A unique set of observations allows the analysis of the environment in which a back-building mesoscale convective system (MCS) developed along a dryline in the semi-arid southern Kalahari region of southern Africa during 28 and 29 December 2023. This MCS resulted in widespread rainfall, roughly equivalent to the December-February average, causing localized flooding and power failures. MCS development occurred along a dryline in a highly unstable, but weakly sheared environment. Near surface moisture gradients exceeded 6 g kg- 1 100 km- 1 in places on the dryline during morning and evening of 28 December but were weaker that afternoon. CAPE values exceeded 1000 J.kg- 1 shortly after sunrise within the first 100 km of the moist side of the dryline and MU-CAPE exceeded 3000 J kg- 1 at 08:00 and 12:00 UTC. Convection started adjacent to the dryline and once individual storm cells merged to form a MCS, new cells continued to develop along the dryline before being steered downstream by mid-level northwesterly winds. Back-building and a strong northeasterly low-level inflow contributed to renewed development along the storm's axis during the night-time, extending its lifespan to at least 13 h. These findings provide new insights into the contribution of MCSs to seasonal rainfall for the southern Kalahari region and a valuable example of mesoscale interactions between a dryline and individual storm cells at night.