Observations and simulations of a long-lived tornadic mesocyclone that formed in a low-CAPE environment with PV banners spawned by the Colorado Front Range

On 22 May 2008 a mesocyclone spawned an EF1 tornado over terrain as high as 2650 m MSL in southeastern Wyoming. The mesocyclone was part of an elongated complex storm system resulting from the merger of two separate cells that developed north of a quasi-stationary warm front. The storm is unusual in that it formed and persisted in an environment with considerable low-level shear and helicity but with little convective available potential energy (<1000 J kg-1) and no convective inhibition. The mesocyclone formed as its parent storm moved over terrain gradually ascending by ~1000 m. It became unusually strong, with low-level shear as high as 84 m s-1 km-1. This mesocyclone could be tracked by the nearest Doppler radar over 90 minutes, during which it travelled 130 km.

This paper examines the characteristics and the environment of this extremely rare supercell storm using both station, upper-air, radar and satellite data, and high-resolution numerical model output. Near-surface radar observations and model output suggest that the formation and maintenance of the mesocyclone in this quasi-moist-neutral environment benefited from two terrain-related factors. One is the channeling of the low-level flow, locally enhancing the helicity. The second and most important factor, in our opinion, is the presence of banners of high potential vorticity as the strong southerly flow sheared around the Colorado Front Range. It is not clear whether the storm ended up in a PV banner by chance, or whether this is the result of low-level convergence associated with the PV banner, but a WRF simulation confirms the development of mesoccyclones within PV banners shedding off the Front Range.