On 12 July 1995, a tornado developed over extreme southern Pinellas County Florida producing $200,000 in damage and injuring 1 person before moving offshore. The tornado/waterspout was within 20 km of the Ruskin FL WSR-88D, thus detailed radar data was available. In addition, detailed satellite, mesoscale surface, model initialization, and representative sounding data was available. Thus, sufficient data was available to access the pre-convective environment, convective initiation, and storm evolution and structure.
The atmosphere was characterized by large static and potential instability, and significant vertical shear in the 0-6 km layer. The tornado parent cell developed rapidly in response to collisions between microscale boundaries. The cell developed supercellular characteristics. This case is unique in that although the parent cell was supercellular, tornadogenesis occurred in a manner normally attributed to non-supercellular tornadoes. In particular, the source of tornadic vorticity in this case was near surface pre-existing vertical vorticity associated with one of the foregoing boundaries. The low-level tornadic circulation was detached from the mid-level mesocyclone. It is unclear what role is attributed to the mid-level mesocyclone to tornadogenesis.
This case is important in that the strategy of anticipating supercell tornadogenesis via descent of the mid-level mesocyclone and associated TVS may not be appropriate in every case. When supercells are apparent, it may be prudent for NWS forecasters to examine surface boundaries external to the storm. Such boundaries may provide sources of vertical vorticity (pre-existing or horizontal vorticity that can be tilted into the vertical) for tornadogenesis.