Flash Flood Forecasting Over Complex Terrain With an Assessment of the Sulphur Mountain NEXRAD in Southern California (2005) / Chapter Skim
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Executive Summary
Executive Summary
Pages 1-8
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as measured by the flash flood warning statistics, the committee found little basis for concerns regarding the operational effectiveness of the Sulphur Mountain radar. The committee's review of atmospheric inversion height statistics off the coast of Southern California and anomalous propagation effects on neighboring NEXRADs indicates that the current placement of the Sulphur Mountain radar above the mean inversion height was justified.
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The proportion of flash flood events with advance warning and the average warning lead time more than doubled following the commissioning of NEXRADs nationwide, including the Sulphur Mountain radar. When the LOX statistics are compared with those of the other 115 WFOs throughout the continental United States, their percentage of flash flood events with advance warnings, at 79 percent, is better than the national average of 69 percent.
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RECOMMENDATIONS Despite the committee's findings that the LOX office is accomplishing its mission and that the Sulphur Mountain radar is adequately sited and functional to fulfill its purpose, there are several ways in which flash flood forecasting and warning can be improved, not only in Southern California and in other regions where NEXRADs are sited in complex terrain, but throughout the country. Enhancing Weather Radar Coverage Providing broad low-level radar coverage, especially in complex terrain, presents a range of challenges.
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The NWS should make necessary hardware and software changes to the NEXRAD system to allow this type of modified scan strategy at the Sulphur Mountain site and other NEXRAD installations nationwide. Recommendation: To extend radar coverage, all available regional real time weather radar data should be made accessible to the NWS WFOs, including Federal Aviation Administration (FAA)
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Evaluation procedures need to be refined to reflect better the skill and value of the warnings. Recommendation: NWS Weather Forecast Offices nationwide, including the Los Angeles-Oxnard Weather Forecast Office, should continue to expand their collaborative efforts with key stakeholders (e.g., county, police, and emergency management officials)
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Radars can observe the evolving weather systems that cause flash floods as they move into areas of concern, and future NEXRAD enhancements will improve the quality of those observations. The measurements of accumulating precipita tion alone may not provide the most effective warning capability, but tech niques such as the Flash Flood Monitoring and Prediction program will provide improvements.
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From page 7... ...
The FFMP system, which requires adaptation to the specific watersheds served by each WFO, would facilitate more specific flash flood warnings. In addition, as part of its new Advanced Hydrologic Prediction Services, the NWS is encouraged to continue its effort to develop and evaluate hydrologic and coupled meteorological-hydrologic models to advance technologies useful for improved flash flood guidance and warnings.
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Basin size, average slope, orientation with respect to the movement of dominant weather pat terns, characteristics of soil, land cover and land use, and channel hydraulic aspects determine the amount of rain that is likely to cause flash flooding and where it may occur. These characteristics, together with local hazard vulnerabilities, can help determine priorities of site selection.
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