Observational Analysis of a Surface-based Bow Echo Transitioning to Elevated Convection Over Complex Terrain

Nocturnal thunderstorms are responsible for the majority of the growing season rainfall in the Great Plains of the United States. These storms often begin as surface-based convective storms in the afternoon, and transition into elevated convective storms after sunset. The transition from surface-based to elevated convection frequently includes a decrease in the severity of the storm. Neither this transition, nor the influence of topography on this transition is well understood. Therefore, this study examines the evolution of a surface-based bow echo that transitioned into an elevated convective storm as it crossed the Black Hills of western South Dakota during the overnight hours of 11-12 July 2017.

The system of interest developed from thunderstorms in southeastern Montana on the afternoon of 11 July 2017. By 0120 UTC on 12 July, the convection had organized into a strong supercell thunderstorm and was located over northeastern Wyoming. As it progressed eastward the supercell evolved into a bow echo, reaching peak intensity of 70 dBz at approximately 0247 UTC as it approached the western slopes of the Black Hills. The bow echo maintained its structure and organization, producing severe wind and hail reports as it crossed the Black Hills over the next hour. As the bow echo crossed the eastern slopes of the Black Hills and emerged onto the surrounding plains between 0400 and 0500 UTC, it diminished in intensity and evolved into a loosely organized cluster of cells. During this time a series of radar fine lines accelerated ahead of the storm, and new convective cells developed following their passage south and east of the original convection. The presence of series of fine lines rather than a single line is suggestive of an undular bore moving ahead of the convective system. The system continued progressing eastward into the overnight hours as a cluster of cells that gradually diminished in intensity. The storm’s early progression and increase in intensity, including the evolution from supercell to bow echo, occurred in a well-mixed boundary layer with approximately 800 J/kg of surface-based CAPE, indicative of surface-based convection. The decrease in severity, evolution from bow echo to cluster of cells, and the evidence of an undular bore as the storm exited the Black Hills occurred as the boundary layer was becoming increasingly stable after sunset, suggesting a transition into elevated convection.

Given the timing of the observed storm evolution, it is hypothesized that the movement of the bow echo off the elevated terrain of the Black Hills played a role in the transition from surface-based to elevated convection. Specifically, changes to the cold pool as the storm moved downslope are suspected to have accelerated the transition as the storm moved into the stable boundary layer east of the Black Hills.

This presentation will discuss a detailed timeline of the convective evolution described above based on analysis of available radar and environmental data. This will focus on the timing of observed changes in storm morphology relative to the underlying terrain while making an observational case for the transition from surface-based-to-elevated convection. These results will then set the stage for case study simulations using the Weather Research and Forecasting (WRF) model to examine cold pool properties and more directly test the hypothesis that the underlying topography assisted in this evolution.