Executive Summary

Estimated bounds on extreme precipitation events are used in the process of determining the flood-induced forces that structures might experience in their lifetimes. Engineering for flood survival is particularly important for high-hazard structures, such as dams above populated areas. There is usually a direct trade-off between cost and safety, with high estimates of precipitation extremes leading to high construction and retrofitting costs.

The most commonly used measure of anticipated extreme rainfall is Probable Maximum Precipitation (PMP), which is defined as “theoretically the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a certain time of year” (WMO 1986). There is no known way to calculate this from first principles, and the procedures used in practice to make estimates are a subject of debate. Recently, proposals have been made to use statistical procedures that provide probabilities of extreme events, rather than PMP.

The procedures used to calculate PMP are based on examinations of the set of storms with the heaviest observed rainfalls and incorporate several implied assumptions. One assumption is that most extreme precipitation events could have happened at a slightly different location in the same region; this leads to a technique called storm transposition, which increases the PMP values for areas near an observed extreme event. It is also considered possible that a given extreme storm could have happened at a time of greater absolute humidity; this leads to a technique



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Estimating Bounds on Extreme Precipitation Events: A Brief Assessment Executive Summary Estimated bounds on extreme precipitation events are used in the process of determining the flood-induced forces that structures might experience in their lifetimes. Engineering for flood survival is particularly important for high-hazard structures, such as dams above populated areas. There is usually a direct trade-off between cost and safety, with high estimates of precipitation extremes leading to high construction and retrofitting costs. The most commonly used measure of anticipated extreme rainfall is Probable Maximum Precipitation (PMP), which is defined as “theoretically the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a certain time of year” (WMO 1986). There is no known way to calculate this from first principles, and the procedures used in practice to make estimates are a subject of debate. Recently, proposals have been made to use statistical procedures that provide probabilities of extreme events, rather than PMP. The procedures used to calculate PMP are based on examinations of the set of storms with the heaviest observed rainfalls and incorporate several implied assumptions. One assumption is that most extreme precipitation events could have happened at a slightly different location in the same region; this leads to a technique called storm transposition, which increases the PMP values for areas near an observed extreme event. It is also considered possible that a given extreme storm could have happened at a time of greater absolute humidity; this leads to a technique

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Estimating Bounds on Extreme Precipitation Events: A Brief Assessment called moisture maximization, which increases the PMP values based on a more extreme extension of an observed event. Competent professionals can obtain different results because these procedures require some subjective judgment. The National Weather Service (NWS) provides maps of PMP for a variety of durations and basin sizes. These generalized maps have fairly smooth contours on a coarse spatial scale. Smoothing adds an additional implicit transposition for some of the most severe storms because higher values are favored over smaller ones in the process. The topography of a particular location may lead to site-specific PMP values that are significantly lower than those that apply to a larger region. However, sufficient data and resources are not available to perform site-specific analyses for all locations, nor are such analyses free of subjectivity. PMP estimates may have a small probability of being exceeded, even though PMP is defined as an absolute upper bound. This probability has been estimated to vary by several orders of magnitude in the United States. The existence and variation of a nonzero exceedance probability have caused confusion with respect to the definition of PMP and questions of equity in its application. Despite flaws in the PMP estimates developed by the NWS, there is no compelling argument for making immediate widespread changes in either PMP methodology or the NWS assessments of PMP, and the Committee recommends its continued use. This recommendation is based in part on lack of a clearly better alternative. However, incremental improvements are possible and should be made. The greatest disparities between generalized and site-specific estimates of PMP appear for short-duration events over small basins. 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 NEXRAD system, which is now being deployed, may prove very helpful for studies of small basins, except in many mountainous areas where radar coverage is inherently poor. In such regions, numerical modeling and paleohydrologic studies may be more useful. The Committee recommends increased research efforts for numerical modeling of extreme storms in mountainous regions. Measuring rainfall is a difficult task owing to the significant variations on very small spatial scales (a few kilometers) and the inaccuracies of raingauges, especially under the windy conditions that accompany most extreme events. Rainfall estimates from the WSR-88D radars, which make up the NEXRAD system, will provide excellent spatial and temporal resolution. Despite the greater technical capabilities of the new equip-

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Estimating Bounds on Extreme Precipitation Events: A Brief Assessment ment, there are unresolved scientific issues in defining the relationship between radar reflectivity and rainfall. The absolute accuracies of the algorithms for rainfall estimates have 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. 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 (USWRP) and the GEWEX Continental-scale International Project (GCIP) could provide many elements of the needed work. Particular emphasis must be placed on mesoscale meteorology, since this is the primary scale on which extreme precipitation events occur. Although further study by the National Research Council of PMP procedures and analyses is not warranted at this time, examinations of some broader research programs (such as the USWRP and GCIP) that concentrate on mesoscale meteorology and hydrometeorology should move forward aggressively. Probability-based methods could provide an alternative to PMP. The currently mature probabilistic methods are largely applicable to station-based (single-point) analyses rather than storm-based or basin-wide analyses. However, precipitation accumulations over a drainage basin are what is needed for flood calculations. Probabilistic techniques applicable to storm-based calculations are not yet well developed. The Committee recommends that strategies be investigated for estimating probabilities of extreme rainfalls, using the best available concepts and methods of meteorology and statistics. The Committee further recommends that attention be focused on storm-based analyses of extreme rainfall.

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Estimating Bounds on Extreme Precipitation Events: A Brief Assessment FIGURE 1 Maximum observed point rainfalls as a function of duration. (Courtesy of John Vogel, National Weather Service.)