Damaging Surface Wind Mechanisms within the 10 June 2003 Saint Louis Bow Echo Event during BAMEX

During mid-afternoon on 10 June 2003, an MCS formed over western Missouri and subsequently moved eastward over the greater St. Louis Metropolitan area. A bow echo formed within the convective line and produced a damage swath more than 80 km in length just to the east of St. Louis, passing directly over the Bow Echo and MCV Experiment (BAMEX) Operations Center. Detailed ground and aerial damage surveys also revealed six tornado damage swaths of F0-F1 intensity that were produced by the convective system.

This paper will present detailed radar and damage survey analyses in an attempt to illustrate the structural relationship between the damage swath and bow echo, as observed by the St. Louis WSR-88D. It will be shown that the primary damage swath of straight-line wind damage was not created by a descending rear-inflow jet at the apex of the bow echo. Rather, it was created by an intense, low-level mesovortex north of the bow echo apex. Observations of mesovortices within bow echoes creating long swaths of straight-line wind damage are not well-documented in the literature.

Detailed analyses of 11 low-level mesovortices formed on the leading edge of the convective system will also be presented. In particular, it will be shown that it is possible to distinguish between tornadic and non-tornadic mesovortices. Specifically, the tornadic vortices are stronger (below 3 km AGL) and longer lived than their non tornadic counterparts. Tornadic vortices consistently strengthen dramatically at low levels and deepen rapidly prior to tornadogenesis. Similar evolution is not observed with the non tornadic vortices. The average time from vortex genesis to tornado genesis was about 12 minutes. The forecasting implications of these findings will be discussed.