Tuesday, 5 October 2004: 5:30 PM
The week of 7 May 2003 was the most active period of severe convection across Missouri. One of the active episodes occurred during the late afternoon and early evening of 6 May 2003 across central and eastern Missouri and moved east-southeastward across parts of east-central Missouri and southwest Illinois, south of the immediate St. Louis metropolitan area. The northern-most storm, which traveled along a nearly east-west warm frontal boundary, revealed High-Precipitation (HP) supercellular characteristics. Other supercells southwest of the HP storm exhibited nearly classic or hybrid supercellular characteristics. Many of the storms across central Missouri produced severe winds (>25 m s-1) and large hail. As the line of thunderstorms approached eastern Missouri, a number of storm mergers occurred within the line resulting in the development of a bow echo and strengthening cold pool. The northern HP storm collapsed during bow echo formation as it encountered developing convection within its inflow region. The strengthening cold pool resulted in a rapid acceleration of the bow echo and subsequent formation of a strong mesovortex near the northern end of the bow echo over southern Jefferson County, Missouri (south of St. Louis). The combination of the bow echo and mesovortex resulted in a prolonged and devastating period (20 to 30 minutes) of severe wind (30 to 45 m s-1) across southern Jefferson County including the town of Desoto, Missouri. Numerous structures including homes, schools, churches, and other businesses were severely damaged, while several injuries were reported to local authorities. Other strong mesovortices formed at or north of the apex of the bow echo over far southwest Illinois. Two mesovortices were responsible for spawning weak tornadoes.
This paper will present detailed radar and damage survey analyses in an attempt to illustrate the relationship between the damage swath, the bow echo and strong mesovortex near the northern end of the 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 first associated with the accelerated bow echo and secondly the strong mesovortex. Detailed analysis of the structure of the northern mesovortex and other mesovortices which formed along the leading edge of the bow will also be presented. Warning and forecast implications of these findings will be discussed.
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