(CKL) were between -5^o and -6^o C. Warm advection and positive vorticity advection occurred east of the trough within a moist, unstable air mass throughout the day. The unstable air mass was widespread over the southeastern United States, as indicated by relatively large values (1000-2000 J kg^-1) of convective available potential energy (CAPE) in the 1200 UTC soundings at Little Rock, Arkansas (LIT), and Jackson, Mississippi (JAN). Combining the LIT upper air data with the observed surface conditions over northern Mississippi at 1800 UTC yielded a CAPE of 2800 J kg ^-1 and a corresponding lifted index of -7^o C. A large number of severe weather events including 16 tornadoes, 199 damaging wind, and 63 large hail events were reported over a 24-hour period. A map of these events is shown in Figure 1.

Figure 2 shows an estimated sounding for northern Alabama at 2200 UTC. Inputto this composite was derived from four soundings, two 1200 UTC upstream NWS soundings (LIT and JAN) and two 0000 UTC soundings at Centreville, Alabama (CKL) and Nashville, Tennessee (BNA), which were launched within 1 hour of the tornado occurrence but were both greater than 180 km distant (see Figure 1 for locations). Although the composite sounding is relatively unstable for the time of year (LI of -4.9^oC and CAPE of 1829 J kg^-1), the instability is less than that of the 1200 UTC LIT sounding modified with observed surface conditions over northern Mississippi. The composite sounding also

The primary focus of this paper to identify the mesoscale events associated with the development of the Huntsville tornado. The most prominent event involved the merger of an active squall-line system with a supercell storm near the time of tornadogenesis. This merger occurred as a result of the different motions of the two systems, as seen in the