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<br />structure with 30 to 90 minutes of multi-supercell structure. <br /> <br />HMS analysis shows a favorable converging low level wind field with pre-trough <br />southwesterly winds and gust-front-generated southerly winds. The two air masses <br />converged at 20 to 40 knots relative speeds providing lift of the unstable air masses as <br />they moved upslope 400 feet to 800 feet into the plateau region around Opal. The <br />atmosphere above this lifting air mass became more unstable as the thermal short wave <br />crossed southwestern Wyoming. Given the atmospheric structure, the diurnal heating <br />cycle helped to set the stage for storm formation but probably acted as a minimal storm <br />trigger. In short, the quantitative setting was ideal for strong thunderstorm formation. <br /> <br />What were tlte aerial coverarre. temooral duratioll alld locatioll aspects of the storm? <br /> <br />Little is known of the size and development sequence of the Opal storm other than the <br />description of a local rancher attesting that the stonn came from the southwest, started <br />about 1610 LDT, and produced flooding rains, hail to 6 inch depths and an impressive <br />lightning display before ending about 1745 LDT (Corrigan and Vogel, ibid). No National <br />Weather Service radar data was available for southwestem Wyoming and satellite photos <br />provided only a limited description of the stonn sequence. To help fill in these gaps in <br />data, HMS used cloud-to-ground lightning data (CG) gathered by the National Lightning <br />Detection Network (NLON) and archived by Global Atmospherics, Inc. <br /> <br />Figure 11 shows the CG data for southwestern Wyoming in a 25 mile radius of Opal, <br />Wyoming from 1500 LOT to 1800 LOT on August 16, 1990. A total of 576 CG's were <br />detected within 25 miles of Opal mostly to the east through southeast. A similar chart for <br />1800 LOT to 2100 LOT detected only 7 CG's within 25 miles of Opal confirming the <br />prime storm time. Table 1 shows a sample ofthe raw lightning data in tabular fonn <br />which HMS used for plotting 15 minute CG charts starting at 1545 LDT. The first <br />column gives the date, the next the time of the CG, the latitude and longitude of the CG, <br />and finally the CG's strength, bearing and distance from the center point. <br /> <br />Figures 14 through 22 show 15 minute plots and analyses of the CG data for the Opal <br />stormes) with strikes discriminated into discrete cells of activity. The stormes) gets <br />started between 1540 LOT and 1600 LOT (see Figure 14) as a group ofCG cells fonn <br />along an arc from south to southeast about 15 miles from Opal. No heavy rain is falling <br />during these times and only 18 CG's are reported in the 15 minute period. These cells <br />appear to be fonning along the gust front which moved through Rock Springs <br />about 1500 LOT. <br /> <br />Rapid development into 3-5 CG cells is noted in the next 15 minutes or by 1615 LDT <br />(see Figure 15). This time period agrees with the observations of the local rancher that <br />the storms started about 1610 LOT. Three of the CG cells appear capable of producing <br />moderate rain by this time. The line or arc of stonllS is moving north-northwest at 20 <br />mph parallel to US 30. <br /> <br />29 <br />