Uncertainties in short-term forecasts of a Mediterranean heavy precipitation event: Assessment with satellite observations

The uncertainties in short-term forecasts of a 5 day episode of heavy precipitation in southeastern France were investigated. The episode took place from 19 to 23 November 2007 resulting in 400 mm of precipitation locally and was fairly typical of events known as "Cevenoles" that frequently affect the region. Several sets of 24 h meso-NH simulations were constructed that differed in their initial and boundary conditions. Comparison with rain gauges showed that simulations initialized from large-scale operational analyses failed to capture the intensity of precipitation associated with convective events and overestimated the amount of precipitation when the conditions were not convective. In contrast, simulations starting from the mesoscale analysis of the French operational mesoscale model ALADIN were more successful in forecasting the amount and location of the precipitation. Satellite observations revealed that this was due to a better prediction of the intensity of the surface wind over the sea during the stratiform regime and a more timely onset of convection over the sea related to a better prediction of the quantity of precipitable water. This study shows the benefits of using satellite observations to verify precipitation forecasts over the sea.