Environmental Evolution of Supercells Interacting with the Appalachian Mountains

Abstract:

The Appalachian Mountains within the eastern United States have a considerable impact on day-to-day weather, including severe convection. However, the impact of the Appalachians on supercell thunderstorms is not well understood, posing a significant short-term forecast challenge across the region. While there have been some individual case studies conducted, there has yet to be a broad analysis of storm-scale modifications of supercells as they interact with complex terrain. To address this gap, this study examines 138 isolated supercells occurring within the central and southern Appalachians between April and July from 2008 to 2019. Each supercell was broadly classified as either a “crosser” or “non-crosser” based on their ability to be maintained during their interaction with terrain; the majority of supercells were not sustained downstream of the Appalachians. The non-crossers were further separated into 4 subsets related to their specific elevation profile over time: (1) forming in higher elevations and dissipating in lower elevations, (2) forming in higher elevations and dissipating in higher elevations, (3) forming in lower elevations and dissipating in lower elevations, (4) forming in higher elevations and dissipating in lower elevations. To identify the environmental controls resulting in crossing or non-crossing storms, near-storm model soundings (either the Rapid Update Cycle [RUC] or the Rapid Refresh [RAP]) were collected for each supercell at three points: (1) upstream of the mountains, (2) near the peak of the terrain, (3) and downstream of the terrain feature using model. These soundings were used to compute a number of different thermodynamic and kinematic parameters. Those that are best able to distinguish between supercell subsets, and the implications for short-term forecasting, will be discussed.