Even though the motion of intense long-lived thunderstorms (e.g., supercells) can be extrapolated reasonably well, nowcasting the occurrence of tornadogenesis is difficult and generally beyond present capabilities. The tornadic storm that struck the city of Huntsville (HSV) in Madison County, Alabama, at 2230 UTC on November 15, 1989, can be described as the end result of a scenario where an isolated supercell storm developed ahead of and, subsequently, interacted and merged with a squall line. The supercell was identified first by National Weather Service radars in central Mississippi 5 hours before reaching HSV, where it produced an F4 intensity tornado that killed 22 people, injured nearly 500, and caused an estimated $100 million in damage.

Considerable progress in our understanding of tornadic storms has been made over the last decade. Doppler radar observations [Brandes, 1984], laboratory simulations [Ward, 1962], and numerical modeling [Klemp and Rotunno, 1983] have indicated the importance of low-level downdraft and outflow in generation and/or amplification of low-level vorticity within supercell storms. Satellite observations have shown that thunderstorms usually tend to intensify when interacting with an outflow boundary produced by adjacent deep convection [Purdom, 1976]. Low-level boundaries also play important roles in nonsupercell tornadic storms [Wakimoto and Wilson, 1989]. Thus vorticity generation and amplification within the lowest 1 km of the atmosphere are crucial to tornadogenesis.

The ability to infer the generation of low-level vorticity prior to tornadogenesis may have practical significance for

The Tornado: Its Structure, Dynamics, Prediction, and Hazards.
Geophysical Monograph 79
Copyright 1993 by the American Geophysical Union.

This case study analysis serves to reemphasize the existence of a high conditional probability of tornado occurrence, given the merger of a gust front (or storm outflow) with an adjacent moderate to strong thunderstorm (first suggested by Cook (1961]). Mesoscale observations of the merger process are used herein to support this hypothesis. Observational data sets include National Weather Service (NWS) composite radar products, local mesonet data, Marshall Space Flight Center (MSFC) cloud-to-ground lightning network data [Goodman et al., 1988a], and visual observations.

Section 2 documents the large-scale environment. Section 3 provides an overview morphology of the squall line and supercell storm. Section 4 presents a detailed mesoscale analysis centered around the time of tornadogenesis. Section 5 discusses other cases where similar squall line and supercell interactions have been observed, and section 6 summarizes the findings.

The observed synoptic environment was of the type commonly observed for widespread severe weather outbreaks. A deepening trough, located over western Oklahoma at 1200 UTC November 15, moved eastward to western Arkansas by 0000 UTC. The 1200 UTC lifted indices at Nashville, Tennessee (BNA) and Centreville, Alabama