A case study of a derecho (windstorm) in the Central Plains

Abstract

During the afternoon of 29 July 1986, three separate supercell thunderstorms developed over central and eastern South Dakota. In their mature stages each storm produced numerous tornadoes; furthermore, one tornado reach F4 status and eventually devastated a power plant near Sioux City, Iowa. Other reports included large hail and an unusually high number of reports of strong winds exceeding 50 knots. During the period, the three supercells eventually merged into one convective complex, and shortly thereafter reached mesoscale convective complex (MCC) status over northwest Iowa; this system was different since many MCCs have become heavy-rain producers while this system continued to produce severe weather many hours after it had reached MCC status (especially involving straight-line winds which exceeded 90 kt in some places). It was also unusual for such a prolonged severe weather event to occur this late in summer. Although this convective system caused no deaths and only a few injuries, extensive crop and property damage followed in its wake. Climatology shows that during the summer months the most favorable region for these derecho storms are over the Central Plains.