Sensitivity of 0–3 km Storm-Relative Helicity and Fixed-Layer SCP to Storm-Motion Perturbations in Significant Tornado Environments

Storm-relative helicity (SRH) and composite severe-weather diagnostics depend on the storm-motion vector assumed in their calculation. This study evaluates storm-motion sensitivity across 3317 reprocessed EF2+ tornado environments in the contiguous United States from 2000 to 2024. Using Bunkers right-moving motion as the baseline, storm motion was displaced by ±2.50 and ±5.00 m s−1 along the axis connecting the Bunkers right- and left-moving vectors. An analytical two-dimensional formulation was also used to determine the maximum absolute SRH response over all horizontal perturbation directions at each prescribed displacement magnitude. Median maximum absolute changes in 0–3 km SRH along the right–left-moving axis were 50.70 and 101.40 m2 s−2 for the ±2.50 and ±5.00 m s−1 perturbations, respectively. The baseline median 0–3 km SRH was 290.92 m2 s−2, and the median fixed-layer supercell composite parameter (SCP) was 7.83. Along the right–left-moving axis, 6.57% and 27.46% of environments crossed the illustrative SCP thresholds of 1 and 4, respectively, within the ±5.00 m s−1 perturbation envelope. These fractions represent conditional responses to prescribed storm-motion perturbations rather than probabilities associated with observed storm-motion errors. The SCP findings apply specifically to the fixed-layer formulation examined here and should not be extrapolated to operational effective-inflow-layer versions without further evaluation.