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<br />1.3 Study Region <br /> <br />The northern and southern boundaries of the region are the borders of the <br />United States with Canada and Mexico. HMR No. 51 provides generalized estimates <br />of PMP for durations and areas east of the 105th meridian. In much of the region <br />between the 103rd and 105th meridians, the PMP maps in HMR No. 51 were stippled <br />to indicate some degree of uncertainty that could be resolved only when the <br />region between the Continental Divide and the 105th meridian was studied. In the <br />present report, PMP criteria for this two-degree-wide region have been included <br />as a result of the present investigations, and the PMP estimates from this report <br />supersede the criteria given in HMR No. 51. The eastern boundary of the study <br />region is the 103rd meridian, while the western boundary is the Continental <br />Divide. <br /> <br />West of the Divide, PMP estimates can be determined from Hydrometeorological <br />Report No. 43, "Probable Maximum Precipitation Estimates, Northwest States" <br />(U.S. Weather Bureau 1966), hereafter referred to as HMR No. 43, from <br />Hydrometeorological Report No. 49, "Probable Maximum Precipitation Estimates, <br />Colorado River and Great Basin Drainages" (Hansen et a!. 1977), hereafter <br />referred to as HMR No. 49, or from Hydrometeorological Report No. 36, "Interim <br />Report -- Probable Maximum Precipitation in California" (U.S. Weather <br />Bureau 1961). Figure 1.1 shows the regions covered by the present report and the <br />other reports mentioned. See Appendix A for a description of the geographic <br />region and scope of each report. <br /> <br />The study region contains all or part of several major river basins. The <br />entire Yellowstone and Powder River basins are within the study region. Only <br />partially within this study region are the upper reaches of the Missouri, North <br />and South Platte, Arkansas, Canadian, Pecos River basins, and the Rio Grande <br />basin. <br /> <br />In summary, the study region extends from the Canadian to the Mexican borders <br />between the Continental Divide and the 103rd meridian. For convenience, the <br />study r~gion will be referred to hereafter in this report as CD-I03. <br /> <br />1.4 Method of Study <br /> <br />Procedures developed for PMP analysis must reflect the varied terrain effects <br />throughout the CD-I03 region. Terrain varies from the relatively flat regions of <br />eastern Montana, Wyoming, Colorado, New Mexico, and western Texas to the <br />mountainous region that approaches the Continental Divide. It was necessary to <br />develop a procedure which would enable this diverse terrain to be analyzed in a <br />consistent fashion. The adopted procedure is similar in basic development to <br />that used in other studies in the western United States. The procedure separates <br />total PMP into convergence and orographic components of precipitation. The <br />convergence portion of the major storms in the region is determined to enable the <br />estimation of convergence PMP throughout the region. <br /> <br />It is necessary to increase the estimates of convergence PMP for variations in <br />orographic effects over the region to determine total PMP. In this report, an <br />orographic factor, Tic, is derived from 10Q-yr 24-hr maps of NOAA Atlas 2 (Miller <br />et aI. 1973). Since the strength of atmospheric forces in the storm varies from <br />the most intense 1-, 2-, 3-, or 6-hr period through the end of the storm, an <br />intensity factor, M, was developed. This factor reduced the effect of orography <br /> <br />3 <br />