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<br /> <br />,. <br /> <br /> <br />CHAPTER II - METEOROIDGY AND HYDROLOGY <br /> <br />4. Antecedent conditions. - An unusual combination of meteorological <br />events early in September 1970 resulted in flood-producing rains <br />over the Upper Colorado Region of Arizona, Colorado, New Mexico <br />and Utah. en 2 September tropical storm "Norma", located in the <br />Pacific Ocean just west of the southern tip of Baja, California, <br />initiated a flow of warm, moist tropical air toward the Upper Color- <br />ado Region. An intense low pressure system aloft just north of <br />Vancouver Island initiated a tlow of unusually cold arctic air <br />towards the Upper Colorado Region. By early afternoon on 5 September, <br />the coincidental movement of these two dissimilar air masses, <br />accompanied by strong southerly winds of 30 to 40 miles per hour, <br />caused heavy rain to develop in the lower 10,000 teet of the air <br />III&8ses. Large amounts of rainfall were produced over a 6 to 12-hour <br />period with the heaviest amounts along south slopes of natural <br />barriers. At Bug Point, Utah, a remote cOlBllllJ1ity near Montezuma <br />Creek in San Juan County, the 6 inches of rain recorded for an 11- <br />hour period set new 12- and 24-hour precipitation records for that <br />part of the state. <br /> <br />5. en 12 September, a week after the record-breaking stona, a cold <br />front moved through the same general area. Weather aBsociated with <br />this front caused from 1 to 3 inches of rain in the area, which <br />again increased streamflow but caused very little flooding. <br /> <br />6. storm precipitation. - After the first wave of the storm, the <br />National Weather Service (formerly U. S. Weather Bureau) conducted <br />a ''bucket survey" of the affected area. Results from this survey, <br />along with rainfall amounts from official rain gages, are contained <br />in the precipitation data shown on Table 1 and the isohyetal map <br />shown on Plate I. Most of the rain occurred during a 12 to 18-hour <br />period beginning before noon on 5 September. <br /> <br />7. From the isoh,yetal IIIap it is noted that the main storm center <br />was over the area of Montezuma and McElmo Creeks with a second <br />center located over the drainage area above Navajo Lake. This is <br />the most intense rain storm recorded for the Montezuma Creek area. <br />U. S. Weather Bureau Technical Paper 40 indicates it had a 12 hour <br />duration frequency rarer than once in 100 years. <br /> <br /> <br /> <br />5 <br />