Tornadoes in Environments with Small Helicity and/or High LCL Heights

Recent studies have suggested that supercell tornado environments are usually associated with large 0–1-km storm-relative helicity (SRH) and relatively low lifting condensation levels (LCL heights). However, occasional tornadoes of significance occur in environments having characteristics that appear less supportive of supercell tornadoes, including small SRH values and/or relatively high LCL heights. Such tornadoes, whether associated with supercell or nonsupercell processes (more precisely termed mesocyclone and nonmesocyclone processes), present a challenge for forecasters. This empirical study uses a database of soundings derived from the Rapid Update Cycle model to examine thermodynamic characteristics of F1 and greater intensity tornado events associated with small SRH and/or high LCL heights. Results strongly suggest that many such tornado events are associated with steep lapse rates in the lowest few kilometers above ground. The low level of free convection heights, small convective inhibition, and sizable convective available potential energy below 3 km were also found to be of possible importance. These thermodynamic characteristics combined would likely reduce resistance to upward accelerations, potentially enhancing ascent for low-level parcels entering thunderstorm updrafts and, hence, low-level stretching. From prior research, if preexisting boundaries were available to provide surface vertical vorticity for stretching, such thermodynamic characteristics could be an important component of tornado events that involve nonmesocyclone processes. These same thermodynamic characteristics may also offer clues for the investigation of mesocyclone tornado events that do not fit well with accepted tornado forecasting parameters from prior studies.