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" .; The geographical areas to which these publications apply include NWS Hydro 35, TP 40 and TP 49 for the 35 states east of the Rocky Mountain area which are essentially free of significant orographic effects. The particular publications to be used depends on the storm duration under examination. The 13 mountain states containing the Rocky Mountains and those areas to the west are covered by NOAA Atlas 2 and other site-specific publications. The procedures described in all the NWS publications are based on statistical evaluations of long-term rainfall-gage records in a region. These evaluations include estimates of the frequency of accumulated rainfall-depth versus storm duration at each rain gage. Rainfall maps were made from these depth-duration values, and isopluviallines were drawn on these maps to define constant rainfall-depth relationships through a region for a specific storm duration. Each of the NWS publications gives a detailed discussion of the derivation of the rainfall-frequency depth-duration relationships. It is strongly recommended that users of NWS data thoroughly read the pertinent sections and be familiar with the applications and limitations of the NWS material. A hypothetical storm developed from NWS data is also referred to as a "balanced storm," because a consistent depth-frequency relation is used for each peak duration interval of the storm. That is, for a hypothetical100-year return period 48- hour-duration storm, the rainfall depths for the peak 30-minute, l-hour, 6-hour, 24- hour or other peak period would each be equal to the 1 OO-year depth for that duration. This consistent frequency-depth-duration relationship throughout a storm would not occur in nature, because of the randomness of the rainfall events. The balanced storm concept, however, does allow for logical construction and arrangement of a storm event for a particular return period. Balanced storms are discussed in more detail later in this section. The Standard Project Storm (SPS) is used primarily by the Corps of Engineers as a standard design criteria to design and evaluate performance of flood control projects. Development of the SPS for states west of the Rocky Mountains requires site-specific criteria, which are not discussed here. The Probable Maximum Precipitation Storm lPMP) is developed primarily from NOAA and NWS criteria. The area east of the 1 05th meridian is covered by HMR 51 and HMR 52. For areas between the 103rd meridian and the continental divide, HMR 55A is used. HMR 49 covers the Colorado River basin and the Great Basin areas. Special studies for site-specific areas are prepared by the NWS upon request. For example HMR 33 was prepared for the design of Chatfield Reservoir and covers the upper South Platte River Basin. 3. HYPOTHETICAL-FREQUENCY-STORM DERIVATION. Development of a storm from NWS data is straightforward and systematic. The individual performing the study must: 1) establish the appropriate storm duration and the time interval for subdividing the storm rainfall, 2) extract the rainfall values from NWS publications for his area of interest, 3) make adjustments to 7-12