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News from the National Academies
Date: Oct. 13, 2004
Contacts: Bill Kearney, Director of Media Relations
Chris Dobbins, Media Relations Assistant
Office of News and Public Information
202-334-2138; e-mail <news@nas.edu>

FOR IMMEDIATE RELEASE

Sulphur Mountain Radar Is Effective at Forecasting Flash Floods,
But National Weather Service Could Improve Use of 'Next Generation Radars'

WASHINGTON -- A National Weather Service radar atop Sulphur Mountain in Southern California is well-placed to detect approaching storms, operates effectively, and helps the Los Angeles-Oxnard forecast office of NWS monitor, predict, and warn of flash floods and other events related to heavy precipitation, says a new report from the National Academies' National Research Council. But there is room for improvement in how NWS uses such radars to forecast flash floods and issue warnings about them across the country – including areas that, like Southern California, have complex, hilly terrain.

"The Sulphur Mountain radar system fulfills its purpose, and local NWS forecasters have indeed benefited from it," said Paul L. Smith, professor emeritus, atmospheric sciences department, South Dakota School of Mines and Technology, Rapid City, and chair of the committee that wrote the report. "Even so, our committee made several recommendations that, if taken, would enhance the usefulness of such radars – especially in the forecasting of flash floods nationwide, which are among nature's worst killers."

Sulphur Mountain in Ventura County is the site of one of the weather service's more than 100 Next Generation Radars (NEXRADs). The U.S. Department of Defense and the Federal Aviation Administration also operate NEXRADs, but only the weather service's weather forecast offices have authority to issue flash-flood watches and warnings. The radars, designed for long-range coverage, measure wind velocity, track the movement and intensity of storms, and generate data and pictures. But gaps in coverage often occur, especially at low elevations, because of the Earth's curvature or when NEXRADs are situated in areas with complex terrain – topography with a variety of features that may block a radar beam's ability to reach low-lying areas. NWS should address this issue, the report says, by making technology adjustments that would allow its radars to scan at lower minimum-elevation angles, which currently are restricted to 0.5 degrees.

Likewise, NWS should consider supplementing its NEXRAD network with short-range radars to improve observation of low-lying areas, the report says. When making decisions about proposed NEXRAD sites, NWS also should conduct assessments of how well the radar would work in particular areas that are at risk for flash floods.

New technologies and techniques that increase the accuracy of flash-flood forecasts and the lead time of warnings should continue to be adopted, the report says. If NWS installed its Flash Flood Monitoring and Prediction program at all weather forecast offices, for example, more specific warnings could be issued. Additionally, NWS should include more detailed information in its database of flash-flood warnings and events to aid decision-making and the evaluation of weather forecasting.

Many people who live near the Sulphur Mountain NEXRAD have questioned its ability to detect precipitation at low elevations. Some also have questioned whether the radar is of much use to meteorologists at the Los Angeles-Oxnard forecast office. The committee performed detailed calculations of low-level radar coverage and analyzed flash-flood warning records from the Los Angeles-Oxnard office.

The Sulphur Mountain radar is located at an elevation of 2,726 feet, an altitude that is necessary to protect the radar beam from disruptions caused by abnormal variations in atmospheric conditions such as temperature and air pressure. At this location, it also provides exclusive radar coverage over a part of the Pacific Ocean from which storms often approach. The radar can detect precipitation below 6,000 feet, and, contrary to what had been previously thought, it can do so up to 78 miles away. This capability has enabled consistent detection of approaching storms and heavy precipitation, the report says. And with rare exceptions, the radar's "availability" in recent years has exceeded the NWS goal, which is a rating greater than 96 percent. Availability is defined as the time that a radar system is operating satisfactorily, expressed as a percentage of the time that it is required to operate.

The Los Angeles-Oxnard forecast office has a commendable record of issuing flash-flood warnings, the committee found. The percentage of flash floods with advance warnings from this office is better than the national average for all NWS weather forecast offices in the continental United States – 79 percent compared with 69 percent. Also, its average lead time for flash-flood warnings, its "false-alarm ratio" for floods that were forecast but never materialized, and its probability of detecting flash-flood events have surpassed goals that NWS set for its western region this year.

To enhance forecasts, flood watches, and warnings, weather forecast offices across the country should have access in real time to all available weather radar data in a given region, the committee said. Information sources should include the FAA, Department of Defense, and local television stations with Doppler radars.

The study was sponsored by the National Oceanic and Atmospheric Administration. The National Research Council is the principal operating agency of the National Academy of Sciences and the National Academy of Engineering. It is a private, nonprofit institution that provides science advice under a congressional charter. A committee roster follows.

Copies of Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California will be available later this fall from the National Academies Press; tel. 202-334-3313 or 1-800-624-6242 or on the Internet at http://www.nap.edu. Reporters may obtain a pre-publication copy from the Office of News and Public Information (contacts listed above).

[ This news release and report are available at http://national-academies.org ]

NATIONAL RESEARCH COUNCIL
Division on Earth and Life Studies
Board on Atmospheric Sciences and Climate

Committee to Assess NEXRAD Flash Flood Forecasting Capabilities
at Sulphur Mountain, California

Paul L. Smith (chair)
Professor Emeritus
Institute of Atmospheric Sciences
South Dakota School of Mines and Technology
Rapid City

Ana Barros
Professor
Department of Civil and Environmental Engineering
Duke University
Durham, N.C.

V. Chandrasekar
Professor
Department of Electrical and Computer Engineering
Colorado State University
Fort Collins

Greg Forbes
Severe Weather Expert
The Weather Channel Inc.
Atlanta

Eve Gruntfest
Professor
Department of Geography and Environmental Studies
University of Colorado
Colorado Springs

Witold Krajewski
Professor of Water Resources Engineering
IIHR-Hydroscience and Engineering
University of Iowa
Iowa City

Thomas Potter
Research Professor Emeritus
Department of Meteorology
University of Utah
Salt Lake City

Rita Roberts
Associate Scientist
Research Applications Laboratory
National Center for Atmospheric Research
Boulder, Colo.

Matthias Steiner
Senior Research Scientist
Department of Civil and Environmental Engineering
Princeton University
Princeton, N.J.

Roger Wakimoto
Professor
Department of Atmospheric Sciences
University of California
Los Angeles

RESEARCH COUNCIL STAFF

Julie Demuth
Study Director