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Introduction

Every year, floods are responsible for more deaths nationwide than any other weather phenomenon, with a 30-year average of more than 120 fatalities per year; of these, most are due to flash floods (NWS, 2004a). From 1996 through 2003, the average number of flash flood events recorded nationwide was nearly 3000 per year (NWS, 2004b). In comparison, only about 1000 tornadoes and 60 related deaths occur in the United States annually (Storm Prediction Center, 2004). Although Davis (2001) and the American Meteorological Society (BAMS, 2000) note that there have been recent improvements in flash flood forecasting due to better radar and satellite observations, flash floods continue to be among nature’s worst killers. Forecasting flash floods is especially challenging because a forecaster must predict not only the occurrence of the event but also its magnitude, for it is the amount of precipitation and the time frame in which it occurs that can transform an ordinary rainfall event into a deadly one. The challenge is particularly great in regions with complex terrain and in which storms approach over water where there are sparse, intermittent surface-level observations.

The National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service (NWS) is the government organization charged with protecting the nation from extreme weather events, including flash floods. The NWS has more than 120 Weather Forecast Offices (WFOs), each of which is responsible for a designated group of county warning areas. Combined, the WFOs are responsible for monitoring the entire United States and its territories. The WFOs have sole official authority to issue flash flood watches and warnings. Their forecast process is extensive, but the crux of flash flood forecasting lies in monitoring storm formation, movement, and precipitation patterns.

In the mid-1990s, a nationwide network of Weather Surveillance Radar-1988 Doppler (WSR-88D) radars was established. Termed the Next



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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California 1 Introduction Every year, floods are responsible for more deaths nationwide than any other weather phenomenon, with a 30-year average of more than 120 fatalities per year; of these, most are due to flash floods (NWS, 2004a). From 1996 through 2003, the average number of flash flood events recorded nationwide was nearly 3000 per year (NWS, 2004b). In comparison, only about 1000 tornadoes and 60 related deaths occur in the United States annually (Storm Prediction Center, 2004). Although Davis (2001) and the American Meteorological Society (BAMS, 2000) note that there have been recent improvements in flash flood forecasting due to better radar and satellite observations, flash floods continue to be among nature’s worst killers. Forecasting flash floods is especially challenging because a forecaster must predict not only the occurrence of the event but also its magnitude, for it is the amount of precipitation and the time frame in which it occurs that can transform an ordinary rainfall event into a deadly one. The challenge is particularly great in regions with complex terrain and in which storms approach over water where there are sparse, intermittent surface-level observations. The National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service (NWS) is the government organization charged with protecting the nation from extreme weather events, including flash floods. The NWS has more than 120 Weather Forecast Offices (WFOs), each of which is responsible for a designated group of county warning areas. Combined, the WFOs are responsible for monitoring the entire United States and its territories. The WFOs have sole official authority to issue flash flood watches and warnings. Their forecast process is extensive, but the crux of flash flood forecasting lies in monitoring storm formation, movement, and precipitation patterns. In the mid-1990s, a nationwide network of Weather Surveillance Radar-1988 Doppler (WSR-88D) radars was established. Termed the Next

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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,

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California including any plausible alternative approaches for flash flood forecasting; and identify lessons that could benefit forecasters, facility planners, and decision makers as they deal with other NEXRAD installations and similar instruments deployed in the future. The Committee to Assess NEXRAD Flash Flood Forecasting Capabilities at Sulphur Mountain, California, carried out this requested study during the first half of 2004. In doing so, the committee assessed the Sulphur Mountain radar and its availability, coverage, and use in flash flood forecasting; it also considered how this example could be applied to other NEXRADs located in regions of complex terrain for improved flash flood forecasting and warning. This report is the result of the considerations and deliberations undertaken by the committee. For the interested reader, Chapters 2 through 4 provide technical background information relevant to this study. Chapter 2 provides basic information about the physical processes of flash floods. Chapter 3 provides an overview of the NWS, its modernization effort, and the general process undertaken by NWS forecasters to issue flash flood forecasts, watches, and warnings. An extensive description of NEXRAD is presented in Chapter 4, including discussions of the radar characteristics and the network and its use for precipitation estimation. The reader more interested in the committee’s analysis, findings, and recommendations should see Chapters 5 through 9. Chapter 5 presents special challenges of low-level radar coverage in regions of complex terrain. Chapters 6 and 7 look specifically at Los Angeles and Ventura Counties, with the warning process presented in Chapter 6 and the analysis of the Sulphur Mountain radar and the flash flood warning performance of the Los Angeles-Oxnard NWS office in Chapter 7. Chapter 8 outlines potential improvements in flash flood warning capabilities, and finally, Chapter 9 provides the concluding thoughts of the committee.