The North Dakota Tornadic Supercells of 18 July 2004 : Issues Concerning High LCL Heights and Evapotranspiration

On the evening of 18 July 2004, several tornadoes occurred with two supercell thunderstorms over eastern North Dakota. The second and smaller in diameter of these storms produced an F4 tornado in an environment with lifting condensation level (LCL) heights that were atypically high according to recent statistical studies about supercell tornado environments. Surface dewpoints were also underforecast by computer models. These two issues are examined in this paper, which provides an overview of this event. The synoptic setting and environment characteristics suggest that evapotranspiration (ET) was responsible in part for enhancing surface moisture. It is likely that ET affected instability and convection initiation. This study also found that the presence of steep low-level lapse rates juxtaposed with low-level convective available potential energy along a surface trough may have contributed to tornado development in a high LCL environment where wind and instability characteristics were otherwise favorable for supporting supercell tornadoes.