Flash Flood Forecasting Over Complex Terrain With an Assessment of the Sulphur Mountain NEXRAD in Southern California (2005) / Chapter Skim
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7 Evaluation of the Sulphur Mountain Radar and Flash Flood Warnings in Los Angeles and Ventura Counties
7 Evaluation of the Sulphur Mountain Radar and Flash Flood Warnings in Los Angeles and Ventura Counties
Pages 91-116
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From page 91... ...
This chapter provides a comprehensive analysis of coverage provided by the Sulphur Mountain NEXRAD and its availability to address these concerns. RADAR COVERAGE Sulphur Mountain Figure 7.1 shows the relief in the 50-km vicinity of the Sulphur Mountain radar location.
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From page 92... ...
To the south, a small South Mountain separates Sulphur Mountain, at an elevation of 829 m, from the coastal Santa Monica Mountains that reach elevations greater than 800 m MSL. The surrounding orography causes blockage of the beam from the Sulphur Mountain NEXRAD in some directions, especially to the north.
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From page 93... ...
reflectivity data from the Sulphur Mountain radar and concluded that our calculations well represent the essential features of the radar coverage. The committee performed the calculations for three antenna elevation angles: 0.5°, 0.0°, and 0.5°.
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From page 94... ...
. In the direction of the open ocean at the same antenna elevation angle and at a range of about 100 km from the radar, the beam "illuminates" a portion of the atmosphere that extends from 1.5 km to slightly more than 1The mean-sea-level contour of Earth's surface curves down and away from any given point.
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From page 95... ...
Some researchers (Brown et al., 2002; Wood et al., 2003a, 2003b) have suggested using 0.0° or even negative elevation angles for radars sited at higher elevation, so the committee explored this concept for the Sulphur Mountain radar.
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From page 96... ...
negative elevation angles could be used to extend the low-level coverage farther out over the water, though with a concomitant increase in beam blockage in some directions. In addition, there would be some increase in side-lobe clutter.
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From page 97... ...
All 1080 of the calculations are available electronically from the following National Academies web site: http://dels.nas.edu/basc/ nexradsm/radar_beam_and_terrain_viewer.html. This interactive presentation provides the user an opportunity to see how the Sulphur Mountain radar beam propagation and power loss change with direction based on three different beam elevation angles.
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From page 98... ...
Adjoining Radars The detailed calculations performed by the committee for the Sulphur Mountain radar are very labor-intensive and time-consuming; thus, the com mittee did not repeat them at this level of detail to analyze the coverage from neighboring NEXRADs (i.e., Vandenberg Air Force Base [AFB] , KVBX; Santa Ana Mountain, KSOX; San Diego, KNKX; Edwards AFB, KEYX)
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From page 99... ...
MSL. Coverage from the KVBX, KSOX, KNKX, and KEYX radars is shown in light yellow; coverage from the Sulphur Mountain radar is shown in dark yellow; and the area covered by the Sulphur Mountain radar but not by any other adjoining radar is shown in light brown.
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From page 100... ...
Figure 7.7 also shows that if the KSOX radar located south of Los Angeles ever fails, the Sulphur Mountain radar provides redundant coverage of the Los Angeles basin. Therefore, the Sulphur Mountain radar is sited in an appropriate position to provide unique, key coverage of the offshore waters.
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From page 101... ...
. From the time of the release of the GAO report in October 1998 through December 2003, the monthly average service availability of the Sulphur Mountain radar fell below the 96.2-percent requirement only 4 months out of 63 (Figure 7.9)
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From page 102... ...
. Accordingly, a directive exists that outlines specific actions to verify various weather watches, warnings, and events, including flash flood warnings (NWS, 2004d)
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From page 103... ...
Of the 116 WFOs, only 22 met the 2004 NWS goal of a 50-minute lead time for flash floods warnings. During 19962003, the mean POD of flash flood events for all stations was 75.5 percent, and the POD for the modal category was between 90 and 95 percent (Figure 7.12)
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From page 104... ...
On the other hand, for the six WFOs with a FAR of less than 20 percent, the number of flash floods within their jurisdiction ranged from 91 to more than 500, indicating that their low FAR scores are due to skill and not to a small sample size. LOX Warning Statistics The Sulphur Mountain radar, along with the other Southern California radars and other aspects of the NWS modernization of the 1990s, has helped to significantly improve the flash flood warning performance of the
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From page 105... ...
The NWS database for 19952003 indicates that 15 of the 57 flash flood events that occurred were unwarned. The LOX WFO supplied information indicating that four of the "missed" events were not actually flash floods; one (occurring on January 20, 1997)
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From page 106... ...
Figure 7.15 portrays the key flash flood warning statistics for the LOX WFO, along with national statistics for comparison purposes, for the period 1996 through 2003. The POD and FAR for LOX are similar to the national averages and exceed the 2004 goals for the NWS Western Region which are also shown in the figure.
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From page 107... ...
This is reflected in the relatively short lead-time goal for the Western Region, where steep orography is a major factor. The NWS database for the LOX WFO for the 19962003 shows 47 flash flood events and 136 flash flood warnings, for an average of about 6 events (as noted in the previous section, see Figure 7.10)
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From page 108... ...
Post sulhur Mountain ercentP 40% Time15 NEXRAD Installation 30% 10 (1995-2003 20% 5 10% 0 0% Average Lead Time Probability of Events with Lead False Alarm Ratio Detection Time > 0 Minutes FIGURE 7.14 Comparison of the LOX WFO's statistical performance measures for flash flood warnings from the period prior to installation of the Sulphur Mountain NEXRAD (19861994) versus the period after its installation (19952003)
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From page 109... ...
In reviewing that appendix, the committee was unable to reconcile all of the events and warnings listed with the databases maintained by the NWS. Table 7.1 presents a composite chronological listing of events and warnings for those two counties since the Sulphur Mountain NEXRAD was commissioned, as contained in the two NWS databases (i.e., one kept at the LOX WFO and the other used by NWS Headquarters in evaluating the national warning program)
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From page 110... ...
LOX Database NWS Database 4 Jan 1995 0850 (1300) Warn LA - 0930 a FF Ventura 1000 FF LA (LT 1:10)
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From page 111... ...
EVALUATION OF THE SUPLHUR MOUNTAIN RADAR 111 GAO Report Remarks Warn LA Carried erroneous 1994 date FF Ventura FF LA b -- Extension of 0850 warning -- Further extension of 0850 warning -- Extension of 1250 warning -- Extension of 1440 warning -- Extension of 1700 warning FF LA Not classed as FF in other databases Warn LA FF Ventura Event actually occurred on 10 Jan Warn Ventura -- May correspond to 0300 event Listed on 9 Jan Warn LA Carried erroneous 1994 date -- b Warn Ventura Extension of 2330/9th warning FF LA May correspond to 0400 event above Warn LA Extension of 0300 warning FF Ventura -- - Extension of 0600 warning -- Extension of 0415 warning FF LA Corresponds to 0800 event -- Extensions of 0834 warning -- Extensions of 0834 warning -- False alarm -- False alarm -- Update of 2105 warning -- Warn Ventura Warn LA -- Corresponds to 2145 FF event? -FF Ventura Corresponds to 2145 FF event FF LA FF LA Warn LA False alarm Warn Ventura False alarm Warn LA False alarm Warn LA False alarm -Warn Ventura -- May correspond to 0745 event -- May correspond to 0600 event FF Ventura continued
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From page 112... ...
-- Warn LA 1800 (2100) -- Warn Ventura 2050 (2300)
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From page 113... ...
Extension of 0905 warning Warn LA Warn Ventura -- Corresponds to 1000 event? Warn Ventura Extension of 0835 warning -- Extension of 0835 warning -- Update of 0910 extension FF LA Corresponds to 1010 event?
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From page 114... ...
For instance, both Los Angeles and Ventura Counties were under continuous flash flood warnings for more than 11 hours on February 2324, 1998. Nevertheless, the GAO report listed two flash flood events that purportedly occurred at least 10 hours after the first warnings as events having zero lead-time warnings; neither of those events appears in the NWS databases, and the extended warnings were still in effect in any case.
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From page 115... ...
Lowering the base scan elevation angle for the Sulphur Mountain radar might yield benefits that go beyond improvements in precipitation pattern identification and tracking (e.g., better observations farther offshore) and quantitative rainfall estimation (e.g., reduced negative effects of partial beam filling and the vertical reflectivity profile)
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From page 116... ...
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 data base of flash flood events and warnings to include more complete and accurate listings of both warnings and events.
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