Warm Sector Tornadoes with Discernible Surface Boundaries and Minimal Deep Layer Shea

On 24 June 2004 a significant tornado outbreak occurred over parts of South Dakota, Minnesota, Nebraska, and Iowa. During the outbreak, a total of 100 tornadoes touched down. The synoptic scale pattern across the central and western United States was characterized by a persistent trough of low pressure aloft with a warm and moist boundary layer across the northern plains, which lead to extreme instability. Thunderstorms developed as a short-wave trough in southwest mid-level flow moved into Nebraska and South Dakota. Thunderstorm initiation occurred in three regions, along a northward moving warm front, along an advancing cold front, and in the warm sector ahead of the synoptic surface low and east of the cold front.

Of the 100 tornadoes reported, 34 occurred in the warm sector ahead of the synoptic surface low. While most of the tornadoes were weak, two tornadoes were of F2 strength. These tornadoes occurred in an area without discernible surface boundaries. A hodograph constructed using observed wind data indicates a significant increase in velocity in the lowest 1 km layer, but with a pronounced lack of turning with height. In addition, the deep-layer shear profile was not supportive of long-track persistent supercells. Thunderstorms were able to produce a significant amount of relatively short-lived weak tornadoes as a result of the significant amount of instability in the lowest 3 km of the atmosphere, along with the increase in 0 to 1 km speed shear.