Résumé
In this paper a heavy rainfall event which causes damages is studied. Rain quantities > 200 mm in 12 hours (100 years return period) were exceeded in some locations of Alpes-Maritimes Department (South –East of France) and of Italian Riviera. Such important quantites are due to two main factors : first is a high water vapour quantity (because of Sea Surface Temperature) available for condensation and precipitation. Second is strong vertical air ascendances caused by vertical thermal instability, important orographic effect,and powerfull cyclonism and vorticity. This event was very well forecasted by the Italian Bolam model, available on Internet, that allows to understand the weight of each component.