The Sequence of Heavy Precipitation and Flash Flooding of 12 and 13 September 2019 in Eastern Spain. Part I: Mesoscale Diagnostic and Sensitivity Analysis of Precipitation

The Mediterranean region is frequently affected by heavy precipitation episodes and subsequent flash flooding. An exemplary case is the heavy precipitation episode that occurred in the regions of Valencia, Murcia, and Almeria (eastern Spain) on 12 and 13 September 2019. Observed rainfall amounts were close to 500 mm in 48 h, causing seven fatalities and estimated economical losses above EUR 425 million. This case exemplifies the challenging aspects of convective-scale forecasting in the Mediterranean region, with kilometer-resolution meteorological fields required over long forecast spans. Understanding the key mesoscale factors acting on the triggering, location, and intensity of the convective systems responsible for extreme accumulations is essential to gain insight into these episodes and contribute toward their accurate hydrometeorological forecasting. Mesoscale diagnosis suggests that local and distant orography, together with air-sea fluxes, were instrumental in developing convection and intensifying precipitation rate. Sensitivity experiments confirm the role of orography in organizing the cyclonic flow over the southeast part of the western Mediterranean, and also acting as a convection-triggering mechanism. Furthermore, results highlight the role of latent heat flux from the Mediterranean Sea in enhancing convective instability at lower levels and moistening the environment. These moist feeding flows substantially contribute to increasing precipitation rates. Such high sensitivity to environmental moisture distribution naturally propagates to the sea surface temperature, which, by means of sensible and latent heat flux exchanges, dominated the evolution of convective activity for the 12-13 September 2019 episode.