A weak tornado touched down briefly near Kings Mountain, North Carolina, on 28 April 2008, damaging mobile homes and vehicles.

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

1.  Introduction

A small tornado occurred in Cleveland County, North Carolina, about
two miles west of Kings Mountain at approximately 1342 UTC (942 am EDT) 
on Monday, the 28th of April 2008 (Fig. 1).  [Note:  All times in this 
report are referenced to Universal Time Coordinated (UTC), which is 
Eastern Daylight Time plus four hours.]  The tornado produced EF-0 
damage on the Enhanced Fujita Scale.  The damage consisted of three 
mobile homes blown off their foundations, three other mobile homes 
with damaged underpinnings, and a power line damaged by a tree limb.
The path length was 200 yd (183 m) and the path width was 110 ft (38 m).  
The duration of the tornado was approximately 11 seconds.

Figure 1.  Marker shows the location of the tornado approximately 
two miles west of Kings Mountain, North Carolina, along Yarborough Road.
Click to enlarge.

The tornado occurred in a small convective precipitation element 
that traveled from Franklin County, Georgia, to Gaston County,
North Carolina.  Figure 2 is the National Severe Storms Laboratory
(NSSL) rotational track analysis depicting the storm’s path across 
the County Warning Area (CWA) of the Greenville - Spartanburg (GSP)
office of the National Weather Service (NWS).

Click here for a loop of KGSP composite reflectivity from 0911 UTC until 
1345 UTC.

Figure 2.  NSSL rotational track showing path of tornadic storm from 
Franklin County, Georgia, to Gaston County, North Carolina.  Click to
enlarge.

The convective storm that produced the Kings Mountain tornado was part 
of a cluster of storms that proceeded to move across central North
Carolina during the middle part of the day, and produced an outbreak of 
tornadoes across the southeastern corner of Virginia in the late 
afternoon (Fig. 3), including the destructive long-track tornado that 
moved across Suffolk County.

Figure 3.  Tornado, hail, and wind damage reports compiled by the 
Storm Prediction Center (SPC) for the 24-hour period ending 1200 UTC 
29 April 2008.

2.  Synoptic Overview

The primary surface feature during the morning of 28 April 2008 was 
a cold front extending from a low pressure system over Pennsylvania 
to the Florida panhandle then offshore.  At 1200 UTC, the surface 
analysis from the Hydrometeorological Prediction Center (HPC) showed
the front moving through the mountains of North Carolina (Fig. 4).   
A sharp 500 mb trough extended from Hudson Bay south through the 
western Great Lakes states to the lower Mississippi River Valley 
(Fig. 5).  The tornado occurred near the right, front quadrant of 
a 300 mb 100 kt wind maximum that extended from Louisiana to 
southwest Kentucky (Fig. 6). 

Figure 4.  1200 UTC 28 April 2008 HPC surface analysis.  Click to
enlarge.

Figure 5.  SPC objective analysis of 500 mb geopotential height, 
temperature, and wind barbs at 1200 UTC on 28 April 2008. Click on 
map to enlarge.

Figure 6.  SPC objective analysis of 300 mb isotachs, streamlines, 
and wind divergence at 1200 UTC on 28 April 2008. Click on map to 
enlarge.

The NWS upper air sounding closest to the tornado occurrence was the 
1200 UTC observation from Greensboro, North Carolina (GSO) (Fig. 7).  
The wind and stability diagnostic variables did not indicate an 
atmosphere conducive to severe convective storm development.  The 
surface-based Convective Available Potential Energy (CAPE) was 
49 J/kg which indicated that strong updraft speeds were not likely.  
The equilibrium level was approximately 3390 m (11,000 ft) AGL.  The 
Lifted Index was +1, and the 700-500 mb lapse rate was -5.5 deg C/km.  
However, the wind field displayed characteristics that are associated 
with tornadic storms in a conditionally unstable atmosphere.  The 
200 mb wind speed was 125 kt, the 500 mb wind speed was 43 kt, and 
the 850 mb wind speed was 37 kt.  Surface to 1 km storm relative 
helicity was 115 m2/s2, and surface to 3 km storm relative helicity 
was 104 m2/s2.  The storm character nomogram (CAPE vs. 0-4 km shear) 
did not even place the storm type indicator in the “ordinary” 
thunderstorm category.

Figure 7.  Skew T - log P and hodograph plot of GSO upper air sounding
at 1200 UTC on 28 April 2008. Click to enlarge.

Forecast soundings from the North American Mesoscale (NAM) model 
at GSP (Fig. 8) and Charlotte, North Carolina (CLT) (Fig. 9) at 
the approximate time the damage occurred did not depict features 
typically associated with tornadogenesis.  Nonetheless, the 
relatively strong speed shear in the lower troposphere provided a 
potential source for low-level vorticity about the vertical axis 
if the shear (horizontal vorticity) could be tilted.

Figure 8.  NAM forecast sounding at GSP, valid at 1300 UTC 28 April 2008.
Click to enlarge.

Figure 9.  NAM forecast sounding at CLT, valid at 1400 UTC 28 April 2008.
Click to enlarge.

The Day 1 Convective Outlook issued at 1236 UTC did not include the 
western Carolinas in the area where severe thunderstorms or tornadoes 
were anticipated.  

3.  Radar Overview

The area affected in Cleveland County was equidistant from the Weather
Surveillance Radar - 1988 Doppler (WSR-88D) located at the NWS office 
at GSP, and the Terminal Doppler Weather Radar (TDWR) located north of 
the Charlotte - Douglas International Airport.  The WSR-88D at GSP is
referred to as the KGSP radar and the TDWR near Charlotte is referred
to as the TCLT radar.

a.  KGSP WSR-88D

A short convective line segment containing 45 to 50 dBZ reflectivity 
entered Franklin County, Georgia, at 0905 UTC.  It was moving toward 
the northeast at approximately 30 kt.  By 0935 UTC, reflectivity 
increased to 55 to 60 dBZ.  The line segment had a slightly concave 
appearance, bowing to the east.  A very weak mesocyclonic circulation 
was evident in both the 0.5 degree and the 1.5 degree storm relative 
motion scans at the southern end of the line by 0940 UTC.

At 1027 UTC, the storm was over western Anderson County, South Carolina,
and it continued moving toward the northeast at 30 kt.  Weak cyclonic 
rotation existed at the southern end of the convective element at the
lowest four elevation scans from the KGSP radar (Fig. 10).  The maximum 
rotational velocity was approximately 11 kt which is in the “weak shear” 
category, but the 6 x 10-2 s-1 rotational shear value was in the 
“minimal mesocyclone” category on the rotational shear nomogram 
(Falk and Parker 1998).

Figure 10.  KGSP storm relative velocity at 1027 UTC 28 April 2008 
at 0.5 degrees, 1.5 degrees, 2.4 degrees, and 3.4 degrees (clockwise 
from upper left).

The weak mesocyclone became diffuse and lost definition as it moved 
across northern Anderson County into Greenville County.  The KGSP 
radar velocity displays were interrupted by range folding as the 
system approached and moved within about 3 nm of KGSP between 
1130 UTC and 1150 UTC.

Indications of a developing mesocyclone became apparent between 1203 UTC 
and 1217 UTC when hints of cyclonic shear or rotation appeared in the 
SRM displays at 2.4 degrees and 3.1 degrees.  Cyclonic shear or rotation 
became evident on the 1.3 degree scan at 1222 UTC about 14 nm northeast 
of KGSP.

From 1226 UTC to 1239 UTC weak cyclonic rotation continued at 1.3 degrees, 
2.4 degrees, and 3.1 degrees.  However, there was no cyclonic shear or 
rotation evident at 0.5 degrees.  At 1243 UTC a subtle indication of 
rotation appeared at 0.5 degrees, and was apparent at 1248 UTC (Fig. 11).  
Rotation persisted in the four lowest scans through the approximate time 
of tornado occurrence at 1331 UTC.

Figure 11.  KGSP 0.5 degree scan of storm relative velocity at 
1248 UTC.  The arrow points toward the developing velocity couplet.

Click here for a loop of storm relative velocity at 0.5 degrees from 
1243 UTC until 1331 UTC.

The rotational velocity at 0.5 degrees was between 10 kt and 12 kt 
through 1305 UTC.  At 1309 UTC, approximately 22 minutes prior to the 
tornado, the rotational velocity at 0.5 degrees increased to 17 kt.  
The storm was about 37 nm northeast of KGSP at this time.

The 0.5 degree rotational velocity remained about 16 or 17 kt through 
1323 UTC then increased to its highest value, 18 kt, at 1327 UTC 
(five minutes prior to the estimated time of tornado occurrence).  
Also at 1327 UTC, the rotational shear at 0.5 degrees reached its 
maximum value of 10 * 10-3 s-1 (“Tornado possible” category on the 
rotational shear nomogram).  At 1331 UTC the 0.5 degree rotational 
velocity was 15 kt.

Figure 12 shows the KGSP 0.5 degree reflectivity at 1318 UTC (approximate 
time a Tornado Warning would have to be issued to meet the lead time goal 
of 11 minutes) and at 1331 UTC (approximate time of tornado occurrence).  
Note the hook echo reflectivity signature at 1318 UTC.  Figure 13 is an 
enlarged 1318 UTC 0.5 degree reflectivity image with arrows depicting a 
conceptual model of the wind flow forming the hook:  A weak rear inflow 
jet and weak inflow on the “front” side of the convective element.

Figure 12.  KGSP 0.5 degree reflectivity at 1318 UTC (left) and 
1331 UTC (right).  Click on images to enlarge.

Figure 13.  An enlarged version of the KGSP 1318 UTC 0.5 degree 
reflectivity image with arrows depicting a conceptual model of the 
wind flow.  Click to enlarge.

3b.  TCLT TDWR

The small convective system that spawned the tornado entered the 
TCLT 55 nm display at approximately 1245 UTC while over northern 
Cherokee County, South Carolina, near Gaffney.  The storm was moving 
toward the northeast at approximately 35 kt.  The reflectivity 
structure at both 0.2 degrees and 1.0 degrees displayed a line echo 
wave pattern (LEWP) suggestive of a mesocyclonic circulation (Fig. 14).

Figure 14.  TCLT 0.2 degree (left) and 1.0 degree (right) reflectivity 
at 1245 UTC 28 April 2008. Click on images to enlarge.

The 0.2 degree storm relative velocity was contaminated by range 
folding so a meaningful evaluation of velocity patterns was not 
possible at 1245 UTC and on subsequent scans at that elevation.  The 
1245 UTC 1.0 degree storm relative velocity contained some range 
folding, but sufficient data were available to make possible a 
cautious evaluation of velocity data.   A very weak rotational 
velocity of 10 kt existed at 1.0 degrees at 1245 UTC (Fig. 15).

Figure 15.  TCLT 1.0 degree storm relative velocity at 1245 UTC 
28 April 2008.

The LEWP persisted as the system moved across the state line into 
Cleveland County, but the structure had less definition by 1309 UTC 
(Fig. 16).  As a matter of fact, the reflectivity pattern assumed a 
rather amorphous appearance, possibly caused by attenuation due to 
precipitation between the radar and the storm of interest.  Range 
folding complicated the location of velocity signatures at 1.0 degree, 
but it appears that the rotational velocity through the time of tornado 
occurrence remained between 10 and 15 kt with the exception of 1309 UTC 
when the rotational velocity peaked at 19 kt.

Figure 16.  TCLT 1.0 degree storm relative motion (top) and reflectivity 
(bottom) at 1309 UTC 28 April 2008.

The LEWP reflectivity pattern displayed a slight tendency to re-organize 
from 1315 UTC until the time of the tornado at approximately 1331 UTC 
(Figs. 17 and 18).  As a matter of fact, a break in the bow just south 
of the LEWP’s comma head hints that a weak rear inflow jet was surging 
into the narrow line resulting in weaker reflectivity at the apex of the 
bow.  The KGSP imagery in Figs. 12 and 13 depict the same process which 
resulted in the hook echo reflectivity signature  just prior to 
tornadogenesis.

Figure 17.  TCLT 1.0 degree reflectivity (top) and storm relative velocity 
(bottom) at 1315 UTC and 1321 UTC 28 April 2008.

Figure 18.  TCLT 1.0 degree reflectivity (top) and storm relative velocity 
(bottom) at 1327 UTC and 1333 UTC 28 April 2008.

Immediately following the estimated time of tornado occurrence, the LEWP 
became very difficult to identify.   The system appeared to be weakening, 
but attenuation might also have contributed to an apparent lack of structure. 

4.  Operational Considerations 

Even though the atmosphere across the central and western Carolinas did 
not provide obvious clues that tornadic storms were possible, forecasters 
at GSP recognized the high shear, low CAPE environment that has been 
associated with small, short-lived tornadoes in the past.  Adding to an 
increased situation awareness for tornadoes was the presence on radar 
of small, quasi-linear convective features indicating the possibility 
of horizontal wind shear and updraft/downdraft interaction that could 
produce local rotation.

The severe weather radar signatures early in the storm’s trip across the 
CWA were sufficiently well defined, given the high shear environment, to 
cause concern.  Even though the reflectivity and velocity signatures on 
both the KGSP and TCLT radars were not profound, the echo characteristics 
continued to attract attention as the storm approached the KGSP RDA and 
continued northeast across Spartanburg and Cherokee counties into 
Cleveland County.  The formation of a weak mesocyclone (detected by SRM 
products) in combination with a well defined LEWP (identified in 
reflectivity products) in a high shear environment were seen as 
precursors to tornado development.  Obviously, radar detection of the 
tornado vortex was impossible.  So, the presence of a tornado would have 
been deduced from a subjective evaluation of the probability of occurrence 
of a 110 ft (38 m) wide vortex lasting 11 seconds.

To focus attention on similar environments that have the potential to 
produce small, short-lived tornadoes, a modification to the traditional 
CAPE vs. shear nomogram is suggested.  From the event discussed in the 
present work and based on experience with similar tornado occurrences, 
it is quite obvious that stability (as measured by traditional indices) 
and available potential energy (as measured by CAPE) play a minor role 
in tornadogenesis of this type.  It seems that only weak updrafts and 
downdrafts are needed to tilt strong horizontal vorticity (a.k.a. low 
level wind shear) into the vertical.  Thus, the requirement for moderate 
to large values of CAPE as a necessary ingredient for small, short-lived 
tornado development should be removed from consideration.  All that is 
needed is lower tropospheric shear of a sufficient magnitude in the 
presence of a tilting mechanism (viz., an updraft or a downdraft).   
With these thoughts in mind, the storm character nomogram should be 
modified as depicted in Figure 19 to increase the situation awareness of 
forecasters during low CAPE, high shear situations.

Figure 19.  Light green area on the revised storm character 
nomogram highlights the CAPE and 0-4 km shear combination 
favorable for small, short lived tornadoes.

Reference

Falk, K., and W. Parker, 1998:  Rotational shear nomogram for tornadoes. 
     Preprints, 19th Conf. on Severe Local Storms, Minneapolis, MN, 
     Amer. Meteor. Soc., 733-735.

Acknowledgements

Pat Moore converted the original report into html code for the 
Internet page.  The severe weather report plot, upper air analyses, 
and upper air soundings were obtained from the archives of the Storm 
Prediction Center.  The surface analysis was obtained from the 
Hydrometeorological Prediction Center.  The NAM forecast sounding 
plots and background for the CAPE - shear Nomogram were obtained 
from the RAOB program.  Some of the radar images were created using 
the Java Nexrad viewer obtained from the National Climatic Data 
Center.  The damage photographs were taken by Larry Gabric during the
storm survey.