Laserfiche WebLink
<br />~L <br /> <br />Corri!!an and Vo!!el (1993) that the storm was an isolated. sin!!le cell storm al!pears <br />incorrect. The storm complex was actuallv a line of multi-cell thunderstorms with <br />several storms \lossibly actin!! like ouasi-st<!tioll ary suvercells for periods of about <br />30-45 minutes. Large quantities of hail which piled up to 5.6 inches deep accompanied <br />the storms and attest to their violence, No quantitative official or cooperative observer <br />measurements of the storm rainfall are available. A rancher did provide an unofficial <br />measurement of about 7 inches ofrain during the 2 hour period. The next section of this <br />storm summary discusses the HMS rainfall estimates. <br /> <br />Call a qualltitative estimate of the stOY/II rainfall be made? <br /> <br />HMS has completed several re.constlUctions of flash flooding events in which.rain and <br />radar observations were not available by using CG lightning data and the HMS <br />Convective Storm Methodology (CSM). These events include the Virginia Canyon stOlID <br />of August 18, 1991; the Rifle flash flood of May 15, ] 991; and the Robideau Creek flash <br />flood of August 10, 1993 (see Section 3.10 for a discussion of the Robideau Creek <br />event). A copy of Henz (1995) describing the re.construction process is included in <br />Appendix C. <br /> <br />In general, the HMS methodology relies on an. estimate of the hourly rain production <br />capability of the atmospheric stlUcture, tlle relationship of CG lightning production to the <br />radar echo structure and evolution and a radar..rainj'itll relationship based on local <br />atmospheric conditions. <br /> <br />HMS uses the equation below to estimat$ the peak hourly rainfall production in 5 and 6 <br />level radar echoes: <br /> <br />Peak hourly rain rate = PWI x (depth ofupdrafl warm layer/1.5 km) x 2 <br /> <br />where PWI is the Surface to 500 mb Precipitable Water Index. <br /> <br />In the case of the Opal StOlll a composite sounding was calculated using the soundings <br />from Lander, Wyoming; Grand Junction, Colorado; and Salt Lake City, Utah at <br />1200 UTC on August 16, 1990 and 0000 UTC on August 17. The surface temperature <br />and dew point were estimated to be 75 and 54 degrees F respectively. The PWI was <br />estimated as 1.04 inches and the depth ofthe updraft wmmer than 0 degrees C was <br />calculated as 2.2 km. Equation 1 becomes: 1.04 inches x 2.2 km/1.5 km x 2 <br />= 3.05 inches/hour. The CSM estimates the peak 30 minute rain rate as 2.15 inches, the <br />peak 10 minute rain rate as 1.28 inches, and the peak 15 minute rain rate as 1.50 inches. <br />These rain rates can be used with the 15 minute CG maps to estimate the storm rainfall. <br /> <br />HMS analyzed the position 0 f each of the peak CG centers for each of the plotted 15 <br />minute periods. The location of the centers is appm'ent in Figures 14 to 22. From <br />1600 LDT until 1630 LDT the peak 30 minute rain rate of2.15 inches was used for areas <br />of overlapping CG centers. A peak 15.minute rain rate of 1.50 inches was assigned to <br /> <br />42 <br />