Damage Survey and Multisensor Analysis of a Cold Season Tornado Outbreak in Southern Brazil

Abstract On 11–12 June 2018, a tornado outbreak in South America caused significant damage in northeastern Argentina and southern Brazil. Unseasonably warm and moist air over the upper La Plata basin led to ample conditional instability, in conjunction with strong wind shear associated with an approaching shortwave trough. A surface trough was the focus of supercell initiation in the outbreak area. In particular, a cyclic supercell produced seven tornadoes with intensities ranging from F0 to at least F3 on the Fujita scale based on damage surveys and digital media. One of the tornadoes rated F3 produced one of the longest and widest damage tracks ever recorded in South America with a pathlength of 51 km and a maximum width of 950 m. In total, 11 tornadoes occurred in this outbreak. The highlight of this work is the documentation of the damage by combining environmental satellite imagery and aerial imagery with datasets rarely available in the region including dual-polarization Doppler radar and ground-level damage analysis. Radar data showed that areas of strong azimuthal shear coincided with tornadic debris signatures (TDSs) that matched the tracks. Deep TDSs were observed in the most intense tornadoes, supporting classifications of F2/F3 or higher on the Fujita scale. Remarkably, there were two instances where at least two tornadoes occurred simultaneously during the event. This marks the first documented case of cyclic tornadogenesis and simultaneous supercell tornadoes in South America, observed through dual-polarization Doppler radar.