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I I I I I I I I I I I I I I I I II I I Introduction During the evening of August 18, 1991 a strong thunderstorm complex formed at the higher elevations of the Colorado Front Range and produced a flash flooding event in Virginia Canyon near Idaho Springs in Clear Creek County and in Central City in neighboring Gilpin County. The flash flooding produced by the intense thunderstorm rainfall briefly closed Interstate 70 and several Clear Creek County roads near Idaho Springs with rock and mud slides. Early rainfall estimates were placed at 2 - 3 inches of rain in less than one hour. Severe lightning accompanied the thunderstorm. The thunderstorm formed just west of the Continental Divide in eastern Grand County and moved south-southeastward into western Jefferson County where the storm dissipated. The storm track covered over 60 miles and crossed the Continental Divide. The storm's duration of 4.5 hours was over twice the average for strong mountain thunderstorms. The slow movement of this storm contributed to the significant rainfall amounts reported. The occurrence of strong thunderstorms in the higher elevations of the Colorado Rockies has been reported previously (Henz, 1974, 1990, and Henz et al, 1989). In most cases, however, the opportunity for a complete hydro-meteorological examination of a storm rarely occurs. It is hoped that this evaluation will assist in furthering our limited knowledge of high elevation flash flooding events. This report will present both a narrative of the meteorological causes of the thunderstorm system and a reconstitution of the storm rainfall over the Virginia Canyon area. The significance of this storm may be measured both on its implications to flash flood watch/warning issuance and in the potential understanding of high elevation flash flooding events. Description of the storm The Virginia Canyon storm occurred during the evening of August 18, 1991 along the Front Range of the Colorado Rockies. The storm formed over the higher terrain adjacent to the Shadow Mountain Reservoir recreation area near Grand Lake, Colorado ( see Figure 1 ) just west of the Continental Divide. Initial radar observations of the storm's formation were indicated by the National Weather Service (NWS) WSR-S7c at Limon, Colorado about 1745L (MDT). The NWS Limon radar is located about 125 miles east- southeast of Grand Lake (Figure 2) or at the western limits of the radar's utility for hydro- meteorology studies. The storm was observed until about 2230L when it dissipated over western Jefferson County and had approached to within about 85 miles of the radar. A radar morphology of the thunderstorm complex's life cycle is presented in Figure 3. The tracings of the storm's two-level radar echo are shown for approximately 30-minute intervals. The two-level echo portion of the storm correlates to a conservative estimate of the precipitating portion of the cloud. Storm movement continued to the southeast at about 15 mph as it crossed the spine of the Continental Divide into Boulder County shortly before 1900L.