The development of multiple low-level mesocyclones within a supercell

On 5 May 2007 over two dozen tornadoes were reported across eastern portions of Nebraska and South Dakota including several tornadoes greater than EF1. Three cyclic supercells developed in eastern Nebraska during the late afternoon and moved across eastern South Dakota and were responsible for multiple EF2 tornadoes in both states. While the mid-level structure was typical of tornadic supercells, near the surface, all three supercells developed two low-level mesocyclones. In at least two cases, both mesocyclones were producing tornadoes at the same time. This development had an impact on both the issuance of tornado warnings and also how to best describe the tornado threat to the public.

The focus of this paper will be on the development of the near surface mesocyclone. It is typically assumed when two mesocyclones are present within one supercell, it is part of the cycling of the storm as one mesocyclone occludes and the second forms resulting in a reinvigoration of the updraft and development of a new tornado. Using the inflow vector as a reference, this conceptual model assumes that the occluding mesocyclone is located downwind of the developing mesocyclone so that the inflow is cut-off to the former. In this case, radar observations and storm surveys show that the downwind mesocyclone developed after the upwind mesocyclone. Further, in at least two cases, the more significant tornado was associated with the downwind mesocyclone. An analysis of the life cycle of the mesocyclone for each storm is done to examine how they evolved. Documenting the unique evolution of the mesocyclones and the environment in which they formed may help forecasters anticipate a different type of storm evolution and issue polygon warnings appropriately. Further research could also be done to determine what unique aspects of the environment favored the development of multiple mesocyclones within the same supercell.