9
Concluding Thoughts

The committee has reviewed a great deal of material related to flash flood forecasting and warning and NEXRAD coverage—mostly dealing with the specific case of the Los Angeles-Oxnard (LOX) Weather Forecast Office (WFO) and the Sulphur Mountain NEXRAD. The overall strategy of using NEXRAD to monitor the development, evolution, and movement of storms and to estimate the resulting precipitation over the radar coverage area seems well suited to the flash flood forecasting and warning mission of the NWS. The principal shortcoming of the system for radars sited in complex terrain, such as that around Los Angeles, is the essentially unavoidable gaps in radar coverage. These gaps are due primarily to the effects of Earth’s curvature or blocking of the radar beam by intervening terrain, and the consequent inability of the beam to reach down into some low-lying areas.

The current restriction of the NEXRAD scans to a minimum elevation angle of 0.5° exacerbates the problem. From the Sulphur Mountain radar altitude of 831 m (2726 ft) the axis of the radar beam at 0.5° elevation angle passes above 1.83-km (6000-ft) altitude beyond a distance of about 75 km from the radar site. The lower edge of the beam, as defined by the half-power or 3-dB point in the antenna pattern, passes above that altitude beyond about 125 km. Thus, precipitation below 6000 ft can be detected only within about 100–125 km from the Sulphur Mountain radar. This problem of detecting low-level precipitation is even worse for other NEXRADs, mostly in the western United States, which are sited at even higher altitudes than the Sulphur Mountain radar.

Despite the shortcomings posed by its elevated site, the Sulphur Mountain radar, in conjunction with its neighboring NEXRADs, consistently has detected heavy precipitation threatening the county warning areas served by the LOX WFO. The radar availability has exceeded, with rare exception, the National Weather Service (NWS) goal of greater than 96 percent avail-



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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California 9 Concluding Thoughts The committee has reviewed a great deal of material related to flash flood forecasting and warning and NEXRAD coverage—mostly dealing with the specific case of the Los Angeles-Oxnard (LOX) Weather Forecast Office (WFO) and the Sulphur Mountain NEXRAD. The overall strategy of using NEXRAD to monitor the development, evolution, and movement of storms and to estimate the resulting precipitation over the radar coverage area seems well suited to the flash flood forecasting and warning mission of the NWS. The principal shortcoming of the system for radars sited in complex terrain, such as that around Los Angeles, is the essentially unavoidable gaps in radar coverage. These gaps are due primarily to the effects of Earth’s curvature or blocking of the radar beam by intervening terrain, and the consequent inability of the beam to reach down into some low-lying areas. The current restriction of the NEXRAD scans to a minimum elevation angle of 0.5° exacerbates the problem. From the Sulphur Mountain radar altitude of 831 m (2726 ft) the axis of the radar beam at 0.5° elevation angle passes above 1.83-km (6000-ft) altitude beyond a distance of about 75 km from the radar site. The lower edge of the beam, as defined by the half-power or 3-dB point in the antenna pattern, passes above that altitude beyond about 125 km. Thus, precipitation below 6000 ft can be detected only within about 100–125 km from the Sulphur Mountain radar. This problem of detecting low-level precipitation is even worse for other NEXRADs, mostly in the western United States, which are sited at even higher altitudes than the Sulphur Mountain radar. Despite the shortcomings posed by its elevated site, the Sulphur Mountain radar, in conjunction with its neighboring NEXRADs, consistently has detected heavy precipitation threatening the county warning areas served by the LOX WFO. The radar availability has exceeded, with rare exception, the National Weather Service (NWS) goal of greater than 96 percent avail-

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California ability over the last several years. The LOX WFO record of flash flood warnings has been better in most respects than the national median performance, and the national database mistakenly includes some “missed events” that either were not flash flood cases or were actually covered by timely warnings. An apparent shortcoming of the LOX office in its average warning lead time compared to the national average is misleading, because warning times tend to be shorter in steep terrain such as that around the Los Angeles area. That is reflected in the NWS Western Region 2004 goal for flash flood warning lead time, which the LOX WFO has routinely exceeded. Based on all of these analyses, 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. Nonetheless, it is clear that better low-level coverage can be achieved from the Sulphur Mountain radar as well as other mountaintop radar sites. The straightforward use of lower minimum elevation angles in the scans would provide coverage down to lower altitudes at greater ranges in any direction not blocked by terrain. In the Sulphur Mountain case, this would extend the low-level coverage farther out over the ocean to the southwest, from where many winter storms approach, and over the main part of Los Angeles to the southeast. At 0.0° elevation, for example, the beam axis would be below 1.83 km (6000 ft) out to 125 km, while the lower edge of the beam would be below that level to well beyond 150 km. This would clearly enhance the ability to sense low-level precipitation to a greater range and should help improve the flash flood warning capability of the LOX office (e.g., by permitting warnings with greater lead times). Similar results can be anticipated for other mountaintop NEXRAD sites, and even slightly negative elevation angles would be useful in many cases. Finally, further improvements in flash flood warning capabilities are on the way, in both the radar and the flash flood forecasting arenas. NEXRAD precipitation products are continually being improved, and the forthcoming polarimetric modification to NEXRAD will improve rainfall estimates, especially in directions where the beam is partially blocked. Moreover, other technologies and instrumentation (e.g., phased-array radars) are being researched and tested; as these become feasible for use in the future, they should be brought to bear on the flash flood forecasting problem as appropriate. In addition, the Flash Flood Monitoring and Prediction (FFMP) program,

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California yet to be adapted for the area served by the LOX office, will help to improve the skill and areal specificity in flash flood forecasting. These and other forthcoming developments promise to improve the already fine record of the LOX office (as well as others) in the flash flood warning efforts.