Right-Moving Supercell Tornadogenesis during Interaction with a Left-Moving Supercell’s Rear-Flank Outflow

Abstract

Abstract On the local afternoon of 29 May 2012, a long-lived, right-moving (RM) supercell formed over northwestern Oklahoma and turned roughly southeastward. For >3 h, as it moved toward the Oklahoma City, Oklahoma, metro area, this supercell remained nontornadic and visually high-based, producing a nearly tornadic gustnado and a swath of significantly severe, sometimes giant hail up to 5 in. (12.7 cm) in diameter. Meanwhile, a left-moving (LM) supercell formed over southwestern Oklahoma about 100 mi (161 km) south-southwest of the RM storm, and moved northeastward, with a rear-flank gust front that became well defined on radar imagery as the LM storm approached southern and central parts of the metro. The authors, who had been observing the RM supercell in the field since genesis, surmised its potential future interaction with the LM storm’s trailing gust front about 1 h beforehand. We repositioned to near the gust front’s extrapolated collision point with the RM mesocyclone, in anticipation of maximized tornado potential, then witnessed a small tornado from the RM mesocyclone immediately following its interception of the boundary. Synchronized radar and photographic images of this remarkable sequence are presented and discussed in context of more recent findings on tornadic supercell–boundary interactions, with implications for operational utility. Significance Statement Supercells—well-organized, rotating thunderstorms mainly found in midlatitudes—commonly produce the largest hail, along with damaging gusts and most tornadoes. In radar imagery and photographs, we show the characteristics and merger of two supercell types: left-moving and right-moving, with respect to winds aloft. As the left-moving storm’s trailing gust front interacted with the right-mover’s mesocyclone, the latter strengthened quickly, soon producing a tornado. Observed evolution of these storms supports idealized numerical and conceptual models for supercell behavior and interactions with storm-scale boundaries, and may be useful in short-fused tornado forecast and warning operations.