A tornado touched down along Kay Road, about 5.5 miles north of Abbeville, South Carolina, on the morning of Monday, 3 May 2010. Image by Tony Sturey, NWS.

Author's Note: The following report has not been subjected to the scientific peer review process.

1.  Introduction

A persistent high-amplitude upper trough over the Plains and a warm, moist 
flow from the Gulf of Mexico ahead of a strong cold front kept active 
weather across the lower Mississippi Valley and southeastern United States 
during the last day of April and the first few days of May 2010.  Numerous 
tornadoes touched down across the mid-Mississippi Valley region on 30 April 
and 1 May, followed by the catastrophic flash flooding across middle 
Tennessee and the Nashville metropolitan area on 1-2 May.  The system was
expected to lose strength as it moved over northeast Georgia and the 
Carolinas on 3 May.

During the early morning hours of 3 May, a quasi-linear convective system 
(QLCS) moved east across north Georgia and into western South Carolina.  As
expected, the QLCS weakened and became less organized as it moved into the 
Lakelands area of Upstate South Carolina between 1100 UTC and 1200 UTC.  
However, a very short linear segment of higher reflectivity developed over 
Elbert County, Georgia, embedded within the larger, weakening QLCS around 
8 am Eastern Daylight Time (EDT), or 1200 Universal Time Coordinated (UTC).  
The newer and smaller QLCS line segment strengthened as it crossed into 
South Carolina, and eventually produced a tornado over northern Abbeville
County near the community of Buck Stand, or about 5.5 miles north of the 
city of Abbeville (Fig. 1) at 1303 UTC.  The tornado struck two residences 
along Kay Road, mainly consisting of damage to roofs, outbuildings and trees.  
The damage was rated at EF0 intensity on the Enhanced Fujita Scale.  The
tornado traveled just under one mile before lifting around 1304 UTC.

[Note:  All times in this report from this point forward are expressed 
in Universal Time Coordinated (UTC), which is EDT plus four hours.  To
convert from UTC to EDT, subtract four hours from the UTC time.]

Figure 1.  Track of the tornado (shown as the thick blue line) along Kay 
Road, near the Buck Stand community in Abbeville County, South Carolina, 
on 3 May 2010.  Click on image to enlarge.

The tornadic QLCS developed in an environment with high shear and low 
convective available potential energy (CAPE).  Forecasters at the Greenville – 
Spartanburg (GSP) National Weather Service office have long associated high 
shear and low CAPE (HSLC) environments with brief tornadoes produced by QLCSs, 
often after a break in the convective line that resembles a "Broken-S" shape.  
Until recently, all the documented tornadoes in HSLC environments over the 
Carolinas associated with breaks in a QLCS (Lee and Jones, 1998; McAvoy 
et al., 2000; Lane and Moore, 2006) occurred in the cool season (i.e., the 
winter months).  A review of the radar data in the minutes leading up to the 
Buck Stand tornado on 3 May revealed a classic "Broken-S" reflectivity pattern, 
which made the event remarkable because it occurred during a normally warm 
time of year.

2.  Synoptic Features and Pre-Storm Environment

The environment across northeast Georgia and the western Upstate of South 
Carolina on the morning of 3 May was characterized by strong deep layer 
shear and upper forcing that favored linear convection.  The Storm Prediction 
Center (SPC) included the area along and to the south and east of a line from 
Elberton, Georgia, to Charlotte, North Carolina, in a slight risk of severe 
weather on the Day 1 Convective Outlook.  At 1200 UTC, a broad, elongated 
region of upper divergence associated with the right entrance region of a 
subtropical jet streak at 300 mb (Fig. 2) stretched from south Alabama to 
the Piedmont of the Carolinas and Virginia.  Water vapor satellite imagery 
showed evidence of a short wave trough over Alabama and Georgia, embedded in 
the brisk southwest to northeast flow (Fig. 3).  The western Carolinas were 
situated on the southeast edge of a mid-level jet streak lifting northeastward 
from a 500 mb trough over the southern Plains (Fig. 4).  The 850 mb analysis 
(Fig. 5) showed the tail end of a low level jet of at least 50 kt over the 
western Carolinas.  The upper air observation taken at Greensboro, North 
Carolina (GSO, Fig. 6) detected the strong low level shear brought about by
the jet streak.  In the layer from the surface to 3 km, the shear was 58 kt, 
which was well above the lower limit thought to be necessary for low level 
mesovortex generation in a QLCS as suggested by the modeling studies of 
Trapp and Weisman (2003).  The storm relative helicity (SRH) in the same 
layer was 167 m2s-2, which was sufficient for rotating updrafts.  However, 
the sounding contained relatively little buoyancy for a pre-frontal air mass 
in early May, as the mixed layer CAPE was only 321 J kg-1.

Click here to view a 19 frame Java loop of GOES-13 water vapor imagery from 
2345 UTC on 2 May to 1745 UTC on 3 May.

Figure 2.  SPC objective analysis of 300 mb wind (kt; barbs), isotachs (kt; 
color fill), streamlines, and divergence (yellow contours) at 1200 UTC on 
3 May.  Click on image to enlarge.

Figure 3.  GOES-13 water vapor imagery at 1145 UTC on 3 May 2010.  Brightness 
temperatures are given by the color table at the bottom.  Click on image to 
enlarge.

Figure 4.  SPC objective analysis of 500 mb geopotential height (dm; dark 
gray contours), wind (kt; barbs), and temperature (degrees C; dashed red 
contours) at 1200 UTC on 3 May 2010.  Click on image to enlarge.

Figure 5.  SPC objective analysis of 850 mb geopotential height (dm; dark 
gray contours), wind (kt; barbs), temperature (degrees C; red dashed 
contours), and dewpoint (above 8 degrees C; green contours) at 1200 UTC 
on 3 May.  Click on image to enlarge.

Figure 6.  Skew-T, log P, diagram of the upper air sounding taken at GSO 
at 1200 UTC on 3 May.  The temperature sounding is shown in red and the 
dewpoint sounding is shown in green.  A hodograph is shown in the upper 
right.  A table showing convective parameters is provided at the bottom.  
Click on image to enlarge.

At the surface at 1200 UTC, a cold front stretched from a low pressure center 
near Mobile, Alabama, northeast across northwest Georgia and west of the 
Appalachian Mountains, to another low pressure center over southern Quebec 
(Fig. 7).  The orientation of the front was roughly parallel to the deep 
layer shear seen in the KGSO sounding.  As a result, thunderstorms developing
along the boundary were expected to grow upscale into a linear convective
system as individual cold pools merged and reinforced the boundary.

Figure 7.  HPC Surface fronts and pressure analysis at 1200 UTC on 
3 May 2010.  Click on image to enlarge.

The mesoanalysis from the SPC confirmed the HSLC environment over northeast 
Georgia and western South Carolina at 1200 UTC immediately ahead of the 
surface front (Fig. 8).  The shear in the layer from the surface to 1 km 
above ground level was in excess of 40 kt across a large part of northeast 
Georgia and western South Carolina.  The SRH in the same layer was in excess 
of 300 m2s-2.  These values were well within the range where rotating updrafts 
and supercell thunderstorms could be expected (Brooks and Craven, 2002).  
However, the surface based CAPE was only on the order of 250 J kg-1.  The 
lifting condensation level was between 500 and 750 m AGL.

Figure 8.  SPC objective mesoscale analysis of (a) surface based CAPE (J kg-1; 
brown contours) and convective inhibition (J kg-1; color fill), (b) lifting 
condensation level (m; green contours), (c) 0-1km shear (kt; blue contours) 
and shear vector (kt; barbs), and (d) 0-1 km SRH (m2s-2; blue contours) and 
storm motion (kt; barbs) at 1200 UTC on 3 May.

3.  Radar Observations of the Abbeville County Storm

The new QLCS line segment that developed over Elbert County was unusually 
short.  The length of the continuous high reflectivity (greater than 45 dBZ) 
was only about 15 miles.  This short linear segment acquired an "S-shape" as 
it moved over southern Anderson County and western Abbeville County, South 
Carolina, through 1235 UTC (Fig. 9).  Evidence of a rear inflow jet behind 
the bowing segment of the short QLCS was less conspicuous compared to other 
documented QLCS tornado events.  A break developed in the short QLCS first 
on the 1.3 degree scan at 1239 UTC and by 1243 UTC the break was complete 
(Fig. 10), by which time the break was also evident on the 0.9 degree scan.  
On the lowest elevation scan, the break developed between 1243 UTC and 
1247 UTC (Fig. 11), and was obvious on the scan at 1252 UTC (Fig. 12).

Click here to view a 16 frame Java loop of base reflectivity on the 0.5 degree 
scan from the KGSP radar from 1210 UTC to 1313 UTC.

Figure 9.  KGSP base reflectivity on the (a) 0.5 degree, (b) 0.9 degree, 
(c) 1.3 degree, and (d) 1.8 degree scans at 1235 UTC.  The location marked 
"Home" is where the tornado touched down at approximately 1303 UTC.  The 
KGSP radar was located approximately 40 nm to the north of the center of 
the higher reflectivity.  Click on image to enlarge.

Figure 10.  As in Fig. 9, except for 1243 UTC.  Click on image to enlarge.

Figure 11.  As in Fig. 9, except for 1247 UTC.  Click on image to enlarge.

Figure 12.  As in Fig. 9, except for 1252 UTC.  Click on image to enlarge.

In the mean time, the rotational velocity on the lowest three elevation scans 
increased gradually and peaked at 36 kt on the 1247 UTC scan (Fig. 13).  The 
distance between the maximum inbound and maximum outbound radial velocity 
was about 2 nm at a range of 40 nm from the radar, which placed the 
circulation in the "moderate mesocyclone" range of the mesocyclone nomogram.  
However, the rotational shear increased much more dramatically between 
1243 UTC and 1256 UTC (Fig. 13).  The rotational shear on the lowest three 
elevation slices peaked at or above 0.040 s-1 at 1256 UTC, which was well 
into the "tornado likely" category on the rotational shear nomogram (and was 
in fact off the chart).  A Severe Thunderstorm Warning was issued for this 
small QLCS segment at 1254 UTC, which included most of the northern and 
eastern parts of Abbeville County (Fig. 14).

Click here to view a 16 frame Java loop of storm relative motion on the 
0.5 degree scan from the KGSP radar from 1210 UTC to 1313 UTC.

Figure 13.  Maximum rotational velocity (a) and maximum rotational shear 
(b) on the lowest three elevation scans from the KGSP  radar from 1214 UTC 
to 1321 UTC on 3 May.  The salmon pink colored vertical bar is the 
approximate time the tornado was on the ground in Abbeville County.  Click 
on image to enlarge.

Figure 14.  Outline of Severe Thunderstorm Warning #0037 (shown in yellow) 
issued at 1254 UTC valid until 1345 UTC.  The location of the tornado 
touchdown at 1303 UTC is shown by the blue dot labeled "T".  The background 
image shows the composite reflectivity from the KGSP radar.  Click on image 
to enlarge.

The rotational shear decreased significantly on the 0.9 degree and 1.3 degree 
scans after 1256 UTC but remained in the "tornado likely" range on the 
0.5 degree scan through 1304 UTC.  The tornado touched down along Kay Road at 
approximately 1303 UTC based on the movement of the velocity couplet on the 
0.5 degree scan (Fig. 15).  This was approximately 11 minutes after the break 
in the QLCS line segment was apparent on the lowest elevation scan.  The 
tornado lifted quickly around 1304 UTC.  No cloud-to-ground lightning was 
detected during the evolution of the QLCS across Abbeville County.

Figure 15.  Storm relative motion on the 0.5 degree scan from the KGSP radar 
at (a) 1247 UTC, (b) 1252 UTC, (c) 1256 UTC, and (d) 1300 UTC on 3 May.  
Values of storm relative motion are given by the color table in the upper 
left, where warmer colors represent motion away from the radar and cooler 
colors represent motion toward the radar.  The location marked "Home" is 
where the tornado touched down at 1303 UTC.  Click on image to enlarge.

Two more breaks in a short QLCS line segments occurred.  One was north of 
Greenwood, South Carolina, at 1321 UTC, and the other was near Joanna in 
eastern Laurens County, South Carolina. However, no additional reports of 
damage were received.  The QLCS continued to move east into Fairfield County 
and produced another tornado about nine miles southeast of Carlisle.  The 
Fairfield County tornado was not associated with a break in a QLCS line 
segment.

4.  Summary 

The larger scale QLCS was on a weakening trend in the hour before the new 
line segment formed west of Abbeville County.   Although the new QLCS line 
segment was very short in comparison to other events, the evolution of the 
reflectivity pattern displayed a classic "Broken-S" signature, with the line 
break clearly discernable more than ten minutes prior to tornadogenesis.  
The maximum rotational shear on the lowest three elevation scans peaked well 
within the "tornado likely" section of the rotational shear nomogram about 
seven minutes prior to tornadogenesis.  The Buck Stand tornado served as a 
reminder that "Broken-S" tornadoes are HSLC environment events and not 
necessarily cool season events.  It just so happens that the overwhelming 
majority of HSLC environments occur in the cool season.

Pictures of the damage from the Buck Stand Tornado

Pictures of the damage observed from the Buck Stand tornado on 3 May2010.  
Click on images to enlarge.

References

Brooks, H. E., and J. P. Craven, 2002:  A database of proximity soundings for 
     significant severe weather.  Preprints, 21st Conf. on Severe Local Storms, 
     San Antonio, TX, Amer. Meteor. Soc.

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.

Lee, L. G., and W. A. Jones, 1998:  Characteristics of WSR-88D velocity and
     reflectivity patterns associated with a cold season non-supercell tornado 
     in upstate South Carolina.  Preprints, 19th Conf. on Severe Local Storms, 
     Minneapolis, MN, Amer. Meteor. Soc., 151-154.

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.

Acknowledgements

The damage survey for the Buck Stand tornado was conducted by Tony Sturey
and Terry Benthall (NWS).  The upper air analyses and the sounding plot were 
obtained from the Storm Prediction Center.  The mesoscale objective analysis 
was prepared by the SPC and obtained from the archive at the NWS office in 
Omaha, Nebraska.  The surface fronts and pressure analysis was obtained from 
the Hydrometeorological Prediction Center.  The satellite imagery and radar 
mosaic imagery was obtained from the University Corporation for Atmospheric 
Research.  The plots of maximum rotational velocity and shear were created 
using Microsoft Excel.  The severe thunderstorm warning graphic was obtained 
from the Iowa Environmental Mesonet.  The tornado track map was made using 
Delorme Street Atlas USA 2009 Plus.