Mesoscale Convective System Development, Synoptic Drivers, and Forecast Challenges of a Catastrophic Coastal Rainfall Event in West Africa

Abstract

Abstract This study elaborates on the (thermo)dynamical drivers of the second highest daily rainfall amount in historical records in the largest Ivorian city Abidjan with over 300 mm, which occurred on 18–19 June 2018 and caused severe socioeconomic damages in this metropolis. Based on unique (sub)daily rainfall data from stations across the District of Abidjan, this extreme amount was highly localized but embedded in a widespread area of daily totals in excess of 100 mm. Leveraging infrared and microwave satellite imagery, ERA5 reanalysis, as well as European Centre for Medium-Range Weather Forecasts deterministic and ensemble predictions, the following results are obtained. First, the extreme event was associated with a westward-propagating, long-lasting coastal mesoscale convective system (MCS), which, remarkably, was in a weakening stage as it entered the District of Abidjan. Then, a moist low-tropospheric vortex developed when the MCSs approached Abidjan. It was generated via vorticity tilting and vortex stretching most likely through interaction with the MCS, causing exceptionally high values of both moisture flux convergence and precipitable water. Finally, according to the extreme forecast index patterns in successive forecasts, an extreme event over the District of Abidjan was not predicted until roughly 12 h before rainfall onset, highlighting considerable challenges in the forecasting of this case. This study emphasizes the need to improve the representation of MCS–vortex interactions in numerical weather prediction models as a means to enhance early warning systems and climate services for extreme flood-inducing precipitation events in West African cities.