The San Angelo, Texas, supercell of 31 May 1995: Visual observations and tornadogenesis

Airborne radar analysis combined with cloud photogrammetry of a supercell that formed on 31 May 1995 near San Angelo, Texas, is presented. The superposition of the Doppler wind syntheses with the cloud pictures provides a unique view of the relationship between the low-level mesocyclone and the developing lowered cloud base. The mesocyclone accompanying the storm went through two periods of intensification. The end of the first period of intensification did not lead to the formation of a tornado. An occlusion downdraft formed but was primarily driven by negative buoyancy rather than a downward-directed perturbation pressure gradient. Recent studies have suggested that cold downdrafts inhibit the formation of tornadoes. The present study supports this hypothesis and provides the first quantitative view of the vertical structure of this downdraft. The mesocyclone evolved through a second period of intensification that produced a weak tornado that was visually apparent from the aircraft. No occlusion downdraft was resolved during this latter period. The unusual aspect of this tornado is that it appeared to develop outside of the low-level mesocyclone. This type of tornadogenesis has not been previously shown.