The association of significant tornadoes with a baroclinic boundary on 2 June 1995

On 2 June 1995, the large-scale environment of eastern New Mexico and western Texas was generally favorable for the occurrence of supercells because of the presence of strong deep shear and storm-relative helicity, as well as sufficient convective available potential energy (CAPE). Indeed, many supercells occurred, but the only storms to produce tornadoes were those supercells that crossed, or developed and persisted on the immediate cool side of a particular outflow boundary generated by earlier convection. Surface conditions, vertical vorticity. and horizontal vorticity near this boundary are documented using conventional and special observations from the VORTEX field program. It is shown that the boundary was locally rich in horizontal vorticity. had somewhat enhanced vertical vorticity, and enhanced CAPE. Theoretical arguments indicate that the observed horizontal vorticity (around 1 × 10^-2 s^-1), largely parallel to the boundary, can be readily produced with the type of buoyancy contrast observed. It is hypothesized that such local enhancement of horizontal vorticity often is required for the occurrence of significant (e.g., F2 or stronger) tornadoes, even in large-scale environments that appear conducive to tornado occurrence without the aid of local influences.