Patterns of Cloud-to-Ground Lightning and Convective Rainfall Associated with Postwildfire Flash Floods and Debris Flows in Complex Terrain of the Western United States

Abstract

Flash flood and debris flow reports from Storm Data and the U.S. Geological Survey (USGS) are used to investigate the relationship between hazardous hydrological responses, convective rainfall, and cloud-to-ground (CG) lightning flash parameters. Basins burned by the Coal Seam and Missionary Ridge wildfires during the summer of 2002 in western Colorado were selected as primary study areas. The North American monsoon (NAM) air mass played a pivotal role in providing low-level moisture over much of Colorado during each of the 12 hydrological events identified. Surface ?e values as high as 354 K were calculated over western Colorado in a composite analysis that also saw a ?e ridge through 500 hPa extending northward into Nebraska and southern South Dakota. Storm-total CG flashes were as high as 718, and the median flash total for the population of events was 256. Mean 5-min CG flash intensity for the events was 18.1 flashes. The mean rainfall intensity associated with the 12 hydrological events was 10.5 mm h?1, and average storm-total rainfall was 14.2 mm. Continuous CG flash sequences (one or more flashes per sequential 5-min interval) were significantly correlated with rainfall intensity, total rainfall, and rainfall duration for events generating postwildfire flash floods and debris flows. Operational utility may be derived from the observation of the mean lag time from the first recorded CG flash in a 7850-km2 study area to the peak rainfall intensity, and the mean lag time from peak CG flash rate to peak rainfall at the hydrological event location. The mean lag times for the 12 events in the study were 165 and 41 min, respectively.