FLOOD06061 - Laserfiche WebLink

Home Browse Search

21 Seasonal values of convergence PMP were read for mid-points of the five least-orographic subregions from these preliminary maps and compared to the l-day, 3-day, and moisture curves shown in figures 2.4a to 2.4e. Smoothing and adjusting of the set of preliminary maps resulted in a consistent series of seasonal curves and maps. The finalized set of 1000~b (lOO-kPa) lD-mi2 (26-km2) 24-hr convergence PMP maps is presented in figures 2.5 to 2.16. Whereas, the initial maps began as extensions of the isohyets in HMR No. 43, the final maps after smoothing no longer maintain the direct association. For some individual months differ- ences in magnitude of up to 1 inch exist at some border locations. The greatest differences in pattern between these two studies occur in April and November, both considered transition months in terms of synoptic storm influences. Final mid-month convergence PMP values were read from figures 2.5 to 2.16 for the least-orographic regions and seasonal curves for these points plotted in terms of percent of the greatest of the 12 values in figures 2.4a to 2.4e (heavy solid lines) for comparison with the data. In figure 2.4a, convergence PMP preserves the summer maximum and broadens the peak, as intended, to include the summer prototype storm over the longer period. A similar remark can be made about the convergence PMP curve in figure 2.4b. In western Utah, figure 2.4c, the convergence PMP curve peaks in September. This is a month later than the eye-smoothed l-day rainfall curve and the curve from HMR No. 43. The PMP maximum in September results from extension beyond the data to consider the influence of late summer tropical cyclones. The peak in convergence PMP in figure 2.4d (Southern Nevad~is noticeably later than the moisture curve and somewhat later than the l-day data, being broadly centered about the 3-day maximum in September. In figure 2.4e (northwest Nevada), the convergence PMP curve has a small amplitude with a broad maximum centered on October. The October maximum is in agreement with the fall prototype storm with westerly inflow in northern California. The resulting 1000-mb (lOO-kPa) convergence PMP maps of figures 2.5 to 2.16 describe a set that is generally consistent with considerations listed at the beginning of this section. With the exception of western Utah and northwest Nevada the patterns show prominent summer maxima similar to maximum moisture, but tend to show much less variation from summer to winter than do the moisture curves in all five regions. The seasonal variation of the conver- gence PMP should be less than the variation of moisture alone since the greater efficiency of storms in the cooler season compensates to some extent for less available moisture.