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2. METEOROLOGY OF KAJOR STORKS IN THE CD-I03 REGION 2.1 Introduction The basic requirement for any study of the upper limits of precipitation within a region is the review of the major storms tbat have occurred in and near the study area. In a region so ~eo~raphically extensive and so topo~raphically diverse as the CD-I03, the causes of major rainstorms have been many and varied. In the southern part of the re~ion some of the major storms of record are a result of tropical storms that bave crossed the Texas Gulf Coast and moved northwestward before recurvin~ eastward. In Montana, the major storms are extratropical cyclones. Important throu~hout the re~ion are extratropical storms that have embedded large convective cells, especially for small area sizes and short durations. In this study, we have made meteorolo~ical analyses of all of these various storm types to ~ain a more complete understandin~ of tbe meteorology of major rainstorms within the CD-I03 re~ion. This chapter describes a number of these storms to provide a basic knowled~e of the causes of major storms in the CD-I03 region. 2.2 Major Stonos of Record A survey was made of all the major storms that have occurred in and near the CD-I03 region. The 82 major storms that occurred in this re~ion are listed in chronological order in table 2.1. Location of the greatest rainfall amount from each of these storms is indicated in figure 2.1. The table provides an identifying storm number, name of location where the storm center occurred, date of occurrence, assignment number from the agency conducting the storm study (COE, USBR, and Atmospheric Environment Service of Canada), and the latitude and longitude of the center of rainfall. The storm identification numbers ~iven in table 2.1 will be used throughout this report to identify tbe individual storms. Table 2.1 also provides a chronological list of 35 additional major storms (supplemental storms, numbers 83-117) tbat occurred in the region just to the east of the CD-I03 region (to 990W). Locations of tbe rainfall centers of these storms are also plotted in figure 2.1. Some of these major storms are important to the estimation of PMP within the CD-I03 re~ion. For most of these storms, depth-area-duration (DAD) data are available from Storm Rainfall in the United States (U.S. Corps of Engineers 1945- ) or reviewed and approved by Bureau of Reclamation storm studies. An exception is the Gibson Dam storm, where a detailed reanalysis of isohyetal maps by the Bureau of Reclamation gave us the DAD data used in this study from a preliminary analysis. Complete storm studies are not available for those storms in which a dash appears under the beading Assignment Numher in Table 2.1, where as a rule, the storms are for sbort durations (Virsylvia, Las Cruces, etc.). It is apparent from examination of fi~ure 2.1 that for large portions of the CD-I03 region there are no major storms in the data base. The state of Wyoming is one such large region. Lack of sufficient storm data has always been a problem for most PMP studies and especially for arid and mountainous regions. One method employed in past hydrometeorological studies to resolve this deficiency is transposition of storms from other locations, i.e., assuming that the precipitation amounts that have occurred in another location could occur in 11