Examining Terrain Effects on the 31 May 1998 Mechanicville, New York Supercell and Tornado

On 31 May 1998, a strong (F3) tornado struck Mechanicville, New York. This tornado was a part of an extreme severe weather outbreak that impacted the northeastern United States. Previous research on the Mechanicville supercell and tornado suggested that terrain may have been an influence on the development and evolution of the severe convection. Specifically, this previous research hypothesized that terrain-channeled flow in the Hudson Valley contributed to increased low-level wind shear and instability in the valley. A lack of observations and capability to perform high-resolution model simulations, however, prevented this hypothesis from being fully tested.

The present study attempts to more robustly evaluate the impact of terrain on the development and evolution of severe convection on this day, using the Weather Research and Forecasting (WRF) model. The model was run with an inner-nest resolution of 1 km and was initialized with North American Regional Reanalysis data. The model produces supercells that then grow upscale, similar to the evolution of the Mechanicville supercell and in approximately the same area. Preliminary results of the simulations support previous hypotheses. Specifically, there was an enhanced corridor of moisture in the Mechanicville area, being advected up the Hudson Valley. Moreover, the magnitude of wind shear was locally enhanced in the valley. These two results suggest that the region in which the tornado occurred was especially favorable for tornadogenesis, in part due to terrain effects on the local environment. The simulations also reveal interesting new details about mesoscale inhomogeneities due to terrain, such as the development of a mesoscale moisture gradient within the Hudson Valley that may have impacted the evolution of the supercell. An analysis of the interaction of these mesoscale features with storm-scale features in these simulations extends previous research and provides insights into how terrain may modify the local environment to enhance the risk of severe weather.