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 influenced the development and evolution of the severe convection. Specifically, LaPenta et al. (2005) 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 more robustly evaluates the impact of terrain on the 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. A warm bubble was inserted into the model, triggering the development of a supercell that closely follows the track of the Mechanicville supercell. The simulated supercell had a substantial increase in low-level vorticity and updraft helicity as it moved into the Hudson Valley area. Preliminary results of the simulations support previous hypotheses regarding terrain-channeling effects. Specifically, there was an enhanced corridor of moisture that was advected northward 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.

The simulations also reveal the structure and evolution of mesoscale inhomogeneities influenced by terrain, such as the development of a mesoscale moisture gradient within the Hudson Valley that may have impacted the evolution of the supercell. Preliminary results from the simulations suggest that this boundary developed as a surface warm front interacted with channeled flow in the Hudson Valley. This boundary has similar characteristics to one that developed during a more recent severe weather case. Therefore, the analysis of the interaction of the boundary with the simulated supercell may generalize to a larger number of local severe weather cases that had similar terrain-influenced mesoscale inhomogeneities.