Remnants of Humberto spawn tornado near
Hickory Tavern, South Carolina, 14 September 2007
Blair Holloway and Patrick D. Moore
This mobile home near Hickory Tavern, South Carolina, was blown off its foundation by a tornado. The tornado also damaged a nearby house. Image taken by Miles Dendy. Used by permission.
Authors' Note: The following report has not been subjected to the scientific peer review process.
An area of thunderstorms developed across north Georgia and moved into western South Carolina on the afternoon of Friday, September 14, 2007. The area of thunderstorms developed around an area of low pressure associated with the remnants of Hurricane Humberto. The thunderstorms produced a tornado in Laurens County, South Carolina, near the town of Hickory Tavern, at 744 pm EDT (2344 UTC). [Note: all times from this point onward will be referenced to coordinated universal time (UTC). To convert to Eastern Daylight Time, subtract four hours from the UTC time.] A survey found a small area of tornado damage about two miles east northeast of Hickory Tavern, along Greenpond Road near the intersection with Shiloh Church Road. One house received significant roof damage, its chimney was toppled, and several windows were blown out. A nearby mobile home also received significant roof damage and was shifted about 20 degrees off its foundation. The damage was rated EF1 on the enhanced Fujita Scale. The National Weather Service issued a Tornado Warning for northern Laurens County at 2321 UTC (721 pm EDT), providing a lead time of 23 minutes.
Preliminary severe weather reports for the 24 hour period ending 8 am (1200 UTC) 15 September 2007. Note that the graphic does not reflect reports received after that time. Click on image to enlarge.
McCaul (1991) determined the majority of hurricanes that made landfall in the period from 1948 to 1986 produced at least one tornado, in some cases as long as five days after landfall. Thus, it was reasonable to expect the remnants of Humberto to hold the potential for at least one tornado. Several studies of tropical cyclone tornadoes have increased forecaster understanding of favorable buoyancy and shear environments across the Southeast (e.g., McCaul 1987, Spratt et al. 1997, McCaul et al. 2004, Schneider and Sharp 2007), including a local study of the tornadoes spawned by the remnants of Frances and Ivan (Lane 2005). Although a wide range of buoyancy values was found in these studies, the minimum value of low level (surface to 3 km) storm relative helicity (SRH) was approximately 150 m2/s2. However, these studies concentrated on outbreaks of tornadoes from remnant cyclones that largely maintained their tropical character during the event.
In the case of remnants of Humberto, it already made the transition to an extratropical low pressure center before the tornadic thunderstorm developed. The extent to which this colored the expectations of forecasters is unknown, but the forecast model guidance on the morning of the event indicated that organized severe weather was unlikely. Clouds and precipitation to the north of a quasi-stationary boundary were thought to be a limiting factor in that instability would be marginal in the location that was expected to have favorable shear. Meanwhile, model guidance showed relatively weak shear where the atmosphere was expected to become the most unstable. Thus, an interesting aspect of the Hickory Tavern tornado was the extent to which the pre-storm environment exceeded expectations and became supportive of supercell thunderstorms.
2. Synoptic Features
Hurricane Humberto made landfall just east of High Island, Texas, at approximately 0700 UTC on 13 September. Humberto weakened to a Tropical Storm over southwestern Louisiana by 1500 UTC, and was further downgraded to a Tropical Depression by 2100 UTC near Alexandria, Louisiana (Fig. 1). The Depression center moved across northeast Louisiana overnight to a position over central Mississippi at 1200 UTC on 14 September (Fig. 2). A quasi-stationary front stretched from the low, east across north Georgia and the middle of South Carolina, to the south of the GSP County Warning Area.
Fig. 1. Best track positions for "Humberto" for the period 12-14 September 2007. Figure prepared by Eric Blake at the Tropical Prediction Center. Click on image to enlarge.
Fig. 2. Hydrometeorological Prediction Center surface fronts and pressure analysis at 1200 UTC 14 September. Click on image to enlarge.
The remnants of Humberto were discernable at 850 mb as a closed low centered over Mississippi at 1200 UTC 14 September (Fig. 3). The strongest wind observed by an upper air sounding was only 25 kts. At 500 mb, only a weak reflection of the low was seen, with relatively light winds aloft (Fig. 4). The upper air sounding taken at 1200 UTC at Peachtree City, Georgia (FFC), showed a forecast Convective Available Potential Energy (CAPE) of 850 J/kg, and 0-3 km SRH of less than 100 m2/s2 (Fig. 5). While the value of buoyancy was in the same range as a tropical cyclone tornado composite sounding determined by McCaul (1991), the value of SRH was substantially less than that and values observed in other tropical cyclone tornado outbreaks. Most parameters suggested a low threat for tornadoes as indicated by Schneider and Sharp (2007), particularly the shear and helicity. However, the wind shear was expected to improve later in the day as the remnant low approached, as evidenced by the upper air sounding taken at 1200 UTC at Birmingham, Alabama (BMX). The Day 1 Convective Outlook from the Storm Prediction Center (SPC) issued at 1300 UTC suggested that an isolated tornado might be possible if enough warming could occur ahead of the remnant low pressure center. The forecast changed little by midday, although the updated Day 1 Convective Outlook issued at 1630 UTC focused mainly on north Alabama and Georgia.
Fig. 3. SPC objective analysis of 850 mb geopotential height, temperature, dewpoint, and wind barbs at 1200 UTC 14 September. Click on image to enlarge.
Fig. 4. SPC objective analysis of 500 mb geopotential height, temperature, and wind at 1200 UTC 14 September. Click on image to enlarge.
Fig. 5. Skew-T log P diagram (upper left) and hodograph (upper right) for upper air sounding at FFC at 1200 UTC 14 September. The tables at the bottom summarize several objective parameters used by the SPC to determine severe weather potential. Click on image to enlarge.
3. Pre-Storm Environment
In fact, the atmosphere became more unstable than expected across northeast Georgia and Upstate South Carolina by the middle part of the afternoon. At 1800 UTC, the remnant surface low was located along the Mississippi - Alabama border, while the quasi-stationary boundary had drifted north across South Carolina (Fig. 6). Most of the convection (Fig. 7) was associated with another low centered along the Alabama - Georgia border. Ahead of the low, clouds thinned over northeast Georgia and Upstate South Carolina, as seen in the visible satellite imagery at 1945 UTC (Fig. 8), which allowed the CAPE to climb above 1000 J/kg ahead of the convection moving across north Georgia. A relatively low lifting condensation level between 750 m and 1000 m above ground level and increasing SRH of nearly 150 m2/s2 favored the development of thunderstorms with supercell characteristics and the potential for a brief tornado, as outlined in the Day 1 Convective Outlook issued by the SPC at 2000 UTC.
Fig. 6. Hydrometeorological Prediction Center surface fronts and pressure analysis at 1800 UTC 14 September. Click on image to enlarge.
Fig. 7. SPC composite reflectivity mosaic at 1835 UTC 14 September. Click on image to enlarge.
Fig. 8. GOES-12 visible satellite imagery at 1945 UTC 14 September.
The convection moved across north Georgia and into western South Carolina through 2100 UTC. After 2100 UTC, the back edge of an area of showers lifted north across Elbert, Abbeville, and Greenwood counties, and was oriented east-west across Anderson County and Laurens County by 2200 UTC. This feature represented a weak surface boundary, behind which the most unstable CAPE approached 2000 J/kg (Fig. 9) and the SRH in the surface to 3 km layer remained in the 100-150 m2/s2 range at 2200 UTC (Fig. 10). A new thunderstorm developed along the boundary over the western tip of Anderson County around 2200 UTC and quickly acquired supercell characteristics as it moved east along the boundary through 2235 UTC (Fig. 11). The supercell was clearly discernable on the visible satellite imagery as an overshooting cloud top over western South Carolina. A well-defined low level inflow notch, weak echo region, and strengthening mesocyclone prompted the issuance of a Tornado Warning for northern Anderson County at 2239 UTC. At the time of the warning, the rotational velocity on the KGSP radar was 23 knots at 4,000 feet AGL. This was higher than observed in earlier storms that produced possible tornado damage in Davidson and Forsyth counties in North Carolina, which increased confidence that a tornado was developing in the Anderson County storm. A funnel cloud was reported through law enforcement near Belton, however it did not touch down.
Fig. 9. SPC objective mesoscale analysis of most unstable CAPE (contoured, J/kg) and lifted parcel level (shaded, m AGL) at 2200 UTC 14 September. Click on image to enlarge.
Fig. 10. SPC objective mesoscale analysis of storm relative helicity in the 0-3 km layer (contoured, m2/s2) and storm motion (barbs) at 2200 UTC 14 September. Click on image to enlarge.
Fig. 11. Composite radar reflectivity from the KGSP WSR-88D radar at 2235 UTC 14 September. The thunderstorm that prompted the first Tornado Warning issuance was located just west of the 'A' in Anderson. The radar is located at the point labelled 'KGSP'. Click on image to enlarge.
4. Radar observations of the Hickory Tavern Storm
As the supercell moved across the eastern end of Anderson County, it featured strong rotation throughout a deep layer, which appeared most prominent on the 2.4 degree elevation slice (Fig. 12). The KGSP radar identified this feature as a mesocyclone. In fact, the storm contained a persistent mesocyclone, as identified by the radar, that lasted nearly two hours as it moved eastward across Anderson, Greenville, and Laurens counties. A Tornado Warning was issued at 2321 UTC for southeastern Greenville and northern Laurens counties. At this time, the storm featured strong low-level rotation at 0.5 degrees (Fig. 13) and a tornado vortex signature (TVS). Other features were also present at the time of issuance including a weak echo region on the 2.4 degree slice and a bounded weak echo region (BWER) above that on the 3.1 degree slice (Fig. 14). Both of these features, in combination with the strong rotation, pointed to the presence of a strong rotating updraft and the increasing risk of this storm producing a tornado. A "velocity enhancement signature" (Schneider and Sharp 2007) was not seen prior to the tornado, although some increase in the outbound velocity was noted on the 4.0 degree scan at 9,100 feet above ground level at 2330 UTC.
Fig. 12. Storm relative motion from the KGSP WSR-88D radar on the 2.4 degree slice at 2307 UTC 14 September. In general, warmer colors represent motion toward the radar while cooler colors represent motion away from the radar. The arrow points to the couplet of inbound and outbound velocity, which the radar algorithm identified as a mesocyclone. Click on image to enlarge.
Fig. 13. Storm relative motion at 2321 UTC from the KGSP radar at (a) 0.5 degrees, (b) 1.3 degrees, (c) 2.4 degrees, and (d) 3.1 degrees. The arrow between Honea Path and Princeton denotes the location of the mesocyclone. The color scale is the same as in Fig. 12. Click on image to enlarge.
Fig. 14. Radar reflectivity at 2321 UTC from the KGSP radar at (a) 0.5 degrees, (b) 1.3 degrees, (c) 2.4 degrees, and (d) 3.1 degrees. The arrow between Honea Path and Princeton denotes the location of the weak echo region and inflow notch. The color scale is the same as in Fig. 11. Click on image to enlarge.
A post-event storm survey conducted by the National Weather Service, determined that a tornado occurred at 2344 UTC about two miles east northeast of Hickory Tavern. The storm had 27 kts of gate-to-gate shear on the 0.5 degree slice at 2344 UTC (Fig. 15), and a persistent mesocyclone with strong and deep rotation that extended above the 3.1 degree slice. The supercell also continued to show other storm structure characteristics consistent with a strong, rotating updraft including a low-level reflectivity appendage and a weak echo region above (Fig. 16). The low-level rotation strengthened on the 0.5 degree slice through 2348 UTC where the storm had 32 kts of shear (Fig. 17), which the radar algorithm identified as a TVS. Furthermore, a BWER was noted in the reflectivity on the 2.4 degree elevation slice at 2348 UTC (Fig. 18).
Fig. 15. As in Fig. 13, except for 2344 UTC. The arrow points to the location of the mesocyclone. The color scale is the same as in Fig. 12. Click on image to enlarge.
Fig. 16. As in Fig. 14, except for 2344 UTC. On the 0.5 degree slice (a), the arrow points to the location of a reflectivity appendage. On the 2.4 degree slice, the arrow points to the weak echo region. Click on image to enlarge.
Fig. 17. Storm relative motion on the 0.5 degree scan at 2348 UTC. The arrow points to the location of the strongest gate-to-gate velocity difference associated with the tornado mesocyclone. Click on image to enlarge.
Fig. 18. Base reflectivity on the 2.4 degree slice at 2348 UTC. The arrow points to the location of the BWER. Click on image to enlarge.
Following the confirmed tornado in the Hickory Tavern area, no other tornado damage was reported. Based on the report of tornado damage, the SPC upgraded the area across the Midlands of South Carolina and the Sandhills of North Carolina to a Slight Risk on the new Day 1 Convective Outlook issued at 0000 UTC 15 September. The Hickory Tavern storm continued moving eastward across Laurens County thereafter, and eventually caused wind damage in the eastern end of the county at 0040 UTC 15 September, about 6 miles east-northeast of Clinton.
Click here to view a 24 frame java loop of base reflectivity and here for a 24 frame java loop of storm relative motion on the 0.5 degree scan from the KGSP radar from 2307 UTC 14 September to 0053 UTC 15 September.
On 14 September 2007, a supercell thunderstorm developed and moved quickly eastward across western South Carolina, prompting a Tornado Warning that included southeastern Greenville and northern Laurens counties. The storm maintained a persistent mesocyclone throughout its life span, along with other distinguishing features that are characteristic of a supercell including a low-level reflectivity appendage, an inflow notch, and a BWER. Tornado damage consistent with EF1 on the Enhanced Fujita Scale was reported at 2344 UTC, approximately 2 miles east northeast of Hickory Tavern, which was 23 minutes after the Tornado Warning was issued. The thunderstorm continued eastward thereafter, and though there was no other tornado damage, wind damage was reported at 0040 UTC 15 September about 6 miles east-northeast of Clinton.
An analysis of the Hickory Tavern tornado yielded additional information about the spectrum of tropical cyclone tornado events. The supercell developed in an environment that was ultimately more favorable than anticipated earlier in the day. Although forecasters are aware of the potential for tropical cyclone remnants to spawn tornadoes, the threat for such development was initially thought to be low due to a relative lack of shear and storm relative helicity. As it turned out, the values of helicity reached a level comparable with other tropical cyclone tornado events. In the end, the ability for the atmosphere to respond favorably in the presence of a remnant circulation, even a few days after landfall and after extratropical transition has occurred, should not be underestimated.
Damage caused by the tornado that touched down northeast of Hickory Tavern, South Carolina, at 7:44 PM on Friday, 14 September, 2007. The image on the left shows where the mobile home was shifted off its foundation. The image on the right shows the damage to the chimney and roof of the adjacent wood-frame house. Images taken by Miles Dendy. Used by permission. Click on images to enlarge.
Lane, J. D., 2005: Environmental aspects of two tornado outbreaks associated with landfalling tropical cyclones. 4th Southeast Severe Storms Symposium, Starkville, MS, Mississippi State University. McCaul, E. W., Jr., 1987: Observations of the Hurricane "Danny" tornado outbreak of 16 August 1985. Mon. Wea. Rev., 115, 1206-1223. McCaul, E. W., Jr., 1991: Buoyancy and shear characteristics of hurricane- tornado environments. Mon. Wea. Rev., 119, 1954-1978. McCaul, E. W., Jr., D. E. Buechler, S. J. Goodman, and M. Cammarata, 2004: Doppler radar and lightning network observations of a severe outbreak of tropical cyclone tornadoes. Mon. Wea. Rev., 132, 1747-1763. Schnieider, D., and S. Sharp, 2007: Radar signatures of tropical cyclone tornadoes in central North Carolina. Wea. Forecasting, 22, 278-286. Spratt, S. M., D. W. Sharp, P. Welsh, A. Sandrik, F. Alsheimer, and C. Paxton, 1997: A WSR-88D assessment of tropical cyclone outer rainband tornadoes. Wea. Forecasting, 12, 479-501.
The authors appreciate the assistance of Jonathan Blaes (NWS Raleigh) with obtaining the SPC mesoscale analysis. Thanks are also given to Miles Dendy for supplying the images of the damage near Hickory Tavern. The upper air soundings, analysis, and severe weather plots were obtained from the Storm Prediction Center. The surface fronts and pressure analyses were obtained from the Hydrometeorological Prediction Center. The satellite imagery was obtained from the University Center for Atmospheric Research. The radar images were created using the Java NEXRAD viewer obtained from the National Climatic Data Center.