FLOOD06061 - Laserfiche WebLink

Home Browse Search

12 One approach to regional patterns was based on maximum I-day precipitation for each month in the least-orographic regions in the Southwest. All long- record (>20 years) stations considered least-orographic within each subregion are listed in table 2.2 and are located by numbered dots in figure 2.1. Max- imum monthly I-day rains were obtained from Technical Paper No. 16 (Jennings 1952) and supplemented by recent records through 1970. Averaged maximum values, by month within each subregion, were helpful but not sufficient to define regional patterns, due primarily to the small number of data points. A further step of adjusting the data to a common elevation and for upwind barriers did not help materially. Additional guidance for regional patterns of 1000-mb (lOO-kPa) convergence PMP came from analysis of moisture potential. The Climatic Atlas (Environ- mental Science Services Administration 1968) presents charts of maximum persisting l2-hr 1000-mb (lOO-kPa) dew points covering the 48 conterminuous states. These charts were used because they portray the broadscale moisture patterns influencing the Southwest. The use of revised moisture charts for the Southwest would not affect the conclusions on moisture patterns based on that Atlas. Figure 2.3 shows examples of schematic charts adapted from the January and August dew point charts from the Atlas. These schematics sug- gest the source of atmospheric moisture for the region. The solid lines are used to imply moisture from the Gulf of Mexico, while the dashed lines sug- gest moisture from Pacific Ocean sources. The change in orientation of the dashed lines between January and August reflects a change from mid- latitude storms in winter and spring to moisture surges from tropical lati- tudes in late summer. The dotted lines represent smoothing in the transition zone between the two moisture sources. The moisture patterns for each of the months give guidance to the pattern of regional variation but not to magnitude of precipitation. They show that the tropical Pacific moisture source has its greatest influence over the southwest region from May through October. The Gulf of Mexico is recognized by many researchers as a source for much of the day-to-day precipitation over the Southwest. However, such rainfall occurrences are not representative of conditions for extreme precipitation (Hansen 1975a, 1975b). Precipitation climatology studies of the Southwest by Schwarz and Hansen (1978) supports this interpretation. 2.2.4 Seasonal Variation Clues to regional patterns of 1000-mb (lOO-kPa) convergence PMP for each month can also be obtained from analyses of seasonal trends in precipitation data at various locations. Therefore, the seasonal variations of the maximum I-day precipitation for the stations in least-orographic subregions shown in figure 2.1 and listed in table 2.2 were analyzed. Seasonal charts, figures 2.4a to 2.4e, show monthly averages within each subregion by open circles, along with an eye-smoothed curve (short dashes). In figure 2.4a to 2.4e the regionally averaged I-day maximum precipitation curves have a summertime maximum in all five regions except northwest Nevada, which shows a summer minimum and bimodal winter and late spring maximum.