling ARs on the U.S. West Coast are heavier than extreme storms anywhere else in the country outside the southeast United States (including those related to landfalling tropical storms and hurricanes). Also, they yield comparable precipitation totals with the southeastern storms, and occur station-by station just as frequently as the extreme precipitation episodes elsewhere,” Dettinger and Ralph concluded in their paper in a recent Bulletin of the American Meteorological Society that explains the R-Cats system.

Today, atmospheric rivers are tracked from space using passive microwave sensors onboard polar orbiting satellites. In 2010, Dettinger said, forecasters were well prepared for the storms because of their improved understanding of the role of atmospheric rivers. Current research adds to the mix ground-based sensing of water vapor and soil moisture, among other factors, with the goal of giving federal, state and local forecasters and water managers a much more precise understanding of the West Coast’s water situation and the potential for floods.

“Knowing about the role of atmospheric rivers has already made a difference,” Dettinger said. “Not so long ago, it was the rare storm you’d hear about a week ahead of time. Now it’s not uncommon for weather forecasters to see storms forming near Japan a week ahead of time. In large part, that’s because they now know a little better what to look for: atmospheric rivers.”

“We’re not trying to replace older methods, just to add on our simple R-Cats categories, which offer a simple way of comparing the size of storms from place to place,” Dettinger said. Our goal was specifically to ask, How big are the biggest historical storms at places all over the country’ in ways that were simple and logical and that still allowed us to compare the absolute sizes of those largest storms. In doing so, as much a surprise to us as to everyone else we’ve ever shown this to, we found that California’s biggest storms are absolutely as big as the storms made by landfalling hurricanes in the American southeast. This is an amazing finding that had been lost in the return-interval and percentile reporting that has been the norm.”

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