On the Nature of Highly Deviant Supercell Motion

Previous studies have shown that right- and left-moving supercells move to the right and to the left of the vertical wind shear, respectively. Most of the time, supercell motion can be predicted to within 5 m/s of the observed motion. However, under certain circumstances, the predicted motion may be in error by much greater than 5 m/s. Observations suggest that this larger error can be partially explained by the distribution of the vertical wind shear, especially in the 0-4- and 4-8-km layers. It is hypothesized that (i) stronger deep-layer vertical wind shear leads to a stronger mesocyclone (mesoanticyclone), and thus to a greater deviation from the mean wind toward the right (left) of the vertical wind shear; and (ii) weaker mid-level vertical wind shear allows for a stronger cold pool, and thus for a tendency for the supercell to become outflow dominated. The latter condition appears to be most common in high-precipitation supercell environments.

An other important consideration in forecasting supercell motion is the choice of an appropriate mean wind depth. For example, a shallower mean wind layer--such as 0-4 km--often works best for low-topped supercells. Conversely, a deeper mean wind layer--such as 0-8 km--may be more representative of a large and tall supercell. Preliminary results indicate that the best mean wind layer is that which is half of the storm's depth. Given this knowledge of the vertical wind shear distribution and an appropriate mean wind layer, one can make modifications to the initial prediction of supercell motion, minimizing the potential for large errors.