A Case of Supercell Intensification along a Preexisting Boundary – Clay County Nebraska Tornado of 22 September 2001

During the early evening hours of 22 September 2001, a supercell thunderstorm developed in south central Nebraska along an advancing cold front. The supercell initially produced an F1 tornado (Fujita 1971) in northern Clay County, Nebraska. As the supercell propagated southeastward through Clay County, it rapidly intensified and produced a second, much stronger, tornado. The tornado produced F3 damage to three farmsteads in rural Clay County, Nebraska, along a 13 km path. The tornado was over one half km in diameter for much of its lifetime. In accordance with research involving low-level boundaries and tornadic storms (Maddox et al. 1980; Rasmussen et al. 1994; Markowski et al. 1998; Rasmussen et al. 2000), it appears the rapid intensification of this supercell and subsequent tornadogenesis was ultimately dependent upon a preexisting mesoscale outflow boundary, and possibly other storm-scale processes. The ambient environment was otherwise characterized by relatively weak mid- to upper-level flow (~20 ms-1) and nearly unidirectional deep layer (0-6 km) shear (Figure 1 and Table 1)