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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />II-ll <br /> <br />')~t.") <br />I.' .. <br />e.transposition of the June 7 to 8, 1964 Montana storm eight <br /> <br /> <br />latitude degrees to the south was in effect a seasonal transposition of about <br /> <br /> <br />four weeks. This difference in time of season is based on a comparison of <br /> <br /> <br />maximum 12-hour persisting dewpoints at the observed and transposed sites. <br /> <br /> <br />In mid-May, the 620F isotherm lies very close to Missoula, while the 680F <br /> <br /> <br />isotherm passes through Denver. It is not until mid-June that the 680F <br /> <br /> <br />isotherm reaches Missoula. It thus appears reasonable to advance the date <br /> <br /> <br />of possible storm occurrence at the transposed site by at least 15 days in <br /> <br /> <br />attempting to compensate for the more advanced season. This would make the <br /> <br /> <br />revised storm date May 23 to 24. By coincidence, when the usual IS-day <br /> <br /> <br />seasonal transposition toward the warmer season is applied to the May 4 to 8, <br /> <br /> <br />1969 storm (Plate 11-2), the storm is being adjusted to May 19 to 23, which <br /> <br /> <br />closely approximates the advanced adjustment date for the transposed June <br /> <br /> <br />1964 storm. <br /> <br />A maximum 12-hour persisting dewpoint of 76.SoF was indicated21 <br />for the advanced date, which is one degree lower than the dewpoirit used in <br />o <br />the Reclamation study. The adjustment factors, computed for 76.5 F and for <br />the inflow barrier heights used in the Reclamation study, were 1.38 and 1.30, <br />which represent a 6-percent reduction from the 1.47 and 1.38 factors used in <br />that study. The maximized transposed June 1964 storm values adjusted to <br />May 23 to 24 are shown in line B of Table 2. While these adjusted values <br />may be below the general level of PMP, they appear to be within the PMP <br />confidence band. <br /> <br />Use of a Major Mountain Historical Storm <br /> <br /> <br />A major mountain storm, which was not included in the USBR study, <br /> <br /> <br />suggested another approach to an acceptable design storm. This was the storm <br /> <br /> <br />of May 29 to 31, 1894, which experienced its heaviest rainfall from a center <br /> <br /> <br />about 12 to 15 miles northwest of Boulder, southward over Idaho Springs to <br /> <br />21 u.s, Environmental Data Service, "C1imatic'At1as of the United States". <br /> <br />IDAMES c- I\IIOOAE <br />