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<br />20 <br /> <br />This formed the nucleus of the scheme for developing Southwest convergence <br />PMP. Since the Northwest PMP report presented monthly maps of convergence <br />PMP (except July to September), these were selected as the point of begin- <br />ing. The California PMP report does not provide a seasonally variable pat- <br />tern of convergence PMP although values are given for October through April. <br />Therefore, some discontinuity existed between the Northwest and the Califor- <br />nia results. Most important was the fact that the patterns of gradients be- <br />tween the two studies were compatible. <br /> <br />The procedure began by simply extending the gradient patterns of 1000-mb <br />(lOO-kPa) convergence PMP from the Northwest into the Southwest. The maxi- <br />mized value at Indio (table 2.1) gives the limiting magnitude for the month <br />of September at that location. The eye-smoothed I-day data curve of figure <br />2.4a was used to get an initial seasonal variation of magnitude at Indio <br />taking the September value as 100%. It was obvious that the deep minimum in <br />spring of this seasonal curve was not in agreement with a consistent pattern <br />of extended gradients from HMRNo. 43. The Indio seasonal curve was modified <br />by increasing the spring values to be more in line with the broad winter- <br />spring minimum shown by the moisture curve in figure 2.4a. <br /> <br />From this beginning the next consideration was how to treat the west slopes <br />of the Rocky Mountains. East of the 105th meridian HMR No. 51 (Schreiner and <br />Riedel 197roshows a tight gradient of PMP having a NE-SW orientation of iso- <br />hyets of PMP. Because the general level of convergence PMP for the Southwest <br />is much less than that shown by HMR No. 51, it is necessary to create a tight <br />gradient somewhere between these two regions. PMP for the mountainous region <br />between the Continental Divide and l05th meridian has yet to be studied in <br />detail. We assume that much of the decrease in magnitude of PMP from HMR <br />No. 51 will be concentrated near the Divide. Therefore, a tighter gradient <br />was maintained along the west slopes of the Rockies than over most of the re- <br />mainder of the Southwestern Region. <br /> <br />Considerations c, d, and e were particularly involved with interpretation <br />of the pattern of PMP gradients during the period of summer maximum precipi- <br />tation, expected to come from a decadent tropical cyclone. The influence of <br />this PMP prototype storm through much of the region is especially important <br />in the southern portion of the region, closest to the source of moisture, and <br />extends from July to September. This causes the isohyets to become aligned <br />more east-west at lower latitudes. An assumption of equal likelihood of the <br />summer prototype general storm between July and early October is supported by <br />monthly distributions of eastern Pacific tropical cyclones (Rosendal 1962, <br />Serra 1971, Baum 1974). Thus a rather broad seasonal maximum in convergence <br />PMP results through the southern portion of the Southwest. <br /> <br />With these considerations in mind, a preliminary set of monthly PMP maps <br />was constructed tying magnitudes and gradients along the north to HMR No. 43, <br />along the west to HMR No. 36, and using the Indio maximized value as a control <br />on the magnitude in the southwest section. Pattern and magnitude in the east- <br />ern sections were controlled to a lesser extent by HMR No. 51. <br /> <br />.di :~ <br /> <br />-.- <br />