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<br />PREFACE TO REVISED EDITION <br /> <br />Hydrometeorological Report (HMR) No. 55 was published in 1984. This report was <br />the first serious attempt to develop a PMP procedure for the highly orographic <br />region between the Continental Divide and the 103rd meridian (CD-I03 region). It <br />superceded Technical Paper No. 38 (U.S. Weather Bureau 1960) west of the 105th <br />meridian, where only broad-scale effects of terrain were considered, and HMR <br />No. 51 (Schreiner and Riedel 1978) between the 103rd and 105th meridians. <br /> <br />The procedure used in HMR No. 55 is highly complex involving a number of <br />subjective decisions based on meteorological experience and understanding. The <br />procedure for orographic intensification in HMR No. 55 represented new thinking <br />and was intended to provide a foundation for a technique that would be applicable <br />to other complex orographic regions. Some of the concepts have since been <br />adopted in NWS HYDRO 39 (Miller et al. 1984) and 41 (Fenn 1985), as well as HMR <br />No. 56 (Zurndorfer et al. 1986). <br /> <br />Since the release of HMR No. 55 in early 1984, considerable controversy has <br />developed regarding potentially high values in both general and local storm PMP <br />estimates at higher elevations. It is these higher elevation storms where <br />detailed observations and knowledge are lacking. In response to these concerns, <br />the National Weather Service and the Bureau of Reclamation authors reexamined <br />those parts of the study that might have influenced the results in these areas of <br />concern. A number of decisions were made in HMR No. 55 that controlled the level <br />of PMP estimates. Upon subsequent review, three areas were found where alternate <br />decisions could be made. In combination, these alternate decisions significantly <br />reduce the PMP estimates for small areas and short durations at higher <br />elevations. These changes have been incorporated into this revised report, to be <br />known as HMR No. 55A. Because some of the changes have resulted in significant <br />differences to the 1984 results, copies of HMR No. 55 should be discarded to <br />avoid confusion. <br /> <br />The following decisions were made: <br /> <br />I. To provide local-storm PMP estimates for the entire CD-I03 region as <br />opposed to the three sheltered geographic zones given in HMR No. 55. In <br />HMR No. 55, we chose to restrict such estimates to the most sheltered <br />zones. It appears reasonable that local-storm estimates can be provided <br />throughout the region and allow the results to delineate the extent of <br />control between local and general storm. This has been done and is <br />discussed in chapter 12. <br /> <br />2. In HMR No. 55, the vertical moisture adjustment for local-storm PMP <br />transposition somewhat departed from past practice. Use of one-half the <br />liquid water variation observed in precipitable water tables (for a <br />saturated pseudo-adiabatic atmosphere) considerably increased the <br />estimates of PMP at higher elevation. The authors have changed this <br />adjustment in HMR No. 55A to conform to previous studies that allow for <br />the full moisture adjustment presented by the change in precipitable <br />water with elevation. <br /> <br />xvi <br />