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.

1.  Introduction

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.

Click here to view a summary of severe weather reports.

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.

Click here to view an 18 frame Java loop of surface fronts and pressure 
analyses.

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.  

Click here to view an 12 frame Java loop of GOES-12 visible satellite 
imagery, and here to view a 22 frame Java loop of GOES-12 water vapor 
satellite imagery.

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.

Click here to view a 26 frame java loop of composite reflectivity from
the KGSP radar from 2102 UTC to 2258 UTC.

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.

4.  Summary 

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.

References

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. 

Acknowledgements

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.