A dynamic and thermodynamic analysis of the 11 December 2017 tornadic supercell in the Highveld of South Africa Lesetja E. Lekoloane1,2, Mary-Jane M. Bopape1, Gift Rambuwani1, Thando Ndarana3, Stephanie Landman1, Puseletso Mofokeng1,4, Morne Gijben1, and Ngwako Mohale1 1South African Weather Service, Pretoria, 0001, South Africa 2Global Change Institute, University of the Witwatersrand, Johannesburg, 2050, South Africa 3Department of Geography, Geoinformatics and Meteorology, University of Pretoria, Pretoria, 0001, South Africa 4School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Johannesburg, 2050, South Africa Correspondence: Lesetja Lekoloane (
[email protected]) Abstract. On 11 December 2017, a tornadic supercell initiated and moved through the northern Highveld region of South Africa for 7 hours. A tornado from this supercell led to extensive damage to infrastructure and caused injury to and displacement of over 1000 people in Vaal Marina, a town located in the extreme south of the Gauteng Province. In this study we conducted an analysis in order to understand the conditions that led to the severity of this supercell, including the formation 5 of a tornado. The dynamics and thermodynamics of two configurations of the Unified Model (UM) were also analysed to assess their performance in predicting this tornadic supercell. It was found that this supercell initiated as part of a cluster of multicellular thunderstorms over a dryline, with three ingredients being important in strengthening and maintaining it for 7 hours: significant surface to mid-level vertical shear, an abundance of low-level warm moisture influx from the tropics and Mozambique Channel, and the relatively dry mid-levels (which enhanced convective instability).