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<br />2. Cloud-to-ground lightning observations archived by Global Atmospherics lnc. as part <br />of the National Lightning Detection Network (NLDN). <br /> <br />HMS employed these data sources in three prior successful reconstructions of flash <br />flooding events in the westem and central Colorado mountains; the Virginia Canyon <br />Flash Flood of August 18, 1991; the Rifle, Colorado Flash Flood of May 15, 1991; and <br />the Robideau Creek Flash Flood of August 10, [993 (see Section 3.10 of the present <br />repoli for a discussion of the Robideau Creek event). In each case no radar data was <br />available to help define the temporal, aerial and intensity distribution of the associated <br />rainfall. A discussion of the technique used to associate the cloud-to-ground lightning <br />data to surface rainfall is repolied in Henz (1994) and the reader is refelTed to this paper <br />in Appendix C. <br /> <br />This storm summary investigates three primary areas of interest about which the <br />following significant questions can still be asked: <br /> <br />1. What were the local meteorological causcs of the thunderstorm event? <br /> <br />2. What were the aerial coverage, temporal duration and location aspects of the storm? <br /> <br />3. Can a quantitative estimate of the storm ramfall be mede? <br /> <br />Possible local meteoroloflical causes of tlte ONJ!l StOrI,-! <br /> <br />Corrigan and Vogel (ibid) describe the general meteorological setting for the Opal slonn <br />well. They identify the approach of a weak upper level thermal short wave into an <br />unstable moist atmosphere with converging low level winds as the large scale factors <br />triggering the storm. These filctors are abetted by the diurnal heating cyclc to produce a <br />single cell thunderstorm in an atmosphere of weak vertica: and horizontal shear. This <br />single cell storm is portrayed as an isolated event which produces the flooding rainfall <br />near Opal. No locations are given for heavy rainfall nor is the place reporting 7 inches of <br />rain identified. No mention, other than the comments of a local rancher, is made of the 5- <br />6 inches of hail which accumulates and covers the ground. No comments are made about <br />the extraordinary lightning production of the storm. In short, the reader is left with more <br />questions than answers. <br /> <br />HMS started the analyses by plotting hourly surface maps for Utah, Idaho and western <br />Wyoming from 1400 LDT umil 1800 LDT. The surface maps show the approach ,md <br />passage of a weak surface low pressure area and associated surface discontinuity with <br />moist southwesterly winds preceding it through Opal between 1500 LDT and 1 800 LOT. <br />Additionally, the Rock Springs airport observes the passage of a surface gust front <br />associated with a northward moving line of cumulonimbus clouds between 1400 LDT <br />and 1500 LDT. In effect, a strong area of surface convergence can be found focusing on <br />Opal near 1600 LDT or about the reported time of the storm's formation. <br /> <br />26 <br />