A Study of Storm and Vortex Morphology during the 'Intensifying Stage' of Severe Wind Mesoscale Convective Systems

Abstract

Since 1993, several staff members from NWS St. Louis and Saint Louis University continue to be actively involved in the study of mesoscale convective systems (MCSs) which produce severe wind damage and non-supercell tornadoes across the Mid-Mississippi Valley Region. The study of seventeen severe wind MCSs have been completed, while an additional five cases are currently being investigated. All of the severe wind MCSs occurred during the warm season (May - September). Damage surveys were completed for 80% of the events studied. Meteorological data for all of the events which produced severe weather were analyzed to determine their morphology, evolutionary characteristics and atmospheric environments in which they evolved. Six of the seventeen events met the criteria to be classified as a derecho while the remaining eleven events evolved into bow echo patterns. Over fifty convective-scale (tornadic and non-tornadic) vortices were analyzed within 150 km of the KLSX or adjacent WSR-88D radar site. Time-height rotational velocity (Vr) traces were used to show their evolutionary characteristics. We specifically focused our study on the ‘intensifying stage' of MCS evolution, where isolated or groups of convective cells begin to fill-in to form a nearly solid linear echo preceding the bowing of the convective line. Based on storm reflectivity patterns during this stage, we were able to classify our severe wind MCS events into four groups or types. Our initial findings show that the external boundaries from earlier convection or a quasi-stationary frontal boundary intersecting the convective line appeared to play an important role in the early stages of convective-scale vortex formation in three of the four groups. Such boundaries were identified by reflectivity fine lines, a line of small (usually weak) isolated cells oriented orthogonal to the larger more intense cells of the convective line, visible satellite imagery or mesoscale surface analyses. In nine of the seventeen cases studied, external boundaries intersected either the far northern (5 cases) or south-central part (4 cases) of the convective line. In five other cases, the convective line did not appear to intersect an external boundary but remained well north of the boundary, travelling parallel to it during the MCS's life span. Nearly 40% of the vortices documented at the convective line-external boundary intersection became tornadic. Observations have shown that the second circulation to develop near the intersection becomes one of the strongest and longest-lived vortices of the group and appears to play a role in enhancing wind damage just south and southeast of the vortex during the MCS's lifespan. Vortices at this intersection often preceded the formation of other convective-scale vortices, near the apex of a bowing segment south of the intersection. We will show comparisons of evolutionary characteristics of convective-scale vortices which intersect external boundaries to those which do not intersect this feature. Additionally, convective-scale vortices which form at the intersection of an external boundary and a convective line will be compared to vortex evolution studies completed by Burgess et al. 1997. It is hoped that these findings will provide forecasters a greater insight into the understandings of external boundary - convective line intersections and vortex evolution.