I. Introduction

The Bibb/Shelby County storm occurred on 21 February, 1997, and produced an isolated area of wind damage and an (F0) tornado (spaced roughly 20 miles apart) in the span of 25 minutes (affected the areas depicted in Figure 1). The initial wind event occurred around 9:10 AM CST (1510Z) in northern Bibb County, near the intersection of Bibb County roads 9 and 24. This storm damaged several mobile homes, and uprooted a number of trees. Event #2 (A weak tornado) occurrred around 9:35 AM (1535Z) in western Shelby County, near Brantleyville. This tornado produced a track roughly one-quarter mile long and 150 yards wide. It knocked down or snapped several dozen trees, a few of which were blown onto the roofs of nearby homes. Shingles were also ripped off some of these dwellings.

The thunderstorm which produced these events was part of a long, broad band of convection which was advancing eastward through Alabama, ahead of a slow-moving cold front. As shown in the following reflectivity images, this particular thunderstorm exhibited a bow echo configuration which persisted through much of its lifetime. More importantly, it spawned in rapid succession two distinct comma head vortices near its northern flank, both of which matured near the times of the wind events.

II. Environment

An examination of the 12Z Birmingham (BMX) sounding taken on the morning of 21 February provides some explanation for the storm's structure and evolution. As revealed by the skew-T and hodograph diagrams, a classic strong shear/low CAPE environment existed over central Alabama on the morning of the 21st. Conditional instability through the depth of the atmosphere was meager, resulting in an unmodified CAPE of only 128 J kg^-1. Storm-relative helicity, on the other hand, was quite impressive; the combination of low-level gradient winds in excess of 50 knots, and an observed storm motion of around 50 knots yielded an estimated helicity value of 522 m²s^-2 with this event. Several investigators (e.g., Johns (1993); Przybylinski (1995)) have noted that the presence of strong shear and low CAPE is conducive to the development of a variety of bow-echo/LEWP configurations during the cool months of the year. One can speculate that the combination of very low CAPE and very strong low level shear present on 21 February may have been the reason for the development of the observed bow-echo/comma head vortex structures instead of supercells, which might be expected with higher values of CAPE.

III. Radar images

The evolution of the Bibb/Shelby County thunderstorm during the time of the two tornadoes is depicted in a sequence of reflectivity and storm-relative velocity images extending from 1511Z to 1531Z. All of these images were taken from the Birmingham (KBMX) WSR-88D, which is located in central Shelby County (Fig. 1).

As seen in the reflectivity products at 1511Z, a bow-echo and accompanying comma head vortex are clearly evident over northern and central Bibb County. Viewing the SRM products at 1511Z,a subtle yet definable velocity couplet is visible at 0.5 and 1.5 degrees, and appears to coincide precisely with the location of the comma head. At this time, the first wind event is inflicting damage over northern Bibb County. Shortly after peaking in strength, the vortex begins to decay. During the next two volume scans (as seen in the 1516Z and 1521Z reflectivity products), the comma head structure moves rapidly east-northeastward and becomes less well-defined. The velocity couplet noted earlier over northern Bibb County appears equally disorganized in the 1521Z SRM products.

While the first comma head is dissipating at the north end of the line, a second incipient vortex is weakly discernable in the 1521Z reflectivity products further south, near the Bibb/Shelby county line. This developing comma head structure becomes more apparent in the higher elevation slices of the 1526Z reflectivity products, and is accompanied by a strong velocity couplet in the 1526Z SRM products. By 1531Z, the second comma head vortex has moved into western Shelby county, and has become well-defined in both the1531Z reflectivity and 1531Z SRM products. In particular, a distinct velocity couplet is observed at 1.5 degrees, exhibiting rotational velocities in excess of 40 knots. This apparently represented the mature point of the vortex, as a weak tornado developed near Brantleyville during this volume scan. Similar to the life cycle of the first comma head, subsequent scans revealed a rapid decay of this second system following the development of the Brantleyville tornado. However, the bow-like configuration of the thunderstorm line persisted for another hour.

Thanks to Bob Kilduff and Ron Murphy for their considerable help with this poster.

Johns, R.H., 1993: Meteorological conditions associated with bow echo development in convective storms. Wea. Forecasting, 8, 294-299.

Przybylinski, R.W., 1995: The bow echo: observations, numerical simulations, and severe weather detection methods. Wea. Forecasting, 10, 203-218.

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