An Update for Predicting Left-Moving Supercell Motion

Abstract

Abstract A sample of 889 left-moving (LM) supercells of varying rotational strength across the United States was examined to determine if improvements could be made in predicting their motion using an existing hodograph-based technique. This technique was previously applied to a sample of only 30 LM supercells, and it was assumed that the same off-hodograph deviation from the mean wind for right-moving (RM) supercells was appropriate for LM supercells. However, our larger sample herein reveals the average deviation for LM supercells is less than the assumed 7.5 m s−1 based on a subset of 207 observed proximity soundings. At the same time, the 0–6-km mean-wind layer is still optimal for the advective component of storm motion (consistent with that for RM supercells). Applying the same methods to a subset of 678 model-derived RUC/RAP proximity soundings generally confirms these results, but with slightly smaller deviations. These findings support decreasing the deviation parameter to 5.0 m s−1 for predicting LM supercell motion (at least for the United States). The sample of LM supercells additionally was subdivided based on strength and duration, and then reevaluated using the observed proximity soundings. The predicted motion of moderate-strength mesoanticyclones had the least error, whereas the strong category had the largest errors by about 1 m s−1. Similarly, mesoanticyclones lasting 60–120 min had the least error in predicted motion. These two findings also are consistent with the results when using the RUC/RAP proximity soundings.