A synoptic and mesoscale diagnosis of a tornado outbreak in the Balearic Islands

A tornadic event occurred over the Balearic Islands (Western Mediterranean) during the evening of 11 September 1996 and the following night. A total o f six tornadoes were observed, affecting populated areas, with an economica l damage of more than 6 million Euro. The meteorological situation in which severe weather developed was characterised at low levels by a low covering all the Western Mediterranean with well-marked warm advection towards the Balearic Islands. At mid and upper levels, a low was located to the southwe st of the Iberian peninsula, producing southwesterly winds over the region. Satellite imagery shows that the first tornado, observed over the Ibiza Is land, was produced by a mature thunderstorm, which presented a well-defined V-shape on the IR images. Tornadoes occurring in Majorca and Minorca islan ds were produced by convective systems, which were in their initial develop ment state. Most of these tornadogenetic convective systems developed over a low-level convergence line formed as a consequence of the existence of a low moving northeastwards along the south of the Balearics and a very small and deep cyclone formed offshore in front of the Valencia coast. Positive interaction between the low-level convergence line and an upper-level jet s treak for producing the lift of low-level parcels has also been identified. (C) 2001 Elsevier Science B.V. All rights reserved.