Synoptic and mesoscale diagnosis of a tornado event in Castellcir, Catalonia, on 18th October 2006

Abstract This paper presents a synoptic and mesoscale analysis of a tornadic event in Castellcir, north of Barcelona (Spain), on 18th October 2006. The tornado caused F2 damage but, fortunately, there were no injuries. No strong forcing for upward vertical motion was identified from the synoptic analysis, but conditions were suitable for the development of convection. Despite the fact that CAPE values were not very high (more than 800 J/kg at 12 UTC), the level of free convection was low enough to favour the vertical development. A pivotal factor was the presence of an easterly low level jet and the shear profile of the hodograph near surface. Also, a thermal boundary was detected using surface observations. Operational radar observations allowed defining the precipitating structure as a mesoscale convective system (MCS) but no clear meso-cyclone was found linked to supercells features. The analysis of lightning data suggested three stages of the MCS, and similar results were obtained from the IR images study. Moreover, at the last stage of the MCS a bow echo which caused a microburst was detected at the northeast of Catalonia.