The Vaison-La-Romaine Flash Flood: Mesoscale Analysis and Predictability Issues

Abstract

On the morning and early afternoon of 22 September 1992, a flash flood (220 mm of rain in 3 h) occurred in the city of Vaison-La-Romaine, located in southeastern France, causing numerous casualties and considerable property damage. It was generated by a combination of several mesoscale convective systems ahead of a slow-moving cold front associated with a cutoff low. The large-scale setting and a mesoscale analysis of the case, together with estimates of radar-derived rain accumulations, are presented. The mesoscale analysis demonstrates the complexity of the case, which involved five precipitation systems. Orographic influences generated a cold pool and focused convective cell development in a confined area, enabling two precipitation systems to be quasi-stationary. Precipitation forecasts by different versions of two models (a fine mesh version of an operational limited area model, and an operational stretched global model) are summarized. They demonstrate the extent to which realistic rainfall forecasts may be produced for extreme meteorological events such as this. One model exhibits a rather precise and realistic distribution and evolution of the precipitation patterns, while all show significant accumulations. Finally, some of the objective pieces of information useful for nowcasting rainfall location and duration for such an event are discussed.