Conclusions and Recommendations

Probable Maximum Precipitation (PMP) is an empirically based concept that has proved to be a useful standard for designing and evaluating spillways for high-hazard dams. However, because it has subjective elements, there are significant problems. Despite flaws in the PMP estimates developed by the National Weather Service (NWS), there is no compelling argument for making immediate widespread changes in either PMP methodology or the NWS's assessments of PMP, and the Committee recommends its continued use. This recommendation is based not simply on a meteorological evaluation but also on engineering and equity considerations and lack of a clearly better alternative. However, incremental improvements are possible and should be made.

The greatest discrepancies between generalized and site-specific PMP estimates are for precipitation events with small spatial and temporal scales. Improvements to PMP estimates are most critical for small-area basins during short events. Time periods of less than 6 hours and drainage areas from 25 to 1000 square kilometers are of particular interest. The Committee recommends a concerted effort to make such improvements. Radar data from the NEXARD system are a critical element of the long-term solution to the problem for small-area basins during short events with the exception of mountainous areas in the western United States. There is little prospect for significant enhancements to the observational side of the mountain rainfall problem. High-resolution mesoscale numerical modeling and paleohydrologic studies provide one avenue for examining heavy rainfall processes in mountainous regions



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Estimating Bounds on Extreme Precipitation Events: A Brief Assessment Conclusions and Recommendations Probable Maximum Precipitation (PMP) is an empirically based concept that has proved to be a useful standard for designing and evaluating spillways for high-hazard dams. However, because it has subjective elements, there are significant problems. Despite flaws in the PMP estimates developed by the National Weather Service (NWS), there is no compelling argument for making immediate widespread changes in either PMP methodology or the NWS's assessments of PMP, and the Committee recommends its continued use. This recommendation is based not simply on a meteorological evaluation but also on engineering and equity considerations and lack of a clearly better alternative. However, incremental improvements are possible and should be made. The greatest discrepancies between generalized and site-specific PMP estimates are for precipitation events with small spatial and temporal scales. Improvements to PMP estimates are most critical for small-area basins during short events. Time periods of less than 6 hours and drainage areas from 25 to 1000 square kilometers are of particular interest. The Committee recommends a concerted effort to make such improvements. Radar data from the NEXARD system are a critical element of the long-term solution to the problem for small-area basins during short events with the exception of mountainous areas in the western United States. There is little prospect for significant enhancements to the observational side of the mountain rainfall problem. High-resolution mesoscale numerical modeling and paleohydrologic studies provide one avenue for examining heavy rainfall processes in mountainous regions

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Estimating Bounds on Extreme Precipitation Events: A Brief Assessment and thus improving PMP estimates. The Committee recommends increased research efforts for numerical modeling of extreme storms in mountainous regions. Radar rainfall estimates from the operational network of WSR-88D radar hold significant potential for improving PMP estimates. However, despite the greater technical capabilities of WSR-88D compared with the radars it replaces, the absolute accuracy of the algorithms for converting radar reflectivity to rainfall estimates has not yet been determined. The Committee recommends that the NWS work to establish the accuracy of WSR-88D rainfall estimates, especially for heavy rainfall events, and develop formal plans for integrating WSR-88D data into a PMP storm catalog. Adding polarization diversity measurement capabilities to the WSR-88D would provide significant potential for improving rainfall rate estimates, especially for heavy rainfall events. Radars also offer great potential for transforming station-based precipitation statistics into storm-based statistics for introduction into the storm-based PMP procedures. The dynamics and thermodynamics of extreme events play an important role that may not be adequately reflected in PMP procedures. Numerical modeling experiments provide an increasingly useful tool for examining tightly framed hypotheses concerning the response of storms to moisture and dynamic variables. In particular, mesoscale models have the potential to test the reasonableness of moisture maximization and storm transposition procedures. Major new research initiatives are needed to improve scientific understanding of extreme rainfall events, especially if the investment in the NWS modernization is to have a strong effect on PMP determinations or probabilistic characterizations of extreme events. The U.S. Weather Research Program (Subcommittee on Atmospheric Research 1992, 1993) and the GEWEX Continental-scale International Project (GCIP) (IGPO 1994, WCRP 1990) could provide many elements of the needed work. Both observational and numerical modeling studies are required, since each will play an important role in improving hydrometeorological procedures used in engineering design. Interdisciplinary research involving atmospheric scientists, hydrologists, and statisticians should prove especially useful. Particular emphasis must be placed on understanding and modeling processes on the mesoscale, since this is the primary scale on which extreme precipitation events occur (NRC 1990). Further NRC study of PMP procedures and analyses is not warranted at this time. However, improvements in PMP estimates may be possible in the near future as a consequence of NWS modernization, particularly the installation of a national network of new weather radars and the development of increasingly sophisticated numerical models. Examinations of some broader research programs (such as the USWRP and GCIP)

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Estimating Bounds on Extreme Precipitation Events: A Brief Assessment that concentrate on mesoscale meteorology and hydrometeorology should move forward aggressively. An alternative strategy to PMP would be to move toward probability-based standards that incorporate current methods of meteorology and statistics. This would allow for direct comparisons of risk versus cost. For application to high-hazard dams, the probability standard could be chosen to give values that are about the same as current values of PMP, except where there is a known need for a very conservative approach. Uncertainty could also be built into any standard. As our database of precipitation measurements increases, this uncertainty would decrease. An effort to develop a probability-based standard would be very useful to other important problems, such as the design of nuclear power plants. The currently mature probabilistic methods are largely applicable to station-based analyses rather than storm-based or basin-wide analyses. However, precipitation accumulations over a drainage basin are the needed ingredients for flood calculations. Probabilistic techniques applicable to storm-based calculations are not yet well developed, though some progress has been made. The Committee recommends that strategies be investigated for estimating probabilities of extreme rainfalls, using the best available concepts and methods of meteorology and statistics. We further recommend that attention focus on storm-based analyses of extreme rainfall.

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