The same supercell thunderstorm that produced the violent tornado that devastated Tuscaloosa, Alabama, tracked across north Georgia and into Rabun County, Georgia, around 11 pm on 27 April 2011. The supercell produced a tornado over Lake Burton that caused at least one fatality. Image created by Brian Tang of the National Center for Atmospheric Research (NCAR) and used by permission.

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

1.  Introduction

An epic tornado outbreak occurred across parts of the eastern United States 
during the afternoon and evening of Wednesday, 27 April 2011, and continued 
into the early morning hours of Thursday, 28 April 2011.  The number of 
tornadoes for the 24 hour period ending at 1200 UTC on 28 April 2011 
shattered the previous record set during the "Super Outbreak" of 3-4 April 
1974 (Fujita 1975).  The death toll was second only to the outbreak of 
18 March 1925, which included the infamous "Tri-State Tornado" (Changnon 
et al. 1966).  The number of strong to violent tornadoes (EF3 intensity or 
greater on the Enhanced Fujita Scale) over the southern Appalachian Mountains 
of North Carolina, Tennessee, and north Georgia (Table 1) was the most since 
the Palm Sunday tornadoes of 1994, and the third instance of multiple violent 
tornadoes in the same day since 1950.  The devastating tornadoes of 27 April 
2011 were part of a larger outbreak of severe weather, mainly across the 
Southeast, that spanned four days from 25 April to 29 April (Fig. 1).

Click here to read the official NWS Service Assessment

Go here for a clickable map of event reviews from other NWS offices

Table 1. F3/EF3 and F4/EF4 tornadoes in the southern Appalachian Mountains of 
North Carolina, Tennessee, and north Georgia since 1950.  Click on image to 
enlarge.

Figure 1.  Preliminary reports of tornadoes, large hail, and thunderstorm wind 
damage collected by the Storm Prediction Center from NWS local storm reports 
for the 24 hour period ending at 1200 UTC on 26 April 2011 (upper left), 
27 April 2011 (upper right), 28 April 2011 (lower left), and 29 April 2011 
(lower right).  Click on images to enlarge.

As in the Super Outbreak, northeast Georgia and western North Carolina did not 
bear the brunt of the tornadic activity on 27-28 April 2011.  However, those 
locations were not entirely spared either.  In fact, the events of 27-28 April 
bore an eerie resemblance to the Super Outbreak over the western Carolinas and 
northeast Georgia (Figs. 2 and 3).  In both events, a supercell thunderstorm 
moved across north Georgia and produced tornadoes in extreme northeast Georgia.  
Furthermore, both events had two tornadoes reported over the northern foothills 
of North Carolina. At least two tornadoes briefly touched down in the mountains 
of North Carolina on 27 April, although several more tornadoes occurred in 
southwest North Carolina during the Super Outbreak.

Click here to view a map of tornado tracks on 27-28 April 2011

Figure 2.  Tornado tracks during the "Super Outbreak" of 3-4 April 1974 
(Fujita 1975).  The 148 tornadoes documented during the outbreak are numbered 
sequentially in the figure.

Figure 3.  Preliminary tornado tracks for 24-29 April 2011 across the 
southeastern United States.  The tracks are color coded by maximum EF-scale 
rating as shown in the legend.  Click on image to enlarge.

The events of 27-28 April 2011 were highly anticipated by the forecasters at 
the Storm Prediction Center (SPC) and at the Greenville - Spartanburg (GSP) 
Weather Forecast Office (WFO), even if the convective mode was not entirely
appreciated until the evening of 27 April.  The potential for severe 
thunderstorms was first mentioned by GSP in the Hazardous Weather Outlook 
issued in the afternoon of 23 April.  By that time, the SPC had already 
included the western part of the Carolinas in the severe weather outlook for 
Day 5 (1200 UTC 27 April to 1200 UTC 28 April), which mentioned the possibility 
of isolated tornadoes.  Every subsequent outlook issued by the SPC had part 
or all of the GSP county warning area (CWA) in a severe weather risk area for 
that time period.  The expected threat gradually increased, with the potential 
for a tornado outbreak in the Tennessee Valley first mentioned in the Day 3 
outlook issued at 0730 UTC on 25 April, and continued on the Day 2 outlooks 
issued at 0600 UTC and 1730 UTC on 26 April.  The threat culminated in a 
moderate risk over the western half of the CWA on the Day 1 outlook issued 
at 0610 UTC on 27 April, with the significant tornado potential extending 
into the southern mountains of North Carolina, northeast Georgia, and the 
western tip of South Carolina.

2.  Tornado Summary

Four distinct supercell thunderstorms moved across north Georgia and western 
North Carolina, producing at least five tornadoes within the GSP CWA during 
the late evening of 27 April 2011 and early morning hours of 28 April 2011 
(Fig. 4).  The first supercell moved across Cocke and Greene counties in 
Tennessee and produced a brief and weak tornado northwest of Hot Springs 
(Madison County), North Carolina.  The second supercell was by far the most 
intense and quite possibly the most impressive storm of the entire outbreak.  
The supercell reached northeast Georgia in the late evening and produced a 
long-track significant tornado that began in northeast Lumpkin County.  The 
tornado remained on the ground across northern White County, the northwest 
corner of Habersham County near Goshen Mountain, and then crossed Rabun 
County from Lake Burton to just northeast of Mountain City, where it lifted 
on the side of Oakey Mountain.  The damage along most of the track from Lumpkin 
to northwest Habersham counties was rated at EF2 on the Enhanced Fujita Scale.  
The damage at Lake Burton was rated at EF3 and caused one fatality and at 
least five injuries, making this tornado the strongest in the GSP CWA since 
the Dudley Shoals (Caldwell County), North Carolina, F4 tornado on 7 May 1998.  
It was also the first significant tornado to strike Rabun County since the Palm 
Sunday Outbreak on 27 March 1994.  The third supercell moved on a nearly 
identical path across Rabun County, Georgia, and across Macon County, North 
Carolina, where it produced a brief, weak tornado in the Scaly Mountain 
community.  The fourth supercell moved along the Blue Ridge and then across the 
northern foothills of North Carolina, eventually producing tornadoes southeast 
of Lenoir in Caldwell County, North Carolina, and west of Taylorsville in 
Alexander County, North Carolina.  The damage from both tornadoes was rated at 
EF1 intensity.

Figure 4.  At least five tornadoes tracked across northeast Georgia and western 
North Carolina on 27-28 April 2011.  The color of the line shows the intensity 
of the tornado.  Click on image to enlarge.

All five tornadoes occurred with a Tornado Watch in effect.  The first tornado 
had no warning in effect, but the other four had a Tornado Warning.  The 
average initial lead time was 17.8 minutes and the average event lead time was 
22.6 minutes.

3. Synoptic and Pre-Storm Environment

The synoptic weather pattern was highly conducive to an outbreak of severe 
thunderstorms over the Southeast on 27-28 April.  A high amplitude upper trough 
at 500 mb was situated over the Plains at 1200 UTC on 27 April, with a short 
wave trough evident over Oklahoma and Texas (Fig. 5).  A mid-level jet streak 
of 80-90 kt was rounding the base of the trough over the southern Plains.  A 
similar arrangement was noted at 700 mb, with a 70 kt wind observation over 
north Texas (Fig. 6).  Water vapor satellite imagery showed evidence of the 
strong mid-level winds moving with the short wave as the area of relatively 
dry air over Texas, Louisiana, and southwest Mississippi (Fig. 7).  At 850 mb, 
a 50 kt low level jet over the area from Louisiana to Georgia advected warm, 
moist air northward from the Gulf of Mexico (Fig. 8).  Morning upper air 
observations over the Southeast showed a capping inversion around 800-850 mb 
and steep mid-level lapse rates from the remnants of an elevated mixed layer.  
At the surface, a broad warm sector covered the entire Southeast and Carolinas 
ahead of a quasi-stationary front stretched across east Texas and Arkansas 
(Fig. 9).  The updated Day 1 Convective Outlook issued at 1300 UTC maintained 
the threat for severe thunderstorms and tornadoes over the western Carolinas
and northeast Georgia.

Click here to view a 25 frame Java loop of GOES-13 water vapor channel 
satellite imagery from 1145 UTC on 27 April to 1145 UTC on 28 April 2011.

Figure 5.  SPC objective analysis of 500 mb geopotential height (dm; dark 
gray contours), temperature (oC; dashed red contours), and wind (kt; barbs) 
at 1200 UTC on 27 April 2011.  Click on image to enlarge.

Figure 6.  SPC objective analysis of 700 mb geopotential height (dm; dark gray 
contours), temperature (0C; dashed red contours), dewpoint (0C greater than or 
equal to -4; green contours), and wind (kt; blue barbs) at 1200 UTC on 27 April 
2011.  Click on image to enlarge.

Figure 7.  GOES-13 water vapor satellite imagery at 1145 UTC on 27 April 2011.  
Brightness temperatures are given by the color scale at the bottom of the figure.
Click on image to enlarge.

Figure 8.  SPC objective analysis of 850 mb geopotential height (dm; dark 
gray contours), temperature (oC; dashed red contours), dewpoint (greater 
than 8 oC; green contours), and wind (kt; barbs) at 1200 UTC on 27 April 2011.  
Click on image to enlarge.

Figure 9.  Hydrometeorological Prediction Center (HPC) surface analysis of 
sea level pressure (mb; black contours) and fronts (traditional symbols) at 
1200 UTC on 27 April 2011.  Click on image to enlarge.

An initial squall line weakened during the middle part of the morning over
eastern Tennessee and eastern Kentucky.  Debris cloudiness from that system 
helped to suppress deep convection across the mountains and foothills of the
Carolinas through the middle part of the day, although a few storms managed
to form on the eastern edge of the cloud boundary over the western Piedmont
of North Carolina.  These storms did not become severe until after leaving
the GSP forecast area.  Meanwhile, partial clearing over northern Alabama, 
northwestern Georgia, and Tennessee allowed for rapid destabilization as 
dewpoints in the upper 60s and lower 70s allowed surface-based parcels to 
break the cap.  The 1630 UTC update to the Day 1 Convective Outlook did not
change the threat for the western Carolinas.  Special upper air soundings 
taken at 1800 UTC at Birmingham, Alabama, and Peachtree City, Georgia (Fig. 10), 
showed moderate to high instability in part due to relatively steep lapse 
rates from about 700 mb to 400 mb and strong shear indicated by long curving 
hodographs.  This led to another refinement of the Day 1 Convective Outlook 
at 1911 UTC.  An initial wave of severe thunderstorms moved northeast up the 
Great Valley of eastern Tennessee through the late afternoon without affecting 
the mountains of North Carolina, perhaps due to a relative lack of buoyancy.  
However, during the early evening, several tornadic supercells moving across 
northern Alabama and southeast Tennessee into an environment with improving 
shear prompted the SPC to issue a "Particularly Dangerous Situation" Tornado 
Watch (#241) for northeast Georgia and most of the western Carolinas.

Click here to view a 14 frame Java loop of GOES-13 visible satellite imagery 
from 1045 UTC to 2333 UTC on 27 April 2011.

Click here to view a 16 frame Java loop of a composite reflectivity mosaic 
from 0900 UTC to 2358 UTC on 27 April 2011.

Figure 10.  Skew-T, log P, diagram and hodograph for the upper air observation 
taken at Peachtree City, Georgia (FFC), at 1800 UTC on 27 April 2011.  The 
temperature sounding is shown by the red line, the dewpoint sounding is 
shown by the green line, and the hypothetical path of a parcel lifted from 
the most unstable layer is shown by the brown dashed line.  A table of 
convective parameters is given at the bottom.  Click on image to enlarge.

By 2200 UTC on 27 April, it was already clear that a remarkable event was 
well underway across the Southeast with numerous tornadic supercells over 
parts of Alabama and Tennessee, although the western Carolinas and northeast 
Georgia had not been affected at that time.  At 0000 UTC on 28 April 2011, a 
short wave trough was analyzed at 500 mb from southern Missouri to the 
Arklatex Region, with a strong mid-level jet streak of at least 90 kt lifting 
out from the trough axis northeastward across the Tennessee Valley (Fig. 11).  
Similar features were noted at 700 mb.  Meanwhile, a southerly low level jet 
of 40-60 kt was observed at 850 mb across Alabama and western Georgia to the 
Tennessee Valley (Fig. 12).  The upper air sounding taken at Peachtree City, 
Georgia (FFC), was representative of the environment immediately ahead of the 
supercells moving northeast from Alabama (Fig. 13).  The sounding was 
moderately buoyant with a convective available potential energy (CAPE) of 
2100 J kg-1 but with a relatively dry boundary layer as compared to locations 
to the west.  The surface dewpoint was only 62 oF which resulted in a lifting 
condensation level (LCL) of greater than 1200 m and limited CAPE below about 
600 mb.  Storm relative helicity (SRH) of greater than 250 m2s-2 in the surface 
to 1 km layer and around 400 m2s-2 in the surface to 3 km layer was highly 
supportive of the development and maintenance of supercell thunderstorms.  The 
surface front moved east as a cold front and stretched from south to north 
across Mississippi to western Tennessee and western Kentucky (Fig. 14).  
Surface observations showed two plumes of moisture: one along the Georgia coast 
and the eastern half of the Carolinas, and the other from the Gulf Coast 
northward across Alabama.  In between the two plumes, a relatively dry patch 
of air was located from central and northeast Georgia to Upstate South Carolina 
and the foothills of North Carolina.  Again, no change was made to the severe 
weather risk for the western Carolinas and northeast Georgia when the Day 1 
Convective Outlook was updated at 0050 UTC on 28 April.

Figure 11.  As in Fig. 5, except for 0000 UTC on 28 April 2011.  Click on 
image to enlarge.

Figure 12.  As in Fig. 8, except for 0000 UTC on 28 April 2011.  Click on image 
to enlarge.

Figure 13.  As in Fig. 10, except for 0000 UTC on 18 April 2011.  Click on
image to enlarge.

Figure 14.  As in Fig. 9, except for 0000 UTC on 28 April 2011.  Click on
image to enlarge.

The objective mesoanalysis from the Storm Prediction Center (SPC) at 0200 UTC 
on 28 April highlighted the effects of the relatively dry air near the surface 
(Fig. 15), which by this time remained across northeast Georgia and the western 
part of the Carolinas.  A plume of higher dewpoint was evident across the 
eastern half of the Carolinas in the southerly flow off the Atlantic, while 
moisture also surged north across Alabama, western Georgia, and southeastern 
Tennessee ahead of the frontal zone.  In between, the dewpoints were only in 
the upper 50s to lower 60s from northeast Tennessee to east central Georgia. 
The surface based CAPE analysis showed a minimum of less than 500 J kg-1 
across this zone (Fig. 16).  The three-hour change in surface based CAPE was 
negative, indicating no increase in instability even though a very active 
surface boundary was approaching from the west.  The SRH in the surface to 
3 km layer was extremely high and greater than 500 m2s-2 across the western 
Carolinas and north Georgia, with modest increases in the previous three 
hours.  The 100 mb mean LCL height showed an area greater than 1250 m covering 
much of northeast and east central Georgia and Upstate South Carolina (Fig. 17).  
The energy-helicity index, supercell composite parameter, and significant 
tornado parameter indices all showed a minimum in the zone from northeast 
Tennessee to east central Georgia.  As a result, the supercellular convection 
across north central Georgia faced a more hostile environment as it moved 
northeast.

Figure 15.  Surface fronts analysis and SPC objective mesoanalysis of surface 
temperature (oF; red contours), surface dewpoint (oF; dashed blue contours and 
color fill > 56), wind (kt; barbs), and mean sea level pressure (mb; black 
contours) at 0200 UTC on 28 April.  Click on image to enlarge.

Figure 16.  SPC objective mesoanalysis at 0200 UTC on 28 April 2011 of 
(a) surface based CAPE (J kg-1; orange contours), surface based convective 
inhibition (CIN, J kg-1; dashed blue contours and color fill), (b) 3 hour 
SBCAPE change (J kg-1; solid red contours positive and dashed blue contours 
negative) and SBCIN (J kg-1; color fill), (c) 0-3 km SRH (m2s-2; blue contours) 
and storm motion (kt; brown barbs), and (d) 3 hour 0-3 km SRH change (m2s-2; 
solid red contours positive and dashed blue contours negative) and current 
storm motion estimate (kt; brown barbs).  Click on image to enlarge.

Figure 17.  SPC objective mesoanalysis at 0200 UTC on 28 April 2011 of 
(a) 100 mb mean parcel LCL (m AGL, contours), (b) 3 km energy-helicity index 
(solid contours greater than 1, dashed contours less than 1), (c) supercell 
composite parameter (solid contours greater than 1, negative contours less 
than 1) and Bunkers storm motion (kt; barbs), and (d) significant tornado 
parameter (solid contours greater than 1, negative contours less than 1) 
and mixed layer CIN (J kg-1; color fill).  Click on image to enlarge.

References

Changnon, S. A., Jr., and R. G. Semonin, 1966:  A great tornado disaster in retrospect.
     Weatherwise, v. 19, n. 2, pp. 56-65.

Fujita, T., 1975: Super Outbreak of tornadoes of April 3–4, 1974.  Map printed by 
     University of Chicago Press. [Available from Wind Science and Engineering 
     Research Center, Texas Tech University, 10th and Akron, Lubbock, TX 79409.]

Acknowledgements

The author wishes to express his thanks to Brian Tang of the National 
Center for Atmospheric Research for sharing his montage of the Tuscaloosa - 
Birmingham supercell.  The storm surveys for the Caldwell County and 
Alexander County tornadoes were conducted by Tony Sturey and Larry Gabric.  
The storm survey for the Lake Burton - Mountain City tornado was conducted 
by Neil Dixon and Terry Benthall.  The surveys of the Goshen Mountain and
Scaly Mountain tornadoes were conducted by Tony Sturey and Terry Benthall.
Steven Nelson (National Weather Service Peachtree City, Georgia) provided
information regarding the track of the tornado across Lumpkin County and
White County, Georgia.  The tornado track maps were created using Google
Earth Pro 2010.