Case Study of a Long-lived Tornadic Mesocyclone in a Low-CAPE Complex-terrain Environment

Abstract

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. 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.