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An Overview of the Bessemer City Tornado
of 13 January 2006
Justin D. Lane and Patrick D. Moore
A tornado ripped the roof from this house under construction in the Barkers Ridge subdivision near Bessemer City, North Carolina, at 810 PM EST on January 13, 2006.
A tornado touched down in Gaston County, North Carolina, just east of Bessemer City (Fig. 1) at approximately 810 PM EST on 13 January 2006 (0110 UTC 14 January), producing damage of F1 intensity on the Fujita Scale. [All times are referred to in Universal Time Coordinated (UTC) in this document, which Eastern Standard Time plus five hours.] The damage path was approximately one-half mile long and 100 yards wide, and stretched in a southwest to northeast direction. The tornado occurred 62 miles east northeast of the Weather Surveillance Radar-88 Doppler (WSR-88D) located at the National Weather Service (NWS) Weather Forecast Office (WFO) in Greer, South Carolina (KGSP, not shown) and 21 miles west of the Terminal Doppler Weather Radar (TDWR) located north of the Charlotte - Douglas International Airport (TCLT, Fig. 1).
Figure 1. Map of Gaston County, North Carolina. Bessemer City is in the west central portion of the county. The approximate location of the tornado is shown by the yellow box. The TCLT radar is located approximately 3 miles north of Paw Creek in Mecklenburg County. Map created with Delorme Street Atlas USA 2006. Click on image to enlarge.
Forecasters in the western Carolinas have for many years recognized environments characterized by strong wind shear, weak instability, and strong deep layer forcing to be favorable for development of non-supercell tornadoes (NST). The Bessemer City tornado developed within a quasi- linear convective system (QLCS) ahead of a cold front accompanying the passage of a strong upper level trough. The QLCS was quite shallow, as radar echo tops associated with the strongest updrafts rarely exceeded 25,000 feet. Radar reflectivity and radial velocity characteristics of the Bessemer City storm were similar to those observed during other non- supercell tornado events in the western Carolinas. Specifically, the reflectivity data displayed a "broken-S" pattern within the QLCS, similar to what has been documented in prior studies of NSTs (McAvoy et al. 2000). However, what sets this event apart from observations of other Broken-S- type NSTs is the availability of data from the Charlotte TDWR, which may shed new light on tornadogenesis in these events.
2. Synoptic Pattern and Stability Characteristics
The 500 mb analysis from the Storm Prediction Center (SPC) at 1200 UTC on 13 January 2006 indicated a highly amplified trough west of the Mississippi River Valley, with diffluent flow downstream of the trough over much of the eastern United States. The surface analysis at 1200 UTC from the Hydrometeorological Prediction Center (HPC) showed a strong cold front extending from low pressure over Lake Superior, through the mid- Mississippi Valley, into the western Gulf of Mexico. By 2100 UTC, the cold front had moved east and extended from the Ohio Valley, through the Great Tennessee Valley into the eastern Gulf of Mexico (Fig. 2).
Figure 2. HPC surface pressure and fronts analysis at 2100 UTC 13 January 2006. Observations are indicated by traditional station model. Click on image to enlarge.
The upper trough at 500 mb moved east by 0000 UTC 14 January, providing a persistent diffluent flow across much of the eastern United States, including the Carolinas (Fig. 3). A regional surface data plot at 0100 UTC 14 January (Fig. 4) shows the wind, temperature, and dew point fields about ten minutes prior to tornado occurrence. The airmass over the Carolinas was very warm and moist for early evening in January. Temperatures were in the lower 60s and dewpoint temperatures were in the upper 50s. An axis of strong convergence was implied along the cold front, with south southeast winds of 10 to 15 knots ahead of the wind shift, and southwest winds of 10 to 20 knots behind the front. Strong moisture convergence was indicated by the Storm Prediction Center (SPC) mesoscale analysis at 0100 UTC (Fig. 5).
Figure 3. SPC objective analysis of 500 mb geopotential height, temperature, and wind for 0000 UTC 14 January. Click on image to enlarge.
Figure 4. Regional surface plot with surface front analysis at 0100 UTC 14 January. Click on image to enlarge.
Figure 5. SPC objective analysis of surface moisture convergence and mixing ratio at 0100 UTC 14 January. Click on image to enlarge.
Despite the moist and relatively warm air mass, instability was limited due to a deep layer of relatively weak temperature lapse rate indicated by the observed Greensboro, North Carolina (GSO), upper air sounding at 0000 UTC on 14 January (Fig. 6). However, a dry air intrusion in the 850 mb to 700 mb layer steepened lapse rates sufficiently for weak destabilization to occur, as the sounding yields 198 J kg-1 of surface- based Convective Available Potential Energy (CAPE). A North American Mesoscale (NAM) initial hour sounding analysis (0000 UTC) showed the stability and wind characteristics at Charlotte (CLT, Fig. 7), which is approximately 17 miles east southeast of the tornado location.
Figure 6. Skew-T log P diagram of the upper air sounding for GSO at 0000 UTC 14 January 2006 (left) and severe weather indices (right). Click on images to enlarge.
Figure 7. Bufkit display of initial hour NAM analysis for CLT. Click on image to enlarge.
The most notable aspect of both soundings is a wind profile characterized by strong veering and a rapid increase in wind speed with height. The 0-1 km storm relative helicity (SRH) at GSO is 283 m2s-2. This is well above the median value of 0-1 km SRH of 137 m2s-2 found to be associated with weak tornadoes based on a study of 916 soundings proximal to tornado occurrence (Thompson et al. 2003). The Day 1 Convective Outlook issued by the SPC at 2000 UTC highlighted the environment characterized by strong shear and weak instability to the east of the Appalachians and included most of the Carolinas in a slight risk of severe thunderstorms.
3. Convective Evolution
During the mid afternoon, a large area of stratiform precipitation with embedded convection moved across the western Carolinas in response to deep synoptic scale forcing associated with the mid and upper level trough (not shown.) However, by late afternoon, a mid level dry slot began to overspread this activity (Fig. 8) resulting in a reduction in precipitation coverage across Upstate South Carolina. Meanwhile, a line of convection began to intensify along the back edge of the deep forcing (coincident with the cold front) across northeast Georgia, possibly as a result of steeper mid-level lapse rates that developed due to the advection of drier mid-level air (not shown.) The convection continued to intensify and organize as it moved across the upper Savannah River Valley into Upstate South Carolina during the early evening.
Figure 8. HPC reanalysis of 500 mb relative humidity at 0000 UTC 14 January. Click on image to enlarge.
By 2327 UTC, the reflectivity image from KGSP (Fig. 9) indicated a QLCS extended across the middle of Upstate South Carolina. At this time, reflectivity data from KGSP revealed a slight bulge in the convective line along the Greenville - Laurens county line. Ten minutes later, the bulging segment had evolved into a break in the QLCS west of Woodruff near the Spartanburg - Laurens county border (Fig. 10). Forecasters at WFO GSP and others have long attributed this "broken-S" signature to the occurrence of weak and occasionally strong tornadoes in the eastern United States (McAvoy et al. 2000, Grumm and Glazewski 2004.) Velocity data at this time (Fig. 11) indicated a line of relatively strong convergence associated with the high reflectivity region of the QLCS, with an area of weak shear across the line-break. This is not unusual. Previous research (McAvoy et al. 2000) has suggested that intense vortices on the mesoscale (i.e., mesocyclones) have preceded "broken-S" tornadoes only in very rare cases.
Figure 9. Base reflectivity on 0.5 degree scan from KGSP radar at 2327 UTC 13 January. Click on image to enlarge.
Figure 10. As in Fig. 9, but for 2337 UTC. Click on image to enlarge.
Figure 11. Radial velocity on 0.5 degree scan from KGSP at 2337 UTC. Click on image to enlarge.
By 2342 UTC, the line-break across southern Spartanburg County persisted (Fig. 12), but was becoming ill-defined. Concurrently, a pronounced bulge was developing in the convective line across Laurens County, northwest of Waterloo. The reflectivity data indicated a narrow channel of weak reflectivity impinging on the back edge of the bulge from the southwest, implying a jet of subsiding, rear-flank inflow. Meanwhile, storm-relative velocity data at 0.5 degrees revealed a weak rotational signature along the axis of the bulging segment (Fig. 13). By 2347 UTC, the rotation associated with the bulge had compressed and strengthened to 0.02 s-1 (Fig. 14). By 2349 UTC, this segment had evolved into a well-defined "broken-S" signature across central Laurens County (Fig. 15).
Figure 12. As in Fig. 9, except for 2342 UTC. Click on image to enlarge.
Figure 13. As in Fig. 11, except for 2342 UTC. Click on image to enlarge.
Figure 14. As in Fig. 11, except for 2347 UTC. Click on image to enlarge.
Figure 15. As in Fig. 9, except for 2352 UTC. Click on image to enlarge.
Despite the distinct appearance of the signature in Laurens County, a tornado apparently did not occur, nor was damage reported with the earlier signature across Spartanburg County. Previous analysis of the "broken-S" pattern has revealed that they are often cyclical in nature. As the QLCS continued to move across Upstate South Carolina, the storm- relative, forward flank inflow into the southern segment appeared to be unimpeded by rain-cooled air, unlike the northern part of the line, which was trailing an expansive area of stratiform precipitation. For that reason, forecasters? attention remained focused on the evolution of the Laurens County segment, as opposed to the Spartanburg County segment. By 0047 UTC, the QLCS had moved within range of the TCLT radar. At this time, the 0.2 degree base reflectivity image from TCLT (Fig. 16) indicated another "broken-S" signature in progress across northeast Cherokee County. Radar loops indicated this was the portion of the QLCS that broke over Spartanburg County earlier in the episode. By 0053 UTC (Fig. 17), this feature had dissipated, although a channel of weaker reflectivity (less than 50 dBz) remained near the North Carolina - South Carolina border between two segments of higher reflectivity. By 0059 UTC (Fig. 18), the reflectivity between the northern and southern segment continued to weaken, while the southern segment had pushed out slightly ahead of the northern one (Fig. 18). This was approximately 10 minutes prior to tornado occurrence near Bessemer City. The 0.2 degree radial velocity image from TCLT at this time (Fig. 19, storm relative velocity was not available) indicated an area of convergence oriented from northeast to southwest extending from a pendant at the southern tip of the northern line segment. The inbound velocities associated with the convergence to the rear of the linear segment are coincident with a channel of minimum reflectivity, suggesting a subsident component to the flow.
Figure 16. Base reflectivity on 0.2 degree scan from TCLT radar at 0047 UTC 14 January. Click on image to enlarge.
Figure 17. As in Fig. 16, except for 0053 UTC. Click on image to enlarge.
Figure 18. As in Fig. 16, except for 0059 UTC.
Figure 19. Radial velocity on 0.2 degree scan from TCLT radar at 0059 UTC.
By 0105 UTC, a "broken-S" signature was evident across western Gaston County (Fig. 20) as the southern segment continued to move northeast slightly faster than the northern segment. However, there is a noticeable difference between the reflectivity pattern in Fig. 20 and those in Figs. 10 and 15. The "broken-S" patterns in Figs. 10 and 15 are of the "distinct" variety that have been documented in the scientific literature. In these cases, the high reflectivity (greater than 50 dBz) associated with the southern line segment extended north and east of the southern tip of the northern segment. This structure is not evident in Fig 20. There was a hint of cyclonic curvature in the reflectivity field at the southern tip of the northern segment. In addition, reflectivity continued to decrease within the channel between the 2 segments. Radial velocity (Fig. 21) indicated an area of low level convergence associated with an apparent descending rear inflow jet extending southwest from the northern segment.
Figure 20. As in Fig. 16, except at 0105 UTC.
Figure 21. As in Fig. 19, except at 0105 UTC.
The 0111 UTC volume scan from the TCLT radar (Fig. 22) was the closest in time to the approximate time of tornado occurrence (likely 0109 to 0110 UTC). The reflectivity data depicted a narrow pendant at the southern tip of the northern segment. A small indentation in the forward flank of the line segment was observed just north of the pendant. This was possibly indicative of an area of easterly storm-relative inflow. It is interesting to note that the reflectivity east of the pendant had decreased to less than 20 dBz at this time. This suggests that the descending rear inflow jet evident in the previous reflectivity images had turned cyclonically around the southern tip of the line segment. The radial velocity data continued to indicate a narrow band of outbound velocities extending southwest from the pendant. Coincident with the pendant, the velocity data revealed a weak rotational signature (Fig. 23). However, the data was difficult to interpret due to the absence of a storm relative velocity product. Subsequent images from TCLT actually depicted a hook-like appendage extending from the northern line segment (Fig. 24), although the significance of this is questionable as this feature became evident several minutes after the tornado had dissipated.
Figure 22. As in Fig. 16, except at 0111 UTC. The letter "T" indicates the approximate location of the tornado.
Figure 23. As in Fig. 19, except at 0111 UTC.
Figure 24. As in Fig. 16, except at 0112 UTC.
A series of images from KGSP prior to and during tornado occurrence (Figs. 25 and 26) reveal some significant differences with TCLT in regard to the detail of the structure of the evolving tornadic storm. Reflectivity images from KGSP did not reveal the cyclonic curvature in the southern tip of the northern segment just prior to tornadogenesis. In addition, the forward flank inflow notch and the hook-like structure that was evident in the TCLT images from 0111 to 0112 UTC was absent in the KGSP reflectivity field. Although part of this may be attributed to the fact that the KGSP radar is much farther away from the storm than the TCLT radar (61 vs. 22 miles), it can also be said that the higher resolution provided by the 5 cm TDWR allows forecasters to see more detailed structure in precipitation systems than the WSR-88D.
Figure 25. As in Fig. 9, but for 0107 UTC 14 January. Click on image to enlarge.
Figure 26. As in Fig. 11, but for 0112 UTC. Click on image to enlarge.
The Bessemer City tornado was somewhat unusual in that it apparently occurred several volume scans after the line-break first became evident. Previous research has shown that tornadogenesis occurs as the line-break is developing, or shortly thereafter, providing at best 5 minutes of lead-time. In this case, a warning issued at the time that the break first became evident in the reflectivity data would have provided approximately 10 minutes of lead time. However, this would have required real-time recognition of the evolving "broken-S" pattern, which was difficult in this particular case.
The Bessemer City tornado of 13 January 2006 shared some of the radar characteristics of previous NST events studied in the western Carolinas and northeast Georgia. However, there were several characteristics that were somewhat unique to this event when compared to the "distinct broken-S" cases that have been previously documented. In the "distinct" cases, the following evolution is typically observed:
The Bessemer City tornado did not follow this evolution. With the benefit of hindsight, one can argue that a break in the QLCS occurs after 0053 UTC. However, examination of the 0053 UTC reflectivity (Fig. 17) reveals that the two high reflectivity segments that were later identified as constituting a "broken-S" pattern existed prior to the "break" in the line, with the apparent "break" occurring within an area of lower reflectivity between the two segments. This made pattern recognition difficult in real-time. In addition, the southern segment did not extend to the north and east of the northern segment. It is interesting to note that while several "distinct broken-S" signatures were observed in radar data on this day, apparently none of them were associated with a tornado. The rear-to-front, descending jet in the Bessemer City storm may have played an important role in tornadogenesis. However, its role was probably much different than that of the rear-flank downdraft in supercell tornadogenesis. The numerical modeling studies of Weisman and Trapp (2003) suggest that the source of intense, low-level rotation in QLCSs is due to downward tilting of environmental, crosswise vorticity by sinking air currents, including the rear-inflow jet associated with organized mesoscale convective systems. Since this is the case, the amount of environmental, storm relative helicity is inconsequential to non-supercell tornado occurrence, because it is a measure of the magnitude of streamwise vorticity. The magnitude of the vertical shear is a more accurate measure of the potential for NSTs. The QLCSs modeled by Weisman and Trapp depicted development of strong low-level mesovortices with 0-2.5 km shear values of 20 m s-1. The magnitude of the 0-2.5 km shear vector calculated from the sounding in Fig. 6 is approximately 26.1 m s-1.
5. Concluding Remarks and Operational Considerations
Detection of non-supercell tornadoes is one of the more difficult challenges facing operational forecasters, especially in low-CAPE environments, because they do not develop in the classic "top-down" manner that has been well-documented in association with supercell tornadoes. It is clear from operational experience that the mechanisms responsible for non-supercell tornadogenesis are different from that of supercell tornadoes, as NSTs are not associated with mesocyclones. Since SRH is an indication of the tendency of thunderstorm updrafts to tilt environmental vorticity to the vertical (and form a mesocyclone), it may have little utility in forecasting the potential for NSTs. Due to the small nature of these tornadoes, their circulations are almost always too small to be resolved by the radar beam. Only in the strongest cases is tornadogenesis preceded by an intense circulation in radar velocity data. Even in these cases, radar-indicated rotation will likely be detected only within several thousand feet of the ground and will precede tornadogenesis only by a matter of a few minutes, if at all.
All reference maps were created with Delorme Street Atlas USA 2006. The surface analysis and NCEP reanalysis graphics (Figs. 2 and 8) were obtained from the Hydrometeorological Prediction Center. The convective outlook graphic, 500 mb analysis, and mesoscale analysis graphics (Figs. 3 and 5) were obtained from the Storm Prediction Center. The regional surface plot and upper air sounding graphics (Figs. 4 and 6) were obtained from the archives at Plymouth State University. The damage survey was conducted by Rodney Hinson. Larry Lee provided a critical review of the manuscript.
Grumm, R. H., and M. Glazewski, 2004: Thunderstorm types associated with the "broken-S" radar signature. Preprints, 22nd Conf. on Severe Local Storms, Hyannis, MA, Amer. Meteor. Soc., CD-Rom, P7.1.
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.
Thompson, R. L., R. Edwards, J. A. Hart, K. L. Elmore and P. Markowski, 2003: Close proximity soundings within supercell environments obtained from the Rapid Update Cycle. Wea. Forecasting, 18, 1243-1261.
Weisman, M. L., and R. J. Trapp, 2003: Low-level mesovortices within squall lines and bow echoes. Part I: Overview and dependence on environmental shear. Mon. Wea. Rev., 131, 2779-2803.
Trapp, R. J., and M. L. Weisman, 2003: Low-level mesovortices within squall lines and bow echoes. Part II: Their genesis and implications. Mon. Wea. Rev., 131, 2804- 2823.