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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California
Generation Radar, or NEXRAD, this radar was a significant improvement over the previous NWS weather radars introduced in 1957 and 1974. There are more than 130 NEXRADs in the United States; with its increased coverage, denser network, greater spatial and temporal resolution, and improved precipitation products, NEXRAD became the major foundation for modern short-term precipitation forecasting. Nonetheless, questions exist about the efficacy of NEXRADs sited in complex terrain because of the potential for (1) gaps in radar coverage due to beam blockage by higher surrounding terrain and (2) lack of radar coverage at low levels of the atmosphere if the radar is sited at a higher altitude relative to surrounding terrain.
In particular, the Sulphur Mountain NEXRAD located north of Los Angeles in Ventura County, California, has been a subject of controversy in terms of its ability to detect precipitation events below 1.83-km (6000-ft) altitude and its use to assist local NWS meteorologists in forecasting and warning of flash flood events. Many analyses of the Sulphur Mountain radar have been conducted (e.g., Paris 1997a, 1997b, 1998, 2001; Rose Institute, 1997, 1998; GAO, 1998; NWS, 2001a; Thompson, 2001; Fox, 2003), but questions persisted about its usefulness. Consequently, in February 2003, an appropriations bill was passed containing a clause—based on a request from U.S. Senator Barbara Boxer—directing that NOAA commission the National Academy of Sciences to conduct an objective study to assess the availability, performance, and capability of the Sulphur Mountain radar to detect heavy precipitation and aid forecasters at the NWS’s Los Angeles-Oxnard WFO in providing flash flood forecasts and warnings (Appendix A). The specific charge to the study committee was to
describe the overall strategy of the NEXRAD radars in support of the NWS flash flood warning and forecast mission and discuss strengths and weaknesses of the system for operations in complex terrain;
assess the availability, performance, and capability of the NWS NEXRAD located on Sulphur Mountain in Ventura County, California, to detect heavy precipitation and aid forecasters at the Los Angeles WFO in providing flash flood warnings and forecasts and fulfilling its other intended purposes;
assess how the Sulphur Mountain NEXRAD radar’s location affects its capability to detect low-level storm events (i.e., below 6000 ft);
provide conclusions about strengths and weaknesses and make recommendations to improve the accuracy and timeliness of flash flood warnings in and around western Los Angeles and Ventura Counties, California,