Analysis of the 2 April 2006 Quasi-Linear Convective System (QLCS) over the Mid-Mississippi Valley Region: Storm Structure and Evolution from WSR-88D data

During the mid-afternoon on 02 April 2006, a line of discrete storms over central Missouri evolved into a nearly solid quasi-linear convective system (QLCS) over extreme eastern Missouri and parts of southwest Illinois, including the Greater St. Louis metropolitan area. Line motion exceeded 30 m s-1 over extreme eastern Missouri and southwest Illinois. Two poorly defined bowing segments formed over the central and northern parts of the QLCS, while discrete storms and a single bowing segment occurred over the southern part of the larger convective line. During the early phase when discrete storms were present over central Missouri, large hail was the predominant severe weather threat. As the storms merged into a nearly solid QLCS over extreme eastern Missouri damaging winds and non-supercell tornadoes became the primary threat. A series of weak non-supercell tornadoes formed near the inflection point of the northern and central bowing segments and caused F0 – F2 damage. Some of the strongest damage F2 happened in Fairview Heights – O'Fallon Illinois area (metro-east) where one fatality and scores of injuries occurred at a shopping mall. A weak tornado developed during the period of convective cell merger in the town of Park Hills, Missouri and caused minor structural damage. Other weak tornadoes occurred over parts of Macoupin and Montgomery counties between Springfield Illinois and St. Louis and were associated with the northern bowing segment. Detailed ground surveys conducted during the following two days revealed that ten tornado damage swaths of F0-F2 intensity were produced by the convective system.

This paper will present detailed radar and damage survey analysis in an attempt to show the relationships between storm mergers and damaging winds over eastern Missouri and subsequent tornado development either near inflection points along the line or near / north of the apex of bowing segments. The surveys revealed that the strongest damage was found along the tracks of the individual mesovortices rather than across the entire bowing segment. An interesting aspect to this case was the shallow depth and weak to moderate rotation documented with the individual mesovortices. In some cases, rotation on the WSR-88D radar at St. Charles KLSX was even non-existent at times during the period of tornado occurrence. In addition to documenting the mesovortex characteristics we will analyze the role of the mesoscale rear inflow jet evolution and its potential contribution to mesovortex development. This was one of the most challenging tornadic QLCS cases that National Weather Service warning forecasters have experienced during the last 15 years.

An overview of the near storm environment which the QLCS evolved from will be presented first. This will be followed by QLCS evolution and an analysis of five low-level mesovortices which formed along the leading edge of the convective system. Warning implications, radar sampling issues, and characteristics of the individual mesovortices will be discussed.