Case Study of a Long-lived Tornadic Mesocyclone in a Low-CAPE Complex-terrain Environment E-Journal of Severe Storms Meteorology

On 22 May 2008 a long-lived mesocyclone spawned an EF2 tornado over terrain as high as 2650 m MSL in southeastern Wyoming. The mesocyclone was part of an elongated, complex storm system that grew rather early in the day near a slow-moving warm front. The mesocyclone is unusual in that it persisted and became tornadic in rather cold (~7°C), saturated surface conditions in an environment with CAPE < 1000 J kg-1 and no surface-based convective inhibition. The mesocyclone intensified as its parent storm moved over terrain gradually ascending by ~1000 m, reaching a radar-estimated low-level horizontal shear as high as 84 m s-1 km-1. This fast-moving mesocyclone could be tracked by the nearest Doppler radar for over 90 min.&#x0D; This paper examines the characteristics and the environment of this mesocyclone using both operational weather data and high-resolution numerical simulations. Near-surface radar observations and model output suggest that the formation and maintenance of the mesocyclone in this low-CAPE environment benefited from two terrain-related factors. One is the observed channeling of the low-level flow, locally enhancing the storm-relative helicity. The second is the presence, suggested by high-resolution simulations, of banners of high potential vorticity generated by the strong southerly flow shearing around the Colorado Front Range.