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VOL. 11, NO.4 WEATHER AND FORECASTING DECEMBER 1996 The Vaison-La-Romaine Flash Flood: Mesoscale Analysis and Predictability Issues STEÂ PHANE SEÂ NEÂ SI,PHILIPPE BOUGEAULT,JEAN-LUC CHEÁ ZE,PHILIPPE COSENTINO, AND ROSE-MAY THEPENIER Groupe de MeÂteÂorologie de Moyenne Echelle, MeÂteÂo-France, Centre National de Recherches MeÂteÂorologiques, Toulouse, France (Manuscript received 8 May 1995, in ®nal form 23 July 1996) ABSTRACT On the morning and early afternoon of 22 September 1992, a ¯ash ¯ood (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 ®ve precipitation systems. Orographic in¯uences generated a cold pool and focused convective cell development in a con®ned area, enabling two precipitation systems to be quasi- stationary. Precipitation forecasts by different versions of two models (a ®ne 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 signi®cant accumulations. Finally, some of the objective pieces of information useful for nowcasting rainfall location and duration for such an event are discussed. 1. Introduction a variety of data sources including pro®ler, aircraft, and Doppler radar data (e.g., McGinley et al. 1991; Brew- Forecasting severe convection and ¯ash ¯ooding ster et al. 1994; Beckman and Polston 1995). remains a dif®cult challenge to national weather ser- The availability of dense and frequent surface data vices and meteorological researchers. Maddox et al. in real time may also be insuf®cient. Even when com- (1979) and Chappell (1986) have studied quasi-sta- prehensive data are available, much effort and much tionary convective events that have led to severe and catastrophic ¯ooding. In analyzing 150 ¯ash ¯ood processing is required in order to use them effectively events over the United States, Maddox et al. (1979) (e.g., Moller et al. 1994). Numerical models are em- were able to de®ne three consistent and typical sce- ployed in only a limited number of experiments ori- narios involving lower- and upper-level patterns as- ented toward explicit real-time simulation of convec- sociated with excessive heavy rainfall. Chappell tion or convective organization types (e.g., Janish et al. (1986) explained the mechanisms enabling rain gen- 1995). In France, severe convection is rare (at least eration to remain stationary. before the recent succession of events), and conse- Forecasters at operational ®eld of®ces rarely bene®t quently few speci®c experiments or case studies have from the most advanced research concerning convec- addressed this issue. To contribute to the understanding tion. Moreover, in most weather services the amount of such extreme events in this region, researchers and of time forecasters can usually spend on a case is lim- forecasters must therefore rely on case studies that have ited, and the observations and numerical analysis and been documented using all the data available off-line. forecast tools available to forecasters today may be The present study evaluates a particular extreme largely insuf®cient for effectively employing recent re- event in a quasi-operational observing and modeling search results in forecast operations. Another consid- framework, and addresses some issues of predictability erable problem in many countries is the lack of high- in this context. For the investigation we employ surface resolution upper-air data for detailed situation analysis observations provided by automated weather stations, and model initialization. To solve this problem implies 5-min and 1-km resolution PPI radar data, and numer- operating assimilation systems capable of coping with ical modeling tools that are slightly more advanced than current operational tools (i.e., the same model but with increased resolution). During the late morning of 22 September 1992, sev- Corresponding author address: SteÂphane SeÂneÂsi, MeÂteÂo-France, eral mesoscale convective systems generated a ¯ash CNRM/GMME/PI, 42. Av. G. Coriolis, F-31057 Toulouse Cedex, France. ¯ood in the city of Vaison-La-Romaine, France, on the E-mail: [email protected] southern foothills of the Alps and not far from the Med- 417 q 1996 American Meteorological Society /3q06 0232 Mp 417 Tuesday Dec 31 03:47 PM AMS: Forecasting (December 96) 0232 Unauthenticated | Downloaded 10/02/21 07:15 AM UTC 418 WEATHER AND FORECASTING VOLUME 11 iterranean coast (Fig. 1). These systems produced 300 mm (11.8 in.) of rain in 24 h (Fig. 2), with 220 mm (8.7 in.) occurring in 3 h near the city. Surrounding areas also recorded intense precipitation (224 mm in 3.5 h in Barnas, ArdeÁche, some 90 km away) and re- ported damage, but not at the magnitude that struck in and around Vaison-La-Romaine where 35 fatalities oc- curred, hundreds of houses were destroyed, and prop- erty damage was estimated at nearly one billion dollars. Benech et al. (1993) state that over a 30-yr period in France a rainfall accumulation of over 300 mm has occurred on only 15 occasions during the later half of the year, and the return period for the value measured in Vaison-La-Romaine (179 mm in 2 h) is estimated between 40 and 150 years, depending on various as- sumptions. As reported by Jacq (1994), 119 cases of rainfall exceeding 190 mm in 24 h have been recorded during the last 37 years in southeastern France. The Mediterranean climate is known for its high precipita- tion rates during autumn (Jacq 1994; Martyn 1992) when unstable synoptic conditions are reinforced by warm and moist advection due to southerly ¯ows driv- FIG. 2. Rain accumulations for 22 September 1992. Thick lines ing Mediterranean air inland. Nevertheless, good doc- represent the 24-h rain accumulation analysis (in mm) beginning at umentation of such dramatic events is rare. MeÂteÂo- 0600 UTC 22 September. The stars indicate the rain gauge network. France internal studies have documented few severe Thin lines are elevation contours at 300 and 900 m. convection cases (four events that caused considerable destruction during the 5-yr period ending in 1991). However, Pircher and Saix (1991) and Barret et al. (1994) do describe similar cases in the same vicinity as the ¯ood we analyze here. The 22 September 1992 event was well forecast by MeÂteÂo-France. (The hydrological watch, however, which is not performed by MeÂteÂo-France, is not dis- cussed here.) According to Benech et al. (1993), 18 h before the ¯ood a warning was issued at a severity level rarely used (no more than 10 times a year for the whole of France), and shortly before the onset of the heavy rainfall, another bulletin was issued forecasting rainfall amounts of 200 mm in 24 h, with rates up to 100 mm in 3 h in the lower RhoÃne Valley and Alps foothills. These authors point out that rainfalls of this magnitude are not by themselves hazardous in all locations. They cite that 450 mm of rainfall caused only minor damage some 100 km away on the day prior to this event. But this case has demonstrated, at least to meteorologists, that the hydrological con®guration of the small catch- ment and subcatchments of the OuveÁze River, which passes through Vaison-La-Romaine, can make this re- gion prone to ¯ash ¯ooding. One can also state that even if the meteorological forecast had been successful, it may not have been accurate enough for hydrological forecasting for this catchment. Ultimately, the quality of the meteorological forecast relied on the forecasters' expertise in interpreting model output (the operational limited area model produced a largely underestimated FIG. 1. Geography and topography of the event area. The contours correspond to 300-, 900-, and 2100-m levels. GL stands for Gulf of but somewhat consistent picture of the situation; see Lion. The sounding locations and main mountain ranges are indi- section 4) and on their knowledge of the synoptic en- cated. The box shows the domain of the mesoscale analysis. vironment associated with such events. /3q06 0232 Mp 418 Tuesday Dec 31 03:47 PM AMS: Forecasting (December 96) 0232 Unauthenticated | Downloaded 10/02/21 07:15 AM UTC DECEMBER 1996 S EÂ NEÂ SI ET AL. 419 FIG. 3.(a) MeÂteÂo-France's of®cial surface analysis for 1200 UTC 21 September. Frontal drawings and observation plots are conventional. Cold front symbols with open triangles signify a cold front aloft. (b) The analysis of mean sea level pressure (solid line at 2-hPa intervals), pseudo-wet-bulb potential temperatures (dashed line at 27C intervals), and wind barbs for 0600 UTC 22 September. In this paper we present the synoptic and mesoscale wind shifts became clearer. Figure 3b illustrates, at aspects of the 22 September 1992 event in sections 2 0600 UTC 22 September 1992, the sharp gradient in and 3, respectively. In section 4 we provide an over- the pseudo-wet-bulb potential temperature ®eld asso- view of the NWP forecast. Section 5 concludes with a ciated with the cold front along the eastern Spanish discussion of some nowcasting issues, namely, the rain- coast and extending into southern France. This gradient fall location and duration nowcast. zone was also clearly delimiting different wind orien- tations.