The 8 June 1995 McLean, Texas, Storm. Part I: Observations of Cyclic Tornadogenesis

Abstract

On 8 June 1995 scientists participating in the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) collected airborne Doppler radar data in a storm that produced a family of tornadoes near McLean, Texas. The Electra Doppler Radar (ELDORA) scanned three significant tornadoes during their formative and mature stages; one of the tornadoes was of F4/F5 intensity. Evidence from pseudo-dual-Doppler analyses of the ELDORA data reveals a process of cyclic tornado formation qualitatively similar to that depicted in previous conceptual models. In particular, the rear-flank gust front appears to play a major role in determining the location of the next vortex in the series. When a tornado forms, a small region (3?5 km wide) of outflow surges ahead of the tornado, producing a local bulge in the gust front. A new vorticity maximum may form near the leading edge of the outflow. In contrast to what is suggested by earlier conceptual models, intersection of the rear-flank gust front with a wind shift along the forward flank does not appear to be a necessary element in the formation of the new vortex. Previous studies have shown that if low-level outflow from the rear flank of a storm surges well ahead of the midlevel updraft, initiation of new deep, moist convection downshear along the gust front may be necessary for storm survival. In contrast, in the McLean storm, the rear gust front did not move ahead of the location of the midlevel updraft. The persistence of the main updraft may have fostered a rapid cyclic process. The first and second tornadoes each, in short time, became separated from the main updraft. In contrast, the third large tornado in the family (the fourth overall) remained with the main updraft and persisted for over 1 h. Reasons for the transition of the cyclic phase into the long-lived phase will be discussed in Part II of this paper.