Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California (2005)

In early 2004, the Committee to Assess NEXRAD Flash Flood Forecasting Capabilities at Sulphur Mountain, California, was formed in response to a 2003 congressional request from U.S. Senator Barbara Boxer (California) and with support from the National Oceanic and Atmospheric Administration (NOAA). The committee’s general task was to assess the effectiveness of operating Next Generation (weather) Radars (NEXRADs) in complex terrain to support the National Weather Service (NWS) in its task of forecasting heavy precipitation events and issuing flash flood forecasts, watches, and warnings. The committee conducted a specific analysis of the Sulphur Mountain NEXRAD located in Ventura County, California, and also considered how flash flood forecasting could be improved for other NEXRADs sited in complex terrain.

After conducting a thorough analysis of the availability of the Sulphur Mountain NEXRAD, the coverage provided by the radar, and the performance of the Los Angeles-Oxnard (LOX) NWS Weather Forecast Office (WFO) 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. In addition, the committee’s examination of the radar availability shows that, from the time of the release in October 1998 of the General Accounting Office report about this radar through December 2003, the average

availability of the Sulphur Mountain radar exceeded the availability requirements established by the NWS.

The committee performed its own calculations of low-level radar coverage using 30-m digital terrain elevation model data, considering the full extent of the radar beam (i.e., not just the axis, as in some prior studies) and assuming standard atmospheric propagation conditions. An integrated analysis of the coverage provided by the Sulphur Mountain NEXRAD and the adjoining Vandenberg Air Force Base, Santa Ana Mountain, Edwards Air Force Base, and San Diego NEXRADs reveals overlapping coverage for much of the area. However, there is an area southwest of the Sulphur Mountain radar over the Pacific Ocean that is covered exclusively by this radar. Low-level coverage of this area from which storms often approach is important for monitoring incoming storms and assessing their flash flood potential before they move onshore. Although the center of the Sulphur Mountain radar beam at the current minimum antenna elevation angle of 0.5° rises above 1.83-km (6000-ft) altitude beyond 75 km from the radar site, half of the radar beam is below that level at this range and has enabled detection of approaching storms. The committee did a hypothetical analysis with the radar antenna lowered to 0.0° and found that this would provide beam-axis coverage at or below 1.83 km (6000 ft) out to 125 km.

The committee examined the LOX WFO’s flash flood warning statistics and found that it has an excellent record of issuing flash flood warnings. 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. LOX’s ratio of flash flood events that were forecast but failed to materialize (called the false alarm ratio) is less than the national average.

The average lead time for flash flood warnings from LOX is less than the national average. The national statistics, however, are skewed by the large numbers of WFOs that forecast for regions with little to no relief in the surrounding topography. A more accurate assessment of the LOX WFO’s performance is gained by comparing LOX with the 2004 goals of the NWS Western Region, which are established by considering the complex terrain in the western United States and thus the greater tendency for rapid onset of flash flooding. LOX’s average lead time, as well as the percentage of forecast events and the false alarm ratio, is superior to the 2004 Western Region

goals. Based on these findings, the committee concludes that the LOX office is performing its flash flood warning mission in an effective manner.

Overall, the committee finds that the Sulphur Mountain radar is appropriately sited to detect approaching storms while avoiding problems with anomalous propagation of the radar signals. The radar is amply functional and has provided crucial support to the Los Angeles-Oxnard forecasters in their mission to monitor, predict, and warn of precipitating events and flash floods.

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.

Providing broad low-level radar coverage, especially in complex terrain, presents a range of challenges. The coverage from radar sites at low levels is limited by Earth’s curvature and often blocked in many directions by the surrounding terrain. Coverage from elevated sites, on the other hand, may not extend down into many valley areas. The latter problem is exacerbated by the current restriction of the NEXRAD system to minimum elevation (tilt) angles of 0.5°.

Recommendation: The NWS should improve nationwide NEXRAD coverage of low-level precipitation and wind, especially for elevated radar sites in complex terrain, through the adoption of a modified scan strategy that will allow scanning at lower elevation angles (Chapter 8¹).

The use of lower, and perhaps even negative, elevation angles would allow monitoring of precipitation and wind at lower altitudes and, hence, would provide a more representative assessment of near-surface rainfall

rates. Flexible selection of elevation angle steps would allow greater ability to avoid terrain blockage and to capture low-level meteorological phenomena. 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) and Department of Defense (DoD) NEXRAD radars; FAA Terminal Doppler Weather Radars (TDWRs) and other surveillance radars equipped to provide weather-echo data; local television station Doppler radars; and operational radars from other organizations (Chapter 8).

High-quality, real-time weather radar data are becoming more widespread in the United States. Although some of these data may not be of the same quality as NEXRAD data, they should be made accessible to NWS forecasters to increase the area and density of coverage of weather radar data—especially in regions of complex terrain—to improve forecasts, watches, and warnings. For radars located along the coast, this could provide a valuable extension of the offshore coverage. The high temporal and spatial resolution of the TDWR data should be of particular value in flash flood forecasting and warning for the major urban areas near which those radars are situated. Data formats and standards must be adhered to and efforts must be made to ensure data quality. This recommendation is consistent with previous National Research Council reports (NRC, 2002, 2003).

Recommendation: The NWS should consider augmenting the NEXRAD network with additional short-range radars to improve observation of low-level meteorological phenomena (Chapter 8).

The NEXRAD is designed for long-range coverage with a single-beam antenna that provides coverage of a large volume of the atmosphere. Although this is adequate for many situations, NEXRAD coverage at low altitudes far away from the radar can be insufficient due to Earth’s curvature and terrain-induced blockage. Additional temporary local problems may arise, for example, as in areas affected by recent wildfires. Some of these problems can be resolved by deploying additional radars, which could be smaller, cheaper, and more easily (and even adaptively) deployable than NEXRAD. To maximize their use, these systems should be networked together, data formats standardized, and metadata established.

Flash floods are meteorological, hydrological, and hydraulic events that often have economic and societal implications. Therefore, an effective flash flood forecaster must adequately monitor and predict an event based on environmental factors and must communicate the warning in a manner that is useful to stakeholders. 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) to enhance the effectiveness of flash flood forecasts, watches, and warnings (Chapter 6).

The committee commends the Los Angeles-Oxnard WFO on its efforts to communicate with local emergency management officials and the media about potential flash flood situations. However, the committee was informed that the criteria for issuing flash flood warnings are not always sufficient to address all the specific needs of a WFO’s end-user community. Existing and new tools—such as geographic information systems (GIS) and the Flash Flood Monitoring and Prediction (FFMP) program, polarimetric radar, and ensemble probabilistic modeling—will make it possible to provide warnings with improved specificity. Moreover, forecasts formulated in a manner analogous to the NWS convective outlooks and with an event severity index analogous to the Fujita scale for tornadoes would provide useful indications of the magnitude of threats. Flash flood forecasts couched in probabilistic terms also would be a useful way to convey uncertainty information. Advanced technologies enable new ways of displaying and communicating individualized warnings to improve the efficiency of the warning process. In the spirit of the recent National Research Council report, Fair Weather (NRC, 2003), this should be done in cooperation with the private sector. In addition, in partnership with the media and emergency management officials, the NWS should continue to educate the public about the risks of flash flooding.

Recommendation: Evaluation of flash flood warnings should be based on their contributions to improved decision making and should employ metrics that take account of the magnitude and scale of the events and the

increasing specificity of the warnings. The NWS should improve the database of flash flood events and warnings to include more complete and accurate listings of both warnings and events (Chapter 7).

The traditional national measures for evaluating flash flood warnings should be augmented to better take into account the myriad needs of the local population at risk. For example, the value attributed to the lead time of a warning should take into account factors such as the response time of a basin. In addition, warnings indicating the magnitude of the threat and designating specific regions within county warning areas are more useful to both the intended responders and others within the same county who are not affected by the warning. Evaluation statistics such as the probability of detection (POD) and the false alarm ratio (FAR) could be stratified by the scale of the events. The warning and event databases should be adapted to capture the increased specificity of the warning capability, in contrast to the current practice of recording warnings and events at the county level, with little indication of magnitude and impact.

Establishing a reliable database of events for verification purposes involves well-recognized difficulties, but recording the warnings issued is (or should be) a straightforward process. Nevertheless, situations involving a series of overlapping warnings, or extensions of warnings, and possibly multiple flash flood events within a county appear not to be well represented in the national database. This results, at least partly, from the rules employed in establishing that database; such rules may be required to ensure uniformity that permits comparisons across the nation. However, a more comprehensive listing would be useful for detailed analyses.

A flash flood by definition is a rapidly evolving event. 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 precipitation alone may not provide the most effective warning capability, but techniques such as the Flash Flood Monitoring and Prediction program will provide improvements. There also are gains to be made from rapidly evolving capabilities for better short-term and fine-scale forecasting, using regional and local numerical models that ingest real-time observations. In addition to these near-term improvements, consideration of hydrologic factors should be an integral part of future radar siting.

Recommendation: To increase the accuracy and lead time of flash flood forecasts and warnings, the NWS should continue to implement new technologies and techniques including (a) the Flash Flood Monitoring and Prediction program at all Weather Forecast Offices, (b) polarimetric modifications to NEXRAD, (c) data assimilation systems that integrate radar and other operational datasets into coupled hydrometeorological and hydrological models, and (d) data fusion systems (Chapter 8).

Extensive opportunities exist for forecasters to take advantage of the rapid advancement of technology to improve forecasts, watches, and warnings. 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. The polarimetric modification would improve the data quality and quantitative precipitation measurement capabilities of NEXRAD. Real-time data assimilation systems that incorporate observations into high-resolution mesoscale numerical models provide rapidly updated wind and precipitation forecasts. Data fusion systems, such as the Auto-Nowcast system, incorporate all available observation datasets together with numerical model output to produce very short range (0- to 2-hour), site-specific forecasts. These advances can produce improved forecasts, including ensemble and probabilistic forecasts, of precipitation rate and accumulation that are essential for flash flood forecasting. To enhance the usefulness of the forecast, quantification of uncertainty (e.g., probability forecasts) should be an integral component of these products (NRC, 2003).

Recommendation: In addition to the original NEXRAD siting considerations, future siting of radars in complex terrain should include detailed assessments of coverage in areas at risk for flash flooding (Chapter 8).

The original NEXRAD siting procedures considered primarily meteorological processes, radar coverage, and ground clutter. Increased understanding of hydrologic processes of runoff production and streamflow response, combined with the application of radar to real-time hydrologic prediction for individual catchments (e.g., FFMP), enables incorporation of hydrologic aspects of the land surface into future siting processes. Readily available detailed digital information on topography and other relevant spatial information now make it possible to analyze a radar’s coverage of the near-

ground portion of the atmosphere using GIS technology. The potential for complete and partial radar beam blockage can be evaluated in the context of hydrologic basins for which coverage is sought. Basin size, average slope, orientation with respect to the movement of dominant weather patterns, 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.