Observations were obtained from Explorer and Doppler radar of a small, isolated cumulonimbus developing in a wind field with relatively little directional shear. The storm displayed a high degree of symmetry about a vertical plane through the center of the storm oriented parallel to the wind shear vector. Single-Doppler observations of this storm reveal a region in which the horizontal component of the wind vector was opposite to that of the mid-level environmental wind, suggesting the presence of a vortex pair circulation. The storm was simulated with a three-dimensional cloud model which reproduced these and some of the other observed storm characteristics. The environmental wind shear in which the storm developed was similar to that of the composite sounding documented by Fankhauser and Mohr (1977) for weak, isolated or scattered storms in northeast Colorado. Therefore, this symmetric structure, involving two counter-rotating vortices, may be a common feature of isolated storms in this area. (Toutenhoofd,V. and Klemp, J.B. 1983).