The Severe Bow Echo Event of 14 June 1998 over the Mid-Mississippi Valley Region: A Case of Vortex Development near the Intersection of a Preexisting Boundary and a Convective Line

Abstract One of the most destructive damaging wind events of the spring and early summer convective season of 1998 over the Mid-Mississippi Valley region occurred during the early morning hours of 14 June 1998 across eastern Missouri and a small part of southwestern Illinois. Extensive property damage occurred across the northern sections of the St. Louis metro area and in the vicinity of the confluence of the Mississippi, Illinois and Missouri Rivers. Peak surface wind gusts ranged between 40 to 50 m s-1 causing countless trees to be snapped or uprooted, falling on numerous homes, vehicles and other structures. Damage resulting from at least three non-supercell tornadoes was also identified after a detailed storm survey was conducted.

The synoptic and mesoscale environment of early 14 June 1998 was characterized by strong southwesterly low-mid level winds ahead of a trough in the Plains, and moderate instability with a convective available potential energy (CAPE) of 2000 J Kg-1. The first mesoscale convective system (MCS) moved southeastward across the Mid-Mississippi Valley region between 0600 and 0900 UTC and laid an outflow boundary which extended northwest to southeast across east-central Missouri. This boundary moved relatively quickly northeastward toward St. Louis and a second developing MCS over northeastern Missouri. The St. Louis WSR-88D VAD wind profile depicted a dramatic increase in the low-level (0-3km) wind shear and SR helicity between 0930 and 1130 UTC as the boundary approached and moved through St. Louis.

The second MCS developed between 0900 and 1000 UTC, as two parallel lines of convection, oriented northeast-southwest, rapidly formed 90 to 120 km northwest of St. Louis. The second line gradually merged with the leading line after 1030 UTC resulting in the intensification of the leading convective line.

After 1045 UTC, one convective cell along the southwestern part of the leading convective line intersected with the northwest-southeast oriented outflow boundary produced earlier by the first MCS. Three vortices rapidly formed just north of the intersection of the convective line and the outflow boundary (on the cool side of the boundary), with the second circulation (Circ. #2) becoming the strongest and longest lived of the three. This vortex initially formed below 2 km, rapidly intensified within the lowest 2 km and gradually deepened to an overall depth of 5 km midway through its life cycle. This circulation lasted over one hour and spawned an F(0) intensity non-supercell tornado.

We will show that Circ. #2 aided in the acceleration of a bowing segment south of the vortex and helped to focus clusters of intense damaging winds across the northern parts of the St. Louis metro area. We will also underscore the importance of identifying external outflow boundaries intersecting convective line segments from WSR-88D reflectivity data, and how these boundaries can change the local low-level wind shear profile leading to dramatic increases in 0-3 km SR helicity. Such boundaries can augment the horizontal vorticity and can help promote convective cell intensification, vortex initiation, growth, and subsequent non-supercell tornadogenesis and/or the production of enhanced wind damage.