Polarimetric X-band Radar Observations of a Failed Occlusion in the 20 May 2013 Moore, Oklahoma EF5 Tornado

On 20 May 2013, the cities of Newcastle and Moore, Oklahoma were impacted by a long-track violent tornado that was rated as an EF5 on the enhanced Fujita scale by the National Weather Service. The University of Oklahoma (OU) Advanced Radar Research Center's PX-1000 transportable, polarimetric, 100-W X-band weather radar was operating in a single-elevation PPI scanning strategy at the OU Westheimer airport throughout the duration of the tornado, collecting high spatial and temporal resolution polarimetric data every 20 seconds at ranges as close as 10 km and heights below 500 m AGL. This dataset allows the study of fine-scale phenomena occurring on very rapid time scales with the advantage of polarimetric data. This paper analyzes a series of debris ejections and their association with rear flank gust front surges that both preceded and followed a loop of the tornado as it weakened over the Moore Medical Center before rapidly accelerating and re-strengthening to the east. The gust front structure, debris/hydrometeor microphysics, and ZDR arc breakdown are explored as strong evidence for a "failed occlusion." Observations are supported by rigorous hand-analysis of critical storm features such as debris characteristics, tornado track relative to the NWS damage survey, and a storm-lifetime vorticity analysis. A conceptual description and illustration of the suspected failed occlusion process is provided, and its implications are discussed. Finally, some considerations regarding debris ejections and gust front surges and their association with damage surveys are presented.