Several non-supercell tornadoes associated with a Derecho developed across parts of north-central Illinois during the late afternoon of 29 June 1998. These tornadoes caused F1 and F2 damage to a number of farmsteads, homes, businesses and large electric transmission lines from 70 km west and northwest of Peoria Illinois to 15 km south of Peoria. In many cases, tornadoes were on the ground from 5 to 10 minutes and had varied damage path lengths from a little as 4 to as much as 10 km. The tornadoes occurred from the apex northward along the northern (cyclonic) side of a large bow echo which formed over eastern Iowa and traversed through a large part of central Illinois into west-central Indiana. Dollar estimates of the tornadic and straight-line wind damage associated with this bowing Mesoscale Convective System (MCS) across central Illinois exceeded 6 million.

The large bowing MCS evolved in a highly unstable, deep layer sheared environment. The MCS traveled along an old east-west low-level boundary generated by a convective complex the preceding day. The first two of many convective-scale vortices formed at the intersection near the northern end of the intense quasi-linear convective system (QLCS) and the old east-west low-level boundary. Both circulations initially took on the characteristics of a tornadocyclone prior to the convective line segment further bowing out. The first circulation exhibited a shorter lifetime compared to the second vortex (C2) and weakened. C2 evolved into a strong mesocyclone and spawned a tornado when the circulation exhibited its strongest rotation and greatest depth. C2 traveled in the vicinity of the east-west low-level boundary and revealed the longest lifetime of all convective-scale vortices documented. During the period of initial bowing, subsequent convective-scale vortices formed from just north of the apex of the bow to the northern line end vortex (C2). Two relatively long-lived convective-scale vortices formed along the leading edge of the bow, during this stage, (C3) just north of the apex, and (C4) along the northern side of the bow. Other convective-scale vortices formed between C4 and C2, however their lifetimes were extremely short (less than 15 minutes).

This presentation will highlight the evolution of the three long-lived convective-scale vortices which spawned non-supercell tornadoes during the early part of bowing. Two of the three circulations spawned tornadoes during the very early stages and again at the midpoint of the vortex lifetime, while the third circulation (C2) spawned a tornado only during the midpoint of its lifetime. Time-height Rotational Velocity (Vr) traces were constructed for all three circulations to show the vortex characteristics and time of tornado occurrence. We will also comment about the importance of the low-level boundary near the northern end of the large bowing segment. This feature has been documented in several other cases and is a critical issue to early vortex growth and longevity with the June 29 event. This case demonstrates the importance of identifying features such as low-level boundaries and their interactions to a larger convective line as an important precursor to potential vortex growth. It is hoped that the June 29 1998 Derecho will also shed light on improving tornado warnings capabilities with a greater understanding as to where tornadic circulations likely form along the convective line and characteristics of these vortices to time of tornado occurrence.