Impacts of Radar Scan Time on Observed 31 May 2013 Supercell Evolution: An Operational Perspective

On 31 May 2013, a supercell produced an EF-3 tornado near El Reno, Oklahoma that was sampled by the Phased Array Radar (PAR) at the National Weather Radar Testbed in Norman, Oklahoma. Given the ample tornado-warning lead time of 27 min provided by the Norman Weather Forecaster Office, an operational question of interest was whether rapid-scan radar data may have aided the tornado warning process. The one-min volume updates of this PAR data set provided the opportunity to address this question from 1) an operational forecaster's perspective and 2) quantitatively.

To gain an operational perspective, a radar data-focused interview was performed with the NWS forecaster who issued the warnings for the El Reno tornado. This interview aided the forecaster's ability to recall and document his real-time tornado warning decision process. Thereafter, he was asked to review the PAR data and to point out and comment on the importance of any differences he saw in supercell evolution. The forecaster identified two primary storm features as being important to his tornado warning process: 1) intensity of rotation in the low and midlevel mesocyclone, and 2) area and magnitude of low-level inflow. While viewing the PAR data, he reported seeing more detail in the evolution of these features. He also noticed interactions of the supercell with other storms that were not sampled by the WSR-88D that he thought would have aided his decisions. In particular, the timing of his decision to end the Tornado Emergency that was in effect for the Oklahoma City area. To quantify differences in the intensity of rotation and inflow sampled by the PAR and the WSR-88D, 1) maximum delta V within each mesocyclone, and 2) the area of velocities 20 m s-1 and higher and mean velocity within the inflow region were computed during the 42-min period preceding tornadogenesis. Analysis showed the advantages of high-temporal resolution data in observing each of these crucial elements. In particular, the PAR provided a more complete picture regarding the intensification and dissipation of each of the three mesocyclone signatures produced during the analysis window. PAR data also sampled periods of increased inflow magnitude that closely corresponded to mesocyclone intensification periods.