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PRECIPITATION 1. BACKGROUND THEORY. Synthetic or hypothetical storms are frequently used by the Corps of Engineers to generate synthetic flood events for evaluation of existing flood conditions and for study of the effects of flood-mitigation components. The degree of dependency on synthetic flood events during a flood evaluation depends generally on the number of gages and the length of hydrologic record available. For reservoir studies there is typically at least one long-record stream gage near the dam site, and thus the storage required for reservoir food control is usually computed by period-of- record simulation techniques using recorded data. Channels, levees, and other local protection projects are more likely to be sized by synthetic storm-flood analyses since these projects often extend over many miles of the stream, and it is rare that gage information would be available throughout the reach. Hypothetical storms are also used to ensure that rare storm-flood events are included in an overall assessment of the proposal. Since large storms that cause major floods are infrequent, the available runoff record quite possibly may not reflect the occurrence of these rare events. Analyses of major reservoirs, although relying mainly on recorded stream data, also require the use of synthetic storms (Standard Project and Probable Maximum Storms) to evaluate the safety of major dam components, such as the spillway and outlet works, and guard against overtopping of the dam. Synthetic precipitation data are also used for analyses of flooding throughout a basin, testing of the hydrologic effects of alternative land uses and flood plain regulations, and evaluation and design of flood control components. This is usually done with a model of the watershed to simulate the rainfall-runoff process, with the rainfall specified by hypothetical storms. 2. DATA SOURCES. The primary sources of hypothetical storm il)formation for the United States are various technical publications (TP) of the National Weather Service (NWS) and hydrometeorological reports (HMR) of the National Oceanic and Atmospheric Administration (NOAA). All publications for development of hypothetical frequency storms feature generalized isopluvial rainfall maps and/or regression equations. Other methods, such as statistical analysis of nearby long-record rain gages to derive hypothetical storms of particular frequencies in lieu of the NWS and NOAA procedures, are used extensively in some parts of the United States but not discussed here. 7-11