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<br />METEOROLOGICAL SYNOPTIC PATTERN <br /> <br />A basic requirement for a rainfall which is to produce a flood <br />is an abundant source of moisutre. The synoptic weather patterns for the <br />few days preceding this flood event met this requirement well. An impressive <br />visual satellite photo taken on Friday morning, September 24 (Figure 1) <br />shows the extensive band of clouds extending from Hurricane Olivia, over the <br />tropical Pacific; northward well into British Columbia. While not all of <br />this moisture was produced by the hurricane, a good portion of it was, and <br />contributed to the saturation of the airmass over Utah during the next four <br />days. Figure 2 shows the increase in moisture over Salt Lake City from OOZ <br />on Saturday, September 25, to 12Z Saturday. From 12Z Saturday through <br />Tuesday, September 28, the air remained essentially saturated from the <br />surface through 400 mb. <br /> <br />By midday Saturday the dying remains of Hurricane Olivia had moved <br />to within 300 nautical miles of the southern'Ca1ifornia coast."",Sate11ite,,' .. <br />pictures at this time (Figure 3) show cloud cover over most of Nevada and <br />Utah while light rain had begun to fall over the area soaking the ground in <br />preparation for large amounts of runoff the following day. Figure 4 shows <br />the precipitable water values across the Western U.S. on Saturday evening <br />(OOZ Sunday). Note the band extending from southern California northeast- <br />ward into southwestern Idaho of values that exceed one inch. <br /> <br />Meanwhile, an upper level low pressure center was moving out of <br />the Gulf of Alaska onto the Pacific Northwest coast. Figures 5 through 7 <br />show this system as it progressed southeastward. The vorticity patterns <br />shown on these progs were not particularly helpful in timing upcoming <br />events.' For example, the chart for Sunday morning (Figure 6) actually <br />showed an area of weak NVA over northern Utah, when the next 6 hours <br />produced the heaviest rainfall of the storm. The mean relative humidity <br />chart for Sunday morning (Figure 8) showed the state under an area of <br />greater than 70 percent mean relative humidity. The 300 mb analysis and <br />maximum wind charts for 12Z Sunday and OOZ Monday (Figures 9 through 12) <br />show that the jet stream axis ran from central and southern California <br />northeastward through. eastern Montana during this period. This feature <br />provided plenty of venting action for the thundershowers which would form <br />along the front and an occasional jet maximum which would additionally <br />enhance the activity over northern Utah at times. Such a jet maximum <br />shows up over southwestern Idaho and northcentral Nevada at 12Z Sunday <br />(Figures 9 and 10), which is a good location to provide divergence aloft <br />and thereby enhance the vertical motion over northwestern Utah. <br /> <br />Surface charts for 12Z Sunday through OOZ Monday (Figures 13 <br />through 15) show a slow moving cold front from central Montana south <br />through Wyoming, then southwest through Utah and into central Nevada. <br />A low pressure center located over central Nevada just north of E1y <br />(Figure 13) appeared to move eastward along the front through 21Z <br />(Figures 14 and 15). .This low was no longer discernab1e by OOZ Monday <br />(Figure 16). Radar displays for this period (Figures 17 through 20) <br />show heavy convection just to the north of the low during this time <br />which corresponds to the period of the largest accumulation of rain <br /> <br />, <br />