13.1
Characteristics of Circulations associated with the 11 February 1999 Tornadic Event over the Mid-Mississippi Valley Region
Ron W. Przybylinski, NOAA/NWSFO, St. Charles, MO; and G. K. Schmocker
Severe storms which quickly develop 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. The overall MCS structure was one of a convective line with leading stratiform precipitation. One of the unique aspects of this case is that tornadoes occurred only at the far northern and southern storms of a nearly linear convective line which extended over 120 km in length. There was an absence of tornadic activity or any type of severe weather across the central portion of the line. The southern tornadic storm traveled at speeds exceeding 30 m s-1, making it very difficult to warn for due to limited sampling of the storms. A comprehensive damage survey, conducted the following day by the Science and Operations Officer and a student, uncovered three tornado damage paths with the southern storm and one tornado track with the northern storm. The most intense 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 with the southern storm were traveling nearly parallel to each other for a duration of five minutes.
The rapidly moving convective line formed at mid-day downwind from an eastward moving cold front over central Missouri and evolved in a strongly sheared environment (25 m s-1 ) with surface-based Convective Available Potential Energy (CAPE) values only reaching 450 J Kg -1 . During the early part of line formation over central Missouri, some of the storms took the shape of a hybrid supercellular structure with pendant echoes near the southern flank of the storm. However, there was a complete absence of any rotation with these storms. Within ten minutes of tornadogenesis, the northern and southern convective storms revealed a small comma-shaped and "S" shaped reflectivity patterns respectively. The first circulation identified with the southern storm originated between 2 and 3 km near the center of the "S" shaped reflectivity pattern, while the vortex with the northern storm was embedded within the "comma-head." A second circulation resembling a "tornadocyclone" rapidly formed after an isolated cell downwind merged with the southern part (bowing part) of the southern "S" shaped storm. A tornado occurred when the tornadocyclone showed upscale (non-descending) growth and intensification of rotation at low-levels. The tornadocyclone paralleled the path of the first circulation.
An overview of the southern storm's reflectivity evolution and characteristics of the first vortex and several tornadocyclones which formed along the leading edge of the southern storm's bowing segment will be presented. We will show that during the intensification of the first core, the first tornadocyclone along the bowing line segment intensified, while a second tornadocyclone formed simultaneously within the first core. During the same period, vortex intensification was observed with the far northern storm. The range of circulations from these two storms at opposite ends of the larger line and absence of any severe weather within the center part of the line makes this case rather unique to investigate. Finally, we will share some thoughts on the topic of situation awareness with cases of this type which evolve during the middle of the winter season.
Session 13, Tornadic Storms
Thursday, 15 August 2002, 10:30 AM-12:00 PM
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