United States Supercell Storms and Their Severity: A 14-Year Radar-Based Climatology

Abstract Supercell storms are recognized to account for a disproportionately large amount of severe weather occurring in the United States and elsewhere. Despite their importance, few studies document their climatological behavior, and those that do focus predominantly or entirely on severe supercells. This study presents an objective and comprehensive radar-based climatology of supercell storms during a 14-yr period over the contiguous United States. Approximately 56 000 supercells are identified, half of which are likely nonsevere. All objectively identified supercells (strongly and persistently rotating storms) are diagnosed as either “right moving” (RM) or “left moving” (LM) based primarily on the deviance of storm motion relative to the 0–6-km environmental wind shear vector. RM supercells outnumber LM supercells at a rate of approximately 3:1 and also live longer. RM supercells are more frequently severe than LM supercells, accounting for ≈99.8% of supercell tornadoes, ≈76.9% of supercell severe hail events, and ≈68.9% of supercell severe wind events. Common to both supercell configurations, numerous sensitivities are identified between storm characteristics and storm severity. Namely, storm severity increases with increasing velocities of storm motion and increasing midlevel rotation. Severity maximizes when storm motion deviates ≈30° left or right of the 0–6-km environmental wind shear vector. Last, severe and nonsevere supercell storm characteristics are compared, and the greatest discriminatory indicators are based on metrics of the depth of high radar reflectivity magnitudes. Significance Statement Rotating thunderstorms (supercells) are responsible for the vast majority of the most significant tornadoes and severe hail events in the United States. Despite their recognized importance, many basic details about them remain unknown, such as their frequency, spatial distribution, and dominant character of motion (right or left moving). In this study, a comprehensive 14-yr statistical analysis of United States supercell storms based on objectively analyzed radar observations is used to address this gap in understanding. Numerous sensitivities between supercell severity and storm characteristics are found and have potential to advance the warning decision-making process.