An Observational Study of a Complex Tornadic Storm in South Dakota on 18 June 2014

A complex storm produced 12 tornadoes over a 2-h period across central South Dakota, and exhibited several traits that warrant a detailed examination. The tornadic storm evolved near the intersection of a warm front and inverted trough axis, along the southern end of a broken line of thunderstorms. Propagation of the storm was strongly influenced by a continuum of merging updrafts associated with the low-level convergence of the eastward moving inverted trough, with the low-level radar reflectivity structure relatively ill-defined. Storm mode fell mostly within the supercellular spectrum, yet featured an elongated/linear updraft orientation. Rapid cyclic mesocyclogenesis (~20 min/per cycle) occurred during much of the event, with virtually all tornadoes originating from one of six well-defined low-level mesocyclones. Observations from 61 wind turbine towers within 10 km of two tornadoes resolved sensitive changes in the low-level environment during the day, including an abrupt increase in storm-relative helicity immediately prior to the tornadoes. Cloud-to-ground lightning was particularly active with this storm, with a 5-min maximum of 297 flashes. Several significant tornadoes (rated EF2+), including an EF4 near the town of Alpena, South Dakota, occurred with the storm, but only two polarimetric Tornado Debris Signature (TDS) were briefly observed by surrounding radars. This case illustrates the sensitivity of the TDS, where precipitation entrainment around the tornado, radar range, and a general absence of damage targets prohibited a reduction in the ρHV fields during the strong tornadoes. Lastly, the research explores some of the new features of the WSR-88D RDA/RPG Build 14, including the increased temporal resolution of velocity data at the lowest elevation angle from the Supplemental Adaptive Intra-Volume Low-Level Scan (SAILS). Challenges with the Storm-Based Auto PRF (also a new feature in Build 14) unexpectedly changing the maximum unambiguous range, masking tornadic circulations, are also discussed.