| description abstract | Eight flash flood events occurred in the Tucson area of southeastern Arizona during the 1990 summer when a high-resolution lightning detection network was operated in the region. A total of 3479 cloud-to-ground lightning flashes was composited with respect to times and locations of these flash floods. The analysis region was a square of 40 km on a side that nearly coincided with a small hydrologic region that drains runoff from high mountains around Tucson and results in streamflow near and through the city. Most lightning in the 40-km-square area occurred between 10 h before the flood and the reported time of the flood. Flashes were most frequent around 2 h prior to the flood, but advance timing was not consistent. The most important factor in determining whether a flash flood report followed lightning was the number of consecutive 5-min periods with two or more flashes in the 40-km-square area. Intensity of the maximum flash rate was not systematically related to the amount of flooding, except that the 2 days with highest lightning frequencies were associated with the most widespread flood effects of the summer in the Tucson area. No precursor was found in positive flashes. While lightning data identified many of the flash flood events and avoided most false detections, the sample size was very small and there were no other cases for an independent test. Streamflow increased abruptly after the occurrence of lightning for two flood periods on 19?20 July and another flood on 24 July. About 2 h after the maximum flash rate, streamflow gauges at three locations in the hydrologic region measured rapid rises in runoff that were indicative of the arrival of flash floods. Additional studies with flashes from an operational lightning detection network need to take into account additional factors, such as different locations, seasons, storm types, and precipitable water in low to midlevels, in order to more fully explore the possibility of lightning strikes providing useful precursors of flash flood events, particularly in rugged semiarid terrain. | |
