An Observational Analysis Quantifying the Distance of Supercell-Boundary Interactions in the Great Plains

Abstract

Several case studies and numerical simulations have hypothesized that baroclinic boundaries provide enhanced horizontal and vertical vorticity, wind shear, helicity, and moisture that induce stronger updrafts, higher reflectivity, and stronger low-level rotation in supercells. However, the distance at which a surface boundary will provide such enhancement is less well-defined. Previous studies have identified enhancement at distances ranging from 10 km to 200 km, and only focused on tornado production and intensity, rather than all forms of severe weather. To better aid short-term forecasts, the observed distances at which supercells produce severe weather in proximity to a boundary needs to be assessed. In this study, the distance between a large number of observed supercells and nearby surface boundaries (including warm fronts, stationary fronts, and outflow boundaries) is measured throughout the lifetime of each storm; the distance at which associated reports of large hail and tornadoes occur is also collected. Statistical analyses assess the sensitivity of report distributions to report type, boundary type, boundary strength, angle of interaction, and direction of storm motion relative to the boundary. Additionally, the range at which each type of severe weather is produced for each boundary is identified to provide a useful operational tool for forecasters. Notably, tornadoes are more likely to be produced closer to a boundary than severe hail. Overall, the observations point to a unique range at which severe weather occurs for each boundary and report type.