A case study of a derecho (windstorm) in the central plains

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 reached 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. An extremely large area of the Central Plains was affected by this system including South Dakota, Minnesota, Nebraska, Iowa, Illinois, and Missouri. This system had similar characteristics to previous MCC events. However, it was different in some respects since many MCC systems have been known to 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 knots in some places). To analyze this event (termed a derecho), the system has been considered from the perspectives of the synoptic and sub-synoptic scales. The information used includes upper-air data, surface observations, satellite pictures, and radar data. Since this system basically occurred between rawinsonde sites the thermodynamic structure of the system itself would be difficult to ascertain without a true mesoscale network; however, since the magnitude of this system was not forecasted by the NWS or NSSFC, a detailed synoptic and sub-synoptic scale analysis of the pre-storm, storm, and post-storm conditions was performed to determine how similar this event was to other systems studied by other researchers. On the sub-synoptic scale, surface observations and radar data were used to determine how the system propagated throughout its history. 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 regions for derecho storms are over the Central Plains.