An Observational Assessment of Off-Hodograph Deviations for use in Operational Supercell Motion Forecasting Methods

Significant advances have been made in the operational understanding and prediction of supercell motion since the late 1990s. One of the key considerations has been the deviation-propagation of updrafts to the right (left) of the vertical wind shear vector for counterclockwise (clockwise) rotating storms. Although operational forecasting methods assume this deviation to be fixed based on observations, modeling studies suggest this deviation is a function of the shear and other thermodynamic properties such as CAPE. The variability of this off-hodograph deviation, D, will be studied herein using a database of 425 observed soundings and supercells.

Preliminary results indicate variations in D are most strongly associated with vertical wind shear, but correlation coefficients are only around 0.15 to 0.25. Meteorologically insignificant correlation coefficients are found with respect to pure thermodynamic variables (CAPE was the largest at r = 0.10). However, various combinations of shear and CAPE do result in some of the largest correlation coefficients with D (e.g., r = 0.24 for VGP and EHI). Nevertheless, this information appears insufficient to improve the error statistics for the various supercell motion forecasting methods, and hence observations still support a fixed D. Finally, although thermodynamic variables such as LCL and LFC did not prove useful for predicting variability in D, they did reveal significant correlation coefficients (r ~ 0.25) with the downshear deviation in supercell motion.