On August 7, 1996, a flood of the Rio Gallego, in Aragon (Spanish central Pyr¿n¿es) wiped out the Biescas' campground, killing more than 80 persons. From the observations of two meteorological radars, this thunderstorm exhibited some of the radar characteristics of a supercell. It spread almost exactly over the Rio Gallego catchment area, up-stream of Biescas, and it remained above this area for about two hours, without any advection. The rainfall reached 100 mm h-1 and the total cumulative amount locally reached 200 mm. The cloud-to-ground (CG) flash density averaged over 5¿5 km2 areas, was exceptionally high, around 2 km-2, and the peak flash rate averaged over 5-minute periods reached 11.6 min-1 within the cell area. The evolution of the CG flash rate and the radar reflectivity were closely correlated. The flash rate reached high values before the arrival of the precipitation at the ground. We observe also a very good agreement between the location of the intense rain at the ground and the high CG flash density. The rainfall water volume per flash was 3¿104 m3, a value in agreement with previous work. We propose that quantitative measurements of the lightning activity may provide valuable information for flash-flood nowcasting and short-term forecasting in mountainous areas. ¿ 1998 American Geophysical Union