FLOOD06061 - Laserfiche WebLink

Home Browse Search

7 2.2 Mid-Month 1000-mb (100-kPa) Convergence PMP Maps, 24 hrs, 10 mi2 (26 km2) 2.2.1 Envelopment of Maximum Observed Rainfalls Record storm rainfall is the underpinning to any PMP study. We need two restrictions to our data sample. First, extreme isolated thunderstorm values are not appropriate for development of general-storm convergence PMP. Such values rather are the basis for the local-storm PMP estimates of chapter 4. Secondly, in this section we are concerned with only the convergence com- ponent of record storm amounts. No consistent method has been found for separating total observed storm precipitation into convergence and orographic components; however, we can restrict the data to observed maxima in 1east- orographic regions of the Southwest. Least-orographic regions are subjectively determined zones (shown in fig. 2.1) outlined on a 1:2,000,000 scale topographic map. The boundary of each subregion depicted on the figure is not significant other than to enclose a group of at least five stations whose precipitation we believe to be least influenced by.orography. An appreciation for the complex terrain and an aid in determining general limits for these subregions was gained by two of the authors (Riedel and Hansen) during a 2-day series of overflights in 1972. We recognize that some substantial orographic features remain within the 1east- orographic boundaries shown in figure 2.1 but stations selected within these subregions were judged not to be significantly influenced by orography. An attempt was made to obtain an equal number of stations in each subregion, but this was difficult to maintain. Station storm totals exceeding 5 inches (127 rom) in 24 hours or less in the subregions were extracted from the histor- ical records. The five storms meeting this criterion are listed in table 2.1. One other storm for Porter, N.M.,.east of the region of interest, is listed for comparison. Meteorological descriptions of each of the events is given in the companion report (Schwarz and Hansen 1978). Each storm total is the result of thunderstorms sustained over a period of 6 hours or more within a more general precipitation storm. This distinguishes them from the isolated thunderstorm events used for local-storm PMP. The locations of storms listed in table 2.1 are shown in figure 2.2. San Luis, Mexico lies just south of the study region. Since the exact duration of the San Luis 1-day storm amount (Secretaria de Recursos Hydro1icos 1970) could not be determined, a duration of 24 hours was used. Two of the 5 values in table 2.1, at Bug Pt., Utah and Dove Ck. 10 SW, Colo., occurred in the September 4-6, 1970 storm. These stations near the edge of an outlined least-orographic region (see fig. 2.1) reported rainfalls of 6.50 inches (165 rom) and 6.00 inches (152 rom), respectively. They are on a high plateau at elevations of 6600 and 6900 feet (2012 and 2103.m) respec- tively. Analysis of orographic PMP in the following chapter shows that some minimum-orographic effect is necessary over this .subregion. Analyses of other notable general storms for the region (i.e. the September 4-7 and 11-13, 1939 and August 28-30, 1951 Arizona storms), disclosed that maximum precipitation for these storms occurred primarily in orographic regions. Total storm amounts were all less than 3 inches (76 rom) at least-orographic stations.