The October‐1987 floods in Catalonia: Synoptic and mesoscale mechanisms

Abstract A heavy rainfall event which produced floods in Catalonia (Spain) during October 1987 is presented. Convection was located over the same area for several days, producing up to 431 mm of rain (67% of the mean annual rainfall). The synoptic‐scale pattern and forcing are studied using objective techniques, while mesoscale features are investigated with satellite imagery and subjective surface analysis. Strong quasi‐geostrophic forcing for vertical motion was diagnosed at low levels white only a weak forcing was evident at 500 hPa. Convective and latent instability characterized the region during the period of heavy rainfall. Also, there was strong low‐level moisture convergence combined with high ‐values of precipitable water over the north‐west Mediterranean Sea. Composite charts delineating the area covered by the indicated synoptic factors seem to be an effective tool for identifying the area where mesoscale focusing mechanisms became effective. Sub‐synoptic analyses show that the main focusing mechanism was a quasi‐stationary convergence line, located near the coast of Catalonia. This lay between the south‐eastern warm and humid flow, caused by a weak low centred north of the Algerian coast, and the cold outflow from a mesohigh located in Catalonia and mainly originated by the orographic blocking of the Pyrenees.