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<br />erate increase over the maximum observed might be warranted. In the <br />case of cyclonic-storm rainfall, amounts measured in observed storms <br />can be multiplied by the ratios of maximum probable precipitable MOis- <br />ture and winds to values recorded in each storm. <br /> <br />Section 4.03. Probable maximum storm derivation <br /> <br />Probable maximum storm precipitation amounts expressed as a depth- <br />area-duration function are obtained generally by enveloping maximized <br />precipitation amounts in all observed storms for each duration and area <br />size. Where storm potential varies arealy or regionally, as discussed in <br />Sections 2.03 and 2.04, ratios of normal precipitation at the location <br />for which the estimate is being made to normal precipitation at the <br />location of each recorded storm should be applied to precipitation <br />amounts for that storm before the enveloping procedure. <br />Figures 4.03 and 4.04 illustrate generalized criteria developed by <br />the U.S. National Weather Service for the eastern United States. Deri- <br />vation of a probable maximum storm hyetograph based on these criteria is <br />illustrated in figure 4.05. Use of such generalized criteria facilitates <br />engineering evaluations and usually assures reliable results. The World <br />Meteorological Organization has published a document which gives consider- <br />able detail on the theory and techniques used by the meteorologist in <br />deriving probable maximum precipitation. It is WHO No. 332. World Meteor- <br />ological Organization, Operational Hydrology Report No.1, -Manual for <br />Estimation of Probable Maximum Precipitation,- 1973. <br /> <br />4-04 <br />