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.