The role of a surface boundary and multiple-cell mergers in the development of the 21 April 2003 tornado in Upstate South Carolina

During the late afternoon and evening of 21 April 2003, several thunderstorms formed along a quasi-stationary surface boundary across northeast Georgia and southern Upstate South Carolina. Many of the cells exhibited weak rotation, though only one maintained a persistent radar defined mesocyclone and Tornadic Vortex Signature (TVS). This storm maintained a mesocyclone for over an hour, nearly coincident with its east-southeast track across Abbeville County. The mesocyclone became progressively stronger, and a brief tornado occurred over the southeast part of the county. While only a small area was affected, the tornado blew down 75 percent of the canopy in a 20-acre section of the northwest slope of Parson’s Mountain, a 300-foot rise from the surrounding terrain.

The development of a weak tornado in association with this multicell severe thunderstorm is examined. It is shown that the synoptic environment was not conducive to tornado formation because of weak low-level wind shear with little veering of the low-level flow away from the boundary. However, there appears to have been sufficient storm relative environmental helicity along the boundary to result in a persistent low-level mesocyclone. Time-height trends of the mesocyclone show that each of at least three cell mergers resulted in the strengthening and deepening of the mesocyclone and eventual tornado formation. The strength of the mesocyclone and the radar-indicated tornado vortex were considerably weaker than is shown in previous work referencing similar tornadoes. In fact, the mesocyclone and TVS became strongest after the tornado occurred. A successful tornado warning decision was made based upon the storm’s course parallel to a well-defined surface boundary, tight low-level circulation, and low lifting condensation levels. This study documents a case in which radar analysis was augmented by knowledge of the near storm environment to issue a successful tornado warning in a synoptic situation that was not considered conducive to tornadogenesis.