Analysis of a large-hail producing supercell and its merger with a nascent supercell on 6 october 2015

This study documents the lifecycle and large-scale environment of a large-hail producing supercell that occurred in southern Brazil (SB) on 6 October 2015. The supercell underwent a merger with a nascent supercell near a Doppler weather radar and rapidly intensified shortly after the interaction. The original (premerger) supercell developed in a high-shear, high-instability pre-cyclogenesis environment and remained isolated until the merger due to the presence of a relatively rare elevated mixed layer that helped suppress widespread convection and provided steep lapse rates for robust updrafts. A small cell developed ~ 30 km to the north of the original supercell and merged in between its inflow and forward-flank regions, resulting in a new (postmerger) supercell that inherited the main features of the intensifying nascent cell. Radar-derived quantities related to supercell intensity reveal that the updraft and mesocyclone intensified significantly within 30 min of the merger, resulting in a large episode of hailfall with 6 + cm hail reported at the surface. After reaching peak intensity, the postmerger supercell and its attendant low-level mesocyclone rapidly decayed, likely as a result of a hailfall-induced outflow surge and large convective inhibition. This case illustrates the importance of mergers between mature and nascent supercells to the short-term forecasting of severe weather, a topic that received little attention in studies and prediction of storms in SB.