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 examined 62 isolated supercells that occurred within the central and southern Appalachians between April and July from 2009 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 (37) were not sustained downstream of the Appalachians. To identify the environmental controls resulting in crossing or non-crossing storms, near-storm model soundings (either the Rapid Update Cycle or the Rapid Refresh) 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. These soundings were used to compute a number of different thermodynamic and kinematic parameters. Results indicate that the lowest 3 km of shear and storm-relative helicity (SRH) appear to best distinguish crossing and non-crossing supercells. Conversely, instability (CAPE and CIN) do not appear to be useful parameters in differentiating between crossers and non-crossers. Forecasters can use the lowest 3 km of SRH and shear, as well as the surface and mixed-layer equivalent potential temperature (θ_e) to aid in the short-term forecasting of isolated crossing supercells. Additionally, syntopic pattern recognition (e.g., the tilt of the upper-level trough, location of the surface low pressure, and the boundary type) of these events may help forecasters identify if supercells will cross the Appalachian Mountains.