Heavy convective rainfall incidents that occurred over western coastal Greece and led to flash floods are analyzed with respect to mesoscale analysis for the period from January 2006 to June 2011. The synoptic scale circulation is examined throughout the troposphere along with satellite images, lightning data and synoptic observations of weather stations. Well-known instability indices are calculated and tested against synoptic observations. Taking into account the severity of the incidents, the performance of the indices was not as good as expected. Further detailed analysis resulted in the development of a new index that incorporates formalized experience of local weather and modeled knowledge of mechanisms of severe thunderstorms. The proposed index named Local Instability Index (LII), is then evaluated and its performance is found to be quite satisfactory.