Detailed WSR-88D Observations of a Cool Season Tornadic Bow Echo Event on 11 February 1999 over the Mid-Mississippi Valley Region: A Unique Tornado Event

Severe storms which rapidly evolve during the winter months in the midwest can be especially challenging due to their rapid changes in storm reflectivity and Doppler velocity structures. The 11 February 1999 tornadic event over parts of east-central Missouri and southwest Illinois was no exception to this fact. As part of on-going COMET-related research, most of our emphasis has been on ‘warm season' bowing MCSs which evolve in relatively high convective available potential energy (CAPE) magnitudes and weak to moderate environmental shear. However on occasion, we may experience and study the evolution of tornadic storms, which evolve during the middle of the winter season. One of the unique aspects of this case is that tornadoes occurred only at the far northern and southern cells of a nearly linear convective line, which extended over 120 km in length. There was an absence of severe weather or tornadic activity across the central portion of the line. The southern tornadic storm traveled at speeds exceeding 30 m s-1, making it very difficult to issue warnings due to limited sampling of the storms. A comprehensive damage survey, conducted the following day by the SOO and a student from St. Louis University, uncovered three tornado damage tracks with the southern storm and one tornado track with the northern storm. The strongest tornadic damage (F2 intensity) occurred with one of the three tornadoes associated with the southern storm. This tornado had an overall path length of 35 km. At one period two tornadoes were traveling nearly parallel to each other for over a period of 5 minutes.

The linear convective line, 120 km in length, formed 100 km downwind from an eastward advancing cold front over central Missouri and evolved in a strongly sheared environment (25 m s-1) with Convective Available Potential Energy (CAPE) values only reaching 450 J Kg-1. Prior to tornadogenesis, the northern and southern convective storms revealed a small comma and "S" shaped reflectivity patterns, respectively. Circulations, containing moderate to strong cyclonic shears and diameters between 5 to 15 km, were located within the comma-head of the northern storm and near the center of the "S" shaped configuration of the southern storm. Small tornadocyclones, meeting TVS criteria, initially formed along the leading edge of the small bowing segments of each storm and traveled nearly parallel to the broader vortices. These tornadic vortices appeared to form in response to shearing instabilities along the rapidly moving small bowing segments. Tornadoes immediately occurred at the time of tornado-cyclogenesis.

We will present the storm reflectivity evolution and characteristics of the southern storm's first circulation and several tornadocyclones which evolved either along the leading edge of the rapidly northeastward moving line segment or within the first core circulation. In particular, we will show that during the intensification of the southern storm's first core, the first tornadocyclone intensified along the line segment while a second tornadocyclone formed simultaneously within the vicinity of the first core. During this same period, vortex growth and evolution was observed with the far northern storm. The range of circulation intensities and diameters from two storms at opposite ends of the larger linear convective line makes this case rather unique for investigation. Finally, we will introduce strategies on how to effectively issue timely and accurate warnings when encountering tornadic cases of this type.