Assessing the Impact of the Evening Transition on the Evolution and Lifetime of Supercell Thunderstorms in the Great Plains

 Predicting the evolution of supercell thunderstorms during and after the evening transition is a known challenge due to an incomplete understanding of how they evolve in response to associated environmental changes. As the low-level environment cools and stabilizes, supercells can dissipate, merge with other convection, grow upscale, or be sustained as either a surface-based or elevated supercell. The goal of this study is therefore to better predict how supercells will evolve during the evening transition by focusing on trends in environmental parameters that will lead to increased skill in forecasting. To quantify the connection between storm evolution and environmental changes during the nocturnal transition, a large number of initially isolated Great Plains supercell thunderstorms occurring between 2005 and 2016 are examined. Each supercell is categorized as either maintained, dissipating, growing upscale, or merging. Changes in the inflow environment are quantified using hourly RUC and RAP proximity soundings between one hour prior to local sunset time and five hours post sunset. Using these soundings, numerous thermodynamic and kinematic parameters are derived, including surface-based and most unstable CAPE and CIN, and low-level and deep-layer shear and storm-relative helicity. In general, the differences were large between evolution categories, but varied depending on the comparison; each classification existed in a unique set of kinematic and thermodynamic parameters. Statistical tests comparing trends and distributions of these parameters were most notable for maintained versus dissipated cases; storm-relative helicity was identified as a key parameter in distinguishing these case types, with maintained supercells containing significantly higher storm-relative helicity values during the nocturnal transition. The benefit of stronger, sustained storm-relative helicity values is inferred to help maintain a robust rotating updraft despite increasing stability, while a decrease (as seen in other supercell evolution categories) would lead to a loss of supercellular characteristics.