An examination of varying supercell environments over the complex terrain of the eastern Tennessee River Valley

The climatological minimum in tornado frequency over the eastern Tennessee River Valley is often attributed to the complex terrain of the region. While tornadoes are relatively uncommon in the eastern Tennessee River Valley (compared to areas further west and south), several significant tornado outbreaks have occurred in this area (e.g., the Super Outbreak of April 1974 and the Veterans Day Weekend Outbreak of November 2002). In addition, there have been several events during the past decade when numerous supercells formed in this area, but the efficiency of these supercells in producing tornadoes varied greatly. Five events in the eastern Tennessee River Valley with varying levels of forecasting and warning difficulties were examined to find any interesting similarities or differences. The 28 April 2002 event was chosen since forecasters expected supercells and tornadoes that day, but ultimately no tornadoes formed despite the presence of several supercells. The 15 May 2003 and 25 April 2006 events were chosen since supercells and tornadoes were not expected on those days, but an isolated tornado-producing supercell formed during each event. The 10-11 November 2002 and 8-9 May 2009 tornado outbreak events were chosen, since the 10-11 November 2002 outbreak produced a relatively low tornado-producing efficiency (with the tornadoes mostly confined to the Cumberland Plateau and a few strong supercells in the Great Tennessee Valley not producing tornadoes) and the 8-9 May 2009 outbreak produced a high tornado-to-supercell ratio (with nine tornadoes reported overall across both the Cumberland Plateau and Great Tennessee Valley).

Overall, this study determined that the events with a high tornado-producing efficiency (15 May 2003, 25 April 2006, and 8-9 May 2009) were characterized by the tornado-producing supercells tracking near a well-defined, preexisting surface boundary, while the events with a low tornado-producing efficiency (28 April 2002 and 10-11 November 2002) had supercells that did not track near well-defined boundaries. During the null event on 28 April 2002, the high LCL heights, lack of low-level directional wind shear, and the paths of the supercells away from a well-defined surface boundary likely caused the lack of tornado development. The 10-11 November 2002 and 8-9 May 2009 events (the two outbreak events in this study) experienced the strongest low-level helicity values in the study, although the low-level instability values were the weakest. While the strongest low-level wind shear was present with the two outbreak events, the higher low-level instability during the other three non-outbreak events may have compensated for the weaker (but still significant) low-level wind shear. While low LCL heights were present near the locations of the tornadoes during both of the outbreak events, the influence of LCL heights during the other two tornado-producing events was inconclusive due to the possibility of frontal augmentation of the RUC40 model depictions. An axis of strong advection of equivalent potential temperatures was present during four of the events near the locations of the observed supercells and tornadoes, with the only exception being the 8-9 May 2009 outbreak event. The observed weak advection (in combination with a mid-level stable layer) may have kept the supercells isolated during this event, and thereby prolific in producing tornadoes by limiting the competition for the available instability.