Abstract
A severe thunderstorm which spawned at least four tornadoes, one of them anticyclonic, formed over central Iowa during the afternoon of 13 June 1976. This storm moved toward the east-northeast, approximately parallel to but slower than the mean tropospheric flow. The anticyclonic tornado (F3) and the most intense (F5) of the cyclonic tornadoes coexisted for 23 min and traveled on nearly parallel, cycloidal-like tracks, with the anticyclonic tornado 3–5 km southeast of the cyclonic. The major emphasis of this paper is on this pair of tornadoes and their relationship to the structure and evolution of the parent thunderstorm.
Radar recorded the development of a hook echo just prior to the genesis of the intense cyclonic tornado. A strengthening mesolow was centered somewhere south of this tornado soon after it formed. The mesolow is believed to have initiated a new updraft; the anticyclonic tornado formed in association with this updraft, south of the cyclonic tornado. It is hypothesized that the mesolow was responsible (through alteration of the storm-scale airflow) for the nearly simultaneous sharp right turns made by these tornadoes. Each of these tornadoes was observed to diminish in intensity soon after becoming associated with heavy rain.
It is argued that the parent thunderstom's distinctive airflow and thermodynamic structure at low levels provided a more favorable setting for the amplification of anticyclonic vorticity than is typical of most severe thunderstorms.
