Two cases of severe weather in Catalonia (Spain). A diagnostic study

A diagnostic study of two cases of severe thunderstorms in Catalonia (the n orth-eastern region of Spain) is presented. The first occurred on 24 August 1993. A squall line, with bow echo configuration in the radar images, cros sed Catalonia from west to east. Strong winds, large hail up to 7 cm in dia meter and heavy rain producing floods were observed. The meteorological sit uation was characterized by an upper-level trough approaching Spain from th e north-west. This upper trough developed a low, which displaced the warm, dry dir located order the Iberian Peninsula towards the east and north-east . The warm, dry air overran the cold, humid Mediterranean air producing a s trong inversion (LID) capped by an elevated mixed layer over the western Me diterranean and also over the Ebro valley. Hand-drawn analyses permitted th e identification of a secondary trough; a combination of this trough and a jet streak located over Spain was responsible for the mesoscale lifting mec hanism that was able to break the LID and maintain the deep convection. The second case occurred on 31 August 1994. A tornado developed over south Cat alonia and large hail, up to 5 cm in diameter also fell. The meteorological situation was also characterised by an upper-level trough advancing from t he west towards Spain. Accurate hand-drawn analyses identified a small ther mal low over Catalonia leading to strong convergence at low levels. Over th e same area, two jet streaks overlapped their left exit and right entrance zones, producing an enhancement of the ascending motion forced by the low-l evel convergence. However the typical environment for supercell development was not present over Catalonia. The tornado was probably produced by a non -supercell thunderstorm.