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that the 1-hour percentages based on this time distribution have occasionally been found to give rainfall values that are not sufficiently conservative, particularly in the Southwestern Division of the Corps of Engineers (SWD). 5. PROBABLE MAXIMUM STORM. The Probable Maximum Storm (PMS) is defined as the most severe ,combination of meteorological occurrences considered reasonably possible in a particular region. It is felt to be an upper limit of flood-producing rainfall (or snowpack melting when applicable) and is used as a design storm where virtually no risk of flooding can be tolerated. The PMS has historically been used in dam designto ensure the adequacy of spillways and top-of-dam elevations for high dams. As with the hypothetical frequency storms, one set of generalized criteria is applied to the majority of the U.S., and a variety of regional criteria, accounting primarily for orographic effects, to the balance. Details for constructing a PMS for a particular region are given in the various Hydrometeorological Reports and Technical Papers listed in the references. The reports, HMR-51 and HMR-52, for the United States east of the 105th meridian, apply to most of the country and are discussed further here. The mechanics of storm breakdown and arrangement presented in these reports are similar to methods for the western United Stated presented in other publications. 5.1. PMS FOR THE EASTERN AND CENTRAL UNITED STATES. The steps for deriving a PMS, using Probable Maximum Precipitation data from HMR-51 and data for the determination of shape, orientation, and distribution from HMR-52. 5.1.1. Determine isohyetal Probable Maximum Precipitation (PMP) values for the study area for desired drainage area sizes (10, 100, 200, 1,000, 5,000, 10,000,20,000 square miles) and for corresponding storm durations (6-,12-,24-,48- , and 72-hours) using the appropriate plates from the report. 5.1.2. Plot 11 family of duration curves (6-, 12-, 24-, 48-, and 72-hours) for PMP intensities versus drainage area size on semi-logarithmic paper. 5.1.3. Interpolate for the desired storm area, determine PMP intensities for each duration and plot on ordinary graph paper. 5.1.4. Determine rainfall in each 6-hour time interval by interpolation, then increment the rainfall by successive subtractions. The PMP will be the maximum value for the selected storm area only; areas greater or less than the selected storm area will show smaller values of PMP. Considerable trail and error computations will normally be required to determine the storm area which maximizes average 7-19 .