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4.1. SPS FOR EASTERN AND CENTRAL UNITED STATES. Deriving the SPS for drainage basins greater than 1000 square miles requires special studies by the National Weather Service. The general criteria in EM 1110-2-1411 are applicable to basins of less than 1000 square miles. The sequence of SPS derivation described in the reference includes: selection of an "index" rainfall, determination of the 24-, 48-, 72- and 96-hour SPS rainfall based on the index and the drainage area under study, adjustment of the rainfall for a basin shape factor, division into incremental rainfall amounts, and arrangement of the incremental rainfall values into the storm sequence. 4.1.1. The SPS index rainfall is determined using Plate 2 of EM 1110-2- 1411. The rainfall depth determined from this figure represents the SPS rainfall for an area of 200 square miles and for a 24-hour duration. Further adjustment of this rainfall value is required, however, because a SPS is a 96-hour storm by definition, and because the drainage area under study will never be exactly 200 square miles. 4.1.2. Plate 9 of EM 1110-2-1411 is used to convert the index rainfall to 24-, 48-, 72-, and 96-hour accumulated depths adjusted for the actual drainage area. The four values taken from the curves in the figure are each multiplied by the index to arrive at the accumulated rainfall values for each duration. 4.1.3. These values are divided into total rain for each 24-hour period and ranked in the sequence 4,2,1,3, and 1 being the highest 24-hour total. The percentage of the total 96-hour storm which each 24-hour period represents is then computed. The SPS criteria are developed using the symmetrical oval-shaped isohyetal pattern shown in Plate 12 of EM 1110-2-1411. The rainfall values computed thus far are for a drainage basin that the symetrical pattern matches perfectly. The fit will always be imperfect in the real world, however, so an adjustment will be required for actual basin shape. That is done by overlaying the storm pattern on the basin, using a planimeter to compute total storm volume, and from this determining the maximum depth that could occur from the most severe arrangement of the pattern. The storm pattern may require several centerings to determine the maximum depth. Unless the drainage area is extremely small, the maximum 96-hour depth will be less than the 96-hour depth already determined. This maximum value of the 96- hour depth is then multiplied by the 24-hour percentages discussed at the start of this subparagraph. The result is the 24-hour SPS rainfall totals adjusted for the shape of the study basin. 4.1,4. DIVISION INTO INCREMENTS AND ARRANGEMENT OF STORM. Each 24-hour period is next subdivided into four 6-hour periods using the criteria in Plate 10 of EM 1110-2-1411. Division into smaller increments uses a simple average of the rainfall in each 6-hour interval, except for the peak 6-hour period of the entire storm. Further breakdown of the peak 6-hour period to one-hour periods is made using the guidelines given in Plate 11 of EM 1110-2-141. It should be noted 7-18