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