The “Broken-S” Radar Signature and its Use as a Nowcasting Indicator for Tornadogenesis in Quasi-Linear Convective Systems

Detection of non-supercell tornadoes remains one of the most significant challenges to
warning operations within the National Weather Service (NWS) as they are often, short-
lived and the tornado signature can exist primarily below the lowest radar elevation scan.
Despite limited documentation in the formal literature, the "Broken-S" radar signature
has become a somewhat reliable indicator for rapid, non-supercellular tornadogenesis
within Quasi-Linear Convective Systems (QLCS) during tornado warning operations.
On 16 November 2016, and 21 January 2017, two separate QLCS tornado outbreaks
occurred across Georgia resulting in a total of 29 tornadoes across the NWS Atlanta
county warning area (CWA), with at least a dozen tornadoes being preceded by the
broken-S signature. Using dual-polarimetric, Doppler radar data from two local WSR-
88Ds details regarding the structure and evolution of the broken-S signature in its relation
to tornadogenesis will be presented. This study also investigates the statistical reliability
of the broken-S signature as a precursor to tornadic development as well as the proportion
of these tornadoes to overall tornadic activity in the NWS Atlanta CWA in recent years.