The High Shear/Low CAPE Severe Weather Event of 21 February 1997

Fig. 8. Same as in Fig. 7 except for storm relative velocity. The gust front is denoted by the dashed blue line. The white circle outlines the book-end vortex.

As the bow echo associated with the Honea Path tornado decayed, the most active portion of the QLCS (SRM loop) shifted back to the northern end of the line. The most intense tornado (F2) on this day occurred at around 2130 UTC near the city of Spartanburg. Once again, the reflectivity data from this time did not reveal a well-developed broken-S structure, at least in the lowest 7000 feet or so (Fig. 10). However, a broken-S pattern was noted in the higher elevation scans (6.2 degrees; 10.1 kft). The broken-S pattern did extend to the lower elevation slices in subsequent radar scans (Fig. 12). However, this was after the tornado had dissipated. This event and the Honea Path event provided anecdotal evidence that tornadogenesis was closely tied to the descent of a rear-inflow jet.

Fig. 9. Same as in Fig. 7 except at 2111 UTC.

Fig. 10. KGSP base reflectivity at (a) 0.5 degrees, (b) 2.4 degrees, (c) 4.3 degrees, and (d) 6.2 degrees at 2130 UTC on 21 February 1997. 

The storm relative velocity image from around the time of the tornado (Fig. 11) revealed an area of cyclonic shear near the area where the tornado occurred. However, the shear was broad and quite weak (rotational velocity of around 25 kt). It was somewhat surprising that a stronger rotational couplet was not observed, since the tornado occurred only 14 nm from the KGSP radar. However, the tornado apparently occurred a minute or two before these images, and was very brief (less than a minute duration). A tornado vortex signature (TVS) may have been observed if the radar had sampled the cell during the time of the tornado.

Fig. 11. Same as in Fig. 10, except storm relative velocity.

After the Spartanburg tornado, there was a relative lull in tornadic activity. However, there was an apparent increase in intensity of downburst damage between 2130 and 2200 UTC. The reflectivity images from 2146 UTC revealed the remnant broken-S structure west of Gaffney that developed in association with the Spartanburg tornado. However, damage during this time was apparently confined to areas south along the line, between Gaffney and Union, where some structural damage occurred. Although there was a slight bowing structure to the QLCS, there was a general absence of notable storm-scale features during this time, despite the ongoing severe weather. However, convection was relatively deep, with high reflectivity cores (i.e., > 50 dBZ) extending as high as 15 kft, among the highest observed on this day. It is possible that these deeper reflectivity cores were mechanically mixing the very high momentum air from aloft to the surface. This downward momentum may have been augmented via microphysical processes (i.e., melting ice). The base velocity data at 2146 UTC (Fig.13) showed a broad area of very high outbound velocities (55 kt to 70 kt) at an elevation as low as 1.7 kft AGL in the vicinity of the QLCS that extended rearward from the gust front.

Fig. 12. KGSP base reflectivity at (a) 0.5 degrees, (b) 1.5 degrees, (c) 2.4 degrees, and (d) 4.3 degrees at 2146 UTC on 21 February 1997.

Fig. 13. Same as in Fig. 12 except base velocity. The dashed blue line delineates the approximate location of the gust front.

One final tornado, which was weak (F1) but fairly long-lived, developed in association with the QLCS as it moved into the North Carolina Piedmont. This event was similar to the first Hall County, Georgia, tornado in that it occurred in association with a well-defined broken-S signature (Figs. 14 and 3). Once again, the “broken-S” structure developed from the top-down, indicating that it was associated with a descending rear inflow jet. This was another event that conformed to the conceptual model of the broken-S signature as described earlier in this section.

Fig. 14. Same as in Fig. 5 except at 2217 UTC. The dashed arrow represents the approximate tornado path.

Fig. 15. Same as in Fig. 5 except at 2227 UTC.

4. Summary

A long-lived, shallow QLCS produced widespread wind damage, several weak tornadoes, and one strong tornado as it moved across north Georgia and the Piedmont of the western Carolinas during the afternoon of 21 February 1997. Although the QLCS exhibited classic bow echo structure (pronounced rear inflow notches, book-end vortices) from time to time, these features were generally not long-lived. Nevertheless, widespread and persistent wind damage was observed along the length of the QCLS throughout most of its lifetime. Because wind damage, some of which was significant, occurred even in the absence of classic storm scale features,  convective mixing of very high momentum air from aloft may have played a key role in damaging wind occurrence. When bow-echo features did develop within the QLCS, they were often accompanied by rapid tornadogenesis. Recent research (i.e., Atkins and St. Laurent, 2008; Weisman and Trapp, 2003) has shown that intense low-level mesovortices can develop within QLCS in strongly sheared environments as the rear inflow jet descends. Most of the tornadoes that occurred on 21 February 1997 were associated with the broken-S reflectivity signature. The broken-S pattern was typically first observed in the mid-levels of the QLCS (i.e., 7.0 kft to 12.0 kft), but then descended rapidly toward the surface in subsequent volume scans, suggestive of a descending rear-inflow jet. As has been observed in other studies, and as suggested by the schematic in Fig. 4, some of the tornadoes occurred well before the fully-formed broken-S was observed at the lowest elevation scans, while some occurred after the signature was identified. This phenomenon may be a simple matter of radar sampling. In other words, the signature may typically develop first in the mid and upper levels of the QLCS, and then descend toward the surface. Therefore, if the storm is at a relatively large distance from the radar, the “break” may only be visible at the lowest elevation scan. Meanwhile, QLCS that are within close proximity of the radar may exhibit a break first at higher elevation scans, with the signature appearing in the lowest scan near the time of tornadogenesis, or even after the tornado has occurred. It is therefore important for operational forecasters to perform a complete vertical assessment of QLCS structure during high shear/low CAPE environments.    

Acknowledgements

All radar imagery was created using GRLevel2 software. The sounding was created with the Universal RAwinsonde OBservation (RAOB) Program. Reference to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not constitute or imply its recommendation, or favoring by the United States Government or NOAA/National Weather Service. Use of information from this publication shall not be used for advertising or product endorsement purposes. 

References

Atkins, N.T. and M. St. Laurent, 2009: Bow Echo Mesovortices. Part II: Their Genesis. Mon. Wea. Rev., 137, 1514-1532.

Lane, J. D., and P. D. Moore, 2006: Observations of a non-supercell tornadic thunderstorm from a Terminal Doppler Weather Radar.  Preprints, 23rd Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc.

McAvoy, B. P., W. A. Jones, and P. D. Moore, 2000:  Investigation of an unusual storm structure associated with weak to occasionally strong tornadoes over the Eastern United States. Preprints, 20th Conf. on Severe Local Storms, Orlando, FL, Amer. Meteor. Soc., 182-185.

Weisman, M. L., and R. J. Trapp, 2003:  Low-level mesovortices with squall lines and bow echoes.  Part I:  Overview and dependence on environmental shear. Mon. Wea. Rev., 131, 2779-2803.