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<br />continue observations for the remaining two hours. Two standing open buckets, one 12 <br />and one 13 inches deep, were filled to overflowing. Bishop (ibid) estimated rainfall for <br />the last half of the storm using observations and descriptions of rainfall intensities from <br />local residents. <br /> <br />Satellite, radar, and atmospheric sounding data attest to the intensity of the Frijole Creek <br />storm. Echo tops reached 45,000 feet msl between 9:00 pm July 2 and 2:00 am on July <br />3. The Denver evening sounding (0000 UTC, 3 July) showed very high precipitable <br />water of 1.32 inches and a lifted index of -5 for a surface temperature of 72 degrees F <br />and surface dewpoint of63 degrees F. Initial estirnates of precipitation potential using <br />the HMS Convective Storm Methodology (CSM) show a 3.4 kilometer warm layer <br />available for coalescence rain formation. The depth exceeds the CSM requirement for a <br />doubling of rainfall potential. CSM analyses show the potential for 6 inches per hour <br />during the height of the storm. Based 011 the observations of]ocal residents, rainfall rates <br />of such intensity probably lasted an average of 2 hours hroughout the 22.7 square mile <br />area of the basin above the railway bridge. Allowing the CSM 60 minute rainfall rate to <br />persist for 2 hours gives a 12-inch total close to observed amounts. Since the storm was <br />apparently "locked in" for about 4 hours, these intense rates may have persisted for 3 to 4- <br />hours over some locations producing isolated 4--hour rain falls approaching 24 inches. <br />Such a total is consistent with Bishop's (ibid) suggestion that much higher cumulative <br />precipitation approaching 20 to 24 inches likely OCCUlTed in the higher elevations <br />southwest of the storm center. Such extreme 4-hour totals are also reasonable given the <br />nearly steady-state fetch of very moist low-level air (precipitable water consistently <br />exceeding 1.3 inches) continuously channeled into the upper reaches of the Frijole Creek <br />Basin. <br /> <br />The meteorological setting that produced the extreme precipitation event in the Frijole <br />Creek Basin on July 2-3, 198 I was similar in some respects to those leading to both the <br />Penrose storm of 1921 and the Big Thompson Canyon stoml of 1976. In all three <br />instances, topography acted to focus low,level moist inflow. One notable difference, <br />however, is in the apparent lack of brisk southeasterly inflow on the Denver and Grand <br />Junction evening soundings. The potential for heavy rainfall was realized as an unstable <br />air mass was forced upward by steeply rising terrain. Heavy tlmnderstorms developed <br />and remained nearly stationary for a few hours under relatively weak mid-level steering <br />winds. ]n fact, winds to 18,000 feet msl were less than 10 knots out of the southwest at <br />Grand Junction at 00 UTC on July 3 and less than is knots out of the west-southwest at <br />Denver for the same date and time. <br /> <br />The heavy thunderstorm that caused flooding in the Frijole Creek Basin, unlike other <br />events considered in the present study, was not an isolated local thunderstorm. Daily <br />precipitation records show that more than I inch of rain also fell on this day at the Rye <br />and Trinidad, Colorado stations suggestil1g that the Frijole event was part of a General <br />Convective Storm (GCS). A GCS, or a complex convective storm system as it is called <br />in HMR 55A, consists of convective stonllS each of which will often last more than 6 <br />hours and cover more than 500 square miles. The total deration of all convective storms <br /> <br />20 <br />