A Tale of Two Mergers: Comparing Examples of Tornado-producing and Non-tornado-producing QLCS-Supercell Mergers in Western South Dakota

Convective storm mergers have long been recognized to lead to changes in storm morphology, intensity and severe weather production.  This can include the onset or cessation of tornado production in supercell thunderstorms.  Recently, several studies have examined mergers between discrete supercell thunderstorms and quasi-linear convective systems (QLCSs) to better understand how interactions between these two different storm types may affect severe weather production and short-term convective evolution.  The present study builds upon this research by performing a detailed analysis of two recent squall line-supercell merger cases in western South Dakota, one of which produced an EF-2 tornado.

Both of the cases examined consisted of a QLCS merging with a relatively discrete supercell.  In the tornadic case, the QLCS was organized as a roughly north/south broken line of cells crossing the Black Hills of western South Dakota.  Just prior to the merger a region of strong westerly (inbound, toward the radar) winds developed within the broken line and merged with the southern rear flank of the supercell.  This region of strong winds appeared to behave analogous to a rear-flank downdraft surge that is often associated with tornadogenesis.  The tornado rapidly formed, persisted for approximately 12 minutes before dissipating as another cell merged with the forward flank of the supercell, cutting it off from undisturbed inflow air.  In the non-tornadic case, the QLCS took the form of a compact bow echo that travelled southwest to northeast across the southern Black Hills, merging with a nearly stationary supercell just north of Rapid City, South Dakota.  Outflow from the QLCS again merged with the southern rear flank of the supercell, evident as a broad region of strong inbound Doppler velocities.  A strong, compact low-level circulation developed, but did not strengthen into a tornado.  Following the merger, the supercell rapidly weakened as the outflow boundary surged northeastward.

This presentation will detail the evolution of these two cases using the radar data from a nearby WSR-88D, as well as available environmental data, including surface and upper air data as well as mesoanalysis fields from the Storm Prediction Center.  Of particular note, the WSR-88D was using the Supplemental Adaptive Intra-Volume Low-Level Scan (SAILS) scanning strategy for both of these events, which produced low-level scans at a much higher frequency than legacy scan strategies.  This allows us to examine the low-level velocity evolution during the merger process in greater detail than prior studies using WSR-88D data.  The differences between the characteristics of the pre-merger storms, geometry and morphology of the merger itself, and other factors will be evaluated to shed light on processes that may have contributed to the differing (tornadic vs. non-tornadic) outcomes in these two events.