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The Northern Gaston County Tornado
of 26 May 2006
Patrick D. Moore
Several trees were blown down by the tornado that moved across northern Gaston County on 26 May 2006. The tornado also damaged the roof of the structure seen in the distance.
Author's Note: The following report has not been subjected to the scientific peer review process.
The active springtime continued across the Carolinas through late May with another severe weather outbreak on Friday, 26 May 2006 (Fig. 1). The National Weather Service (NWS) Weather Forecast Office (WFO) in Greer, South Carolina (GSP), issued 25 Severe Thunderstorm Warnings, one Tornado Warning, and one Flash Flood Warning on that day. In particular, a series of strong to severe multicell thunderstorms moved across the southern Piedmont of North Carolina during the late afternoon and early evening. The fourth and last in the series of severe storms produced a tornado across the extreme northwestern part of Gaston County in a rural area between Cherryville and High Shoals. The tornado touched down about four miles east northeast of Cherryville, near the intersection of Hepzibeth Road and Saint Marks Church Road, at approximately 614 pm EDT (2214 UTC). [All times in this document are referred to in Universal Time Coordinated (UTC), which is Eastern Daylight Time plus four hours.] The tornado produced intermittent damage along a path 2.2 miles long and 40 yards wide that ended near the intersection of Landers Chapel Road and Gaston-Webb Chapel Road. A damage survey rated the tornado at F1 intensity on the Fujita Scale. A Tornado Warning was issued by WFO GSP at 2209 UTC for northern Gaston County, providing a lead time of 5 minutes.
(Click here to view a summary of severe weather reports for 26 May 2006.
Figure 1. Wind damage, large hail, and tornado reports for 26 May 2006. Click on image to enlarge.
The events of 26 May are interesting because the initial threat for tornadoes was perceived to be low. Environmental clues that morning suggested the most likely threat to be damaging straight line winds. After the first multicell thunderstorm formed in the middle part of the afternoon, preferential development of new cells to the west of the initial cell acted to strengthen the boundary between the environmental air and the rain-cooled air at low levels, as the new cells moved to the east. The boundary further acted to strengthen the low level vorticity ingested by successive cells, culminating in the development of a low-level mesocyclone and the production of the weak tornado over extreme northwestern Gaston County.
2. Synoptic Features and Pre-Storm Environment
A severe weather episode triggered in part by the passage of a strong short wave was a recurring theme during the Spring of 2006 and the events of 26 May were no exception. The upper air analysis from the Storm Prediction Center (SPC) at 1200 UTC on that day showed the axis of a short wave trough at 500 mb over the Cumberland Plateau and Tennessee Valley (Fig. 2). A belt of stronger winds at that level, shown by the wind barbs in the figure, was expected to provide favorable wind shear as it translated across the Carolinas later in the day with the passage of the upper trough. The surface analysis from the Hydrometeorological Prediction Center (HPC) at 1200 UTC showed a cold front pushing east across the Ohio Valley and a lee trough extending down across the Piedmont of North Carolina (Fig. 3). Forecasters at GSP expected the passage of the short wave, combined with the lee trough and outflow from a decaying mesoscale convective system over northeast Tennessee, to trigger a round of severe thunderstorms in the afternoon given sufficient instability, which was highlighted in the early morning Area Forecast Discussion and Severe Weather Outlook.
Figure 2. SPC objective analysis of 500 mb geopotential height, temperature, and wind at 1200 UTC 26 May. Click on image to enlarge.
Figure 3. HPC surface pressure and fronts analysis at 1200 UTC 26 May. The lee trough over North Carolina is shown by a dashed orange line. Click on image to enlarge.
In spite of its closer proximity to the southern Piedmont, the upper air sounding at Greensboro, North Carolina (GSO), at 1200 UTC was not particularly supportive of severe weather, probably due to contamination from the remnants of earlier storms that moved out of northeast Tennessee and across the Piedmont Triad during the early morning hours. Instead, the sounding at Peachtree City, Georgia (FFC), was more indicative of the expected environment across the western Carolinas (Fig. 4). The modified sounding yielded a surface-based convective available potential energy (CAPE, a measure of the potential energy of rising air parcels related to buoyancy) of greater than 2000 J/kg (a moderate amount), while shear of 21 kt in the surface to 6 km layer, though not strong, was expected to be sufficient to provide for some organization of storms. The favorable environment prompted the Storm Prediction Center (SPC) forecast a Slight Risk of severe thunderstorms for all of the Carolinas on the Convective Outlook issued at 1300 UTC. At the time, the primary threat for severe weather was expected to be damaging wind and large hail.
Figure 4. Skew-T log P diagram (upper left) and hodograph (upper right) for upper air sounding at FFC at 1200 UTC 26 May. The tables at the bottom summarize several objective parameters used by the SPC to determine severe weather potential. Click on image to enlarge.
Visible imagery from the GOES-12 satellite at 1245 UTC suggested the presence of a boundary moving east across the Piedmont, manifested as the sharp edge between the clouds over the eastern part of the Piedmont and the clear sky over the Midlands of South Carolina and Sandhills of North Carolina (Fig. 5). The passage of the boundary across the western Piedmont earlier in the morning, followed by debris clouds from overnight convection, effectively delayed the destabilization of the air mass across the western Carolinas during the late morning. A Mesoscale Discussion issued by the SPC at 1542 UTC highlighted the cloudiness over the mountains as a mitigating factor in convective development through the middle part of the day, but mentioned the possibility of rotation in the stronger updrafts. In spite of the slow start, the potential for severe thunderstorms still existed later in the day over the Carolinas, as discussed in the updated Convective Outlook issued by the SPC at 1630 UTC.
Figure 5. Visible satellite image from GOES-12 at 1245 UTC 26 May.
The debris clouds moved east of the Foothills by 1745 UTC which allowed for the boundary layer to warm quickly. The 1800 UTC surface analysis continued to show the lee trough over the Piedmont, while a closer look at surface observations showed a pool of slightly higher dewpoints in the mid-60s from Anderson and Greenwood northeast to Charlotte (Fig. 6). By 1800 UTC, the last of the convective inhibition (CIN, a measure of resistance to convection at low levels) had been eliminated by boundary layer heating, which raised the surface-based CAPE above 1500 J/kg east of the mountains (Fig. 7). Most signs continued to support the conclusion that damaging wind gusts were the primary threat. Water vapor imagery from GOES-12 at 1745 UTC showed a surge of mid-level dry air across the Tennessee Valley and southern Applachians (Fig. 8). An intermediate upper air sounding taken at FFC at 1800 UTC showed the mid-level drying along with moderate instability and a deep sub-cloud layer (the layer from the surface up to the level of free convection at 1647 m above ground level). The SPC analysis of downdraft CAPE showed a large area of greater than 1000 J/kg across the Carolinas (Fig. 9). Downdraft CAPE over 1000 J/kg is considered significant to the production of damaging wind gusts. The perceived tornado threat was comparatively low owing to weak values of storm relative helicity (SRH, in both the surface - 1 km and surface - 3 km layers) and high lifting condensation level.
Figure 6. Surface observations plot at 1743 UTC 26 May. The traditional station model is used. Click on image to enlarge.
Figure 7. SPC objective analysis of surface based CAPE and CIN for 1800 UTC 26 May. Click on image to enlarge.
Figure 8. Water vapor image from GOES-12 at 1745 UTC 26 May. Click on image to view a 5 frame Java loop of water vapor imagery.
Figure 9. SPC objective analysis of DCAPE at 1800 UTC 26 May. Click on image to enlarge.
Deep convection initiated just northeast of Asheville at 1745 UTC, perhaps due to enhanced instability on the southern edge of the Black Mountains. The initial shower moved off the Blue Ridge after 1800 UTC, developing into a thunderstorm as it moved over southern McDowell County. The first Severe Thunderstorm Warning was issued for northeast Rutherford County and northern Cleveland County at 1906 UTC as the thunderstorm gained strength. The SPC acknowledged the threat for more severe thunderstorms along the lee trough in the updated Day 1 Convective Outlook issued at 1943 UTC. More strong thunderstorms developing over the western part of Upstate South Carolina led the SPC to surmise correctly that an outbreak was underway, and a Severe Thunderstorm Watch was issued at 1955 UTC for nearly all of western North Carolina, western South Carolina, and extreme northeast Georgia.
3. Radar observations of the Southern Piedmont Storms
The initial severe thunderstorm developed into a multicell cluster of storms (Cell 1) as it moved east across the northern part of Cleveland County through 1930 UTC (Fig. 10), producing large hail near Lawndale. Additional warnings were issued for southern Lincoln County at 1944 UTC and northern Gaston County at 1957 UTC as a new multicell cluster of thunderstorms (Cell 2) developed to the west (upshear) of the first cluster (Fig. 11). Wind damage and one-half inch diameter hail were reported in Cherryville (northwest Gaston County) at 2000 UTC with the passage of the first two multicell clusters. By that time, a third multicell (Cell 3) developed near Lawndale in northern Cleveland County on the western (upshear) flank of the original complex of multicells (Fig. 12). Movement and propagation of the multicell clusters continued along the Lincoln - Gaston county line to the area near the southern end of Lake Norman, prompting the issuance of a Severe Thunderstorm Warning for northern Mecklenburg County at 2010 UTC.
Figure 10. Radar reflectivity on 0.5 degree scan from the KGSP WSR88-D at 1930 UTC. The radar is located off the bottom left corner of the image. The TCLT radar location is shown in northern Mecklenburg County. Multicell clusters are annotated. Click on image to enlarge.
Figure 11. As in Figure 10, except for 1945 UTC. Click on image to enlarge.
Figure 12. As in Figure 10, except for 2000 UTC. Click on image to enlarge.
Multicell 3 followed the same track as the previous multicell clusters, eventually producing large hail across the area from Dellview in extreme northwestern Gaston County to Crouse in far southwestern Lincoln County between 2035 UTC and 2045 UTC. Around that time, a fourth and final multicell (Cell 4) developed once again over northern Cleveland County (Fig. 13), resulting in another Severe Thunderstorm Warning for southern Lincoln County at 2048 UTC. Multicell 4 quickly grew to severe intensity as it moved east through 2113 UTC (Fig. 14), prompting another Severe Thunderstorm Warning for northern Gaston County at 2117 UTC.
Figure 13. As in Fig. 10, except for 2049 UTC. Click on image to enlarge.
Figure 14. As in Fig. 10, except for 2113 UTC. Click on image to enlarge.
This final multicell complex was persistent with frequent new updraft development on the upshear flank. The repeated movement of cells prompted the issuance of a Flash Flood Warning for northern Gaston County at 2140 UTC. Additionally, the Severe Thunderstorm Warning for Lincoln County was reissued for a third time at 2145 UTC. This cell quickly acquired severe characteristics as it moved north of Cherryville around 2202 UTC (Fig. 15).
Figure 15. Radar reflectivity at 0.5 degree scan from the KGSP radar at 2202 UTC. Click on image to enlarge.
Rotation within the main updraft of multicell 4 developed quickly at 2202 UTC, resulting in both a radar-defined and operator-defined mesocyclone (not shown). The lowest level base velocity image from the KGSP radar showed evidence of strong rotation on the 2207 UTC scan (Fig. 16), indicated by the red-green couplet just east of Cherryville.
Figure 16. Doppler velocity at 0.5 degree scan from the KGSP radar at 2207 UTC. The red colors show motion of the target away from the radar, while the green colors show motion of the target toward the radar. Click on image to enlarge.
The rapid spin-up of the mesocyclone and the low level rotation signature, combined with corroborating evidence from the Terminal Doppler Weather Radar located north of the Charlotte - Douglas Airport, led to the issuance of a Tornado Warning for northern Gaston County at 2209 UTC. Subsequent radar images showed low level rotation weakening as quickly as it had developed as the storm moved across northern Gaston County.
The initial convective cell appeared to have strengthened in a zone of low level convergence to the lee of the mountains, perhaps as a result of confluence in the westerly flow downwind of the mountain barrier. The convergence zone was manifested by an east-to-west line of enhanced cumulus seen on the visible satellite imagery at 2000 UTC, stretching from northern Cleveland County, across northern Rutherford County, to northern Buncombe County, in the wake of the initial multicell (Fig. 17). The surface observations at that time clearly show the convergence in the flow between the stronger southwest winds at Rutherfordton (KFQD) and Shelby (KEHO), and the weaker west-southwest winds at Hickory (KHKY).
Figure 17. Visible satellite imagery from GOES-12 at 2000 UTC. Surface observations are indicated with the traditional station model. Note the satellite presentation of the expanding cloud anvil associated with multicell cluster (cell 1) in vicinity of KIPJ. Also note the line of enhanced cumulus extending westward from cell 3 back to the north of KAVL. Click on image to enlarge.
New cells appeared to develop preferentially over northern Cleveland County on the upshear (in this case, upwind) side of the multicell storms over Lincoln and Gaston counties. The speculation is that interaction between the westward-directed cold pool from the mature cells over Lincoln and Gaston counties (Fig. 18), the westerly wind shear observed in the upper air sounding at GSO, and the low level convergent zone in the previous figure resulted in a region of favored updraft growth over northern Cleveland County. Cumulus growing in the convergence zone and moving east over northern Rutherford County developed rapidly when they encountered this region over northern Cleveland County.
Figure 18. Composite reflectivity from KGSP radar at 2058 UTC. Surface observations are indicated by the traditional station model. The dashed grey line indicates the outflow boundary from the multicell clusters over the southern Piedmont. Note the low level convergence over northern Rutherford County as suggested by surface observations. Click on image to enlarge.
In reality, the shear was stronger than expected from the morning upper air soundings. The SPC objective analysis of 0-6 km shear at 2200 UTC showed moderate amounts of shear (greater than 30 kt), which resulted in environmental storm relative helicity in the 0-1 km layer of greater than 150 m2/s2. The storm relative helicity of parcels lifted over the cold pool in the vicinity of new cell development over northern Cleveland County was likely much greater. The moderate environmental shear combined with the greater potential for generating vorticity north of the cold pool boundary in northwestern Gaston County may have allowed multicell 4 to develop enough rotation to support the development of the tornado.
A tornado of F1 intensity touched down in extreme northwern Gaston County, North Carolina, to the east of Cherryville, at 2214 UTC on 26 May 2006. The tornado produced an intermittent damage path approximately 2.9 miles long and 40 yards wide, beginning near the intersection of Hepzibeth Road and St. Marks Church Road (Fig. 19). Winds were estimated at 75 mph. The damage included a carport that had been blown into the top of a tree and twisted pieces of a barn roof 0.6 miles away from the property. A Tornado Warning was issued by the National Weather Service at 2209 UTC (609 pm EDT) for northern Gaston County, providing a lead time of 5 minutes.
Figure 19. Approximate path of damage from the tornado that moved across northern Gaston County on 26 May 2006. The purple colored line is the damage path. Click on image to enlarge.
The rapid growth of new cells in the presence of moderate shear, maximizing low level storm relative helicity as they moved to the north of an outflow boundary, may have contributed significantly to the production of the tornado.
The graphics for the storm reports (Fig. 1), 500 mb analyses (Fig. 2), upper air soundings (Fig. 4), and mesoscale analysis graphics (Figs. 7 and 9) were obtained from the Storm Prediction Center. The surface fronts analysis (Fig. 3) was obtained from the Hydrometeorological Prediction Center. The surface observations plot (Fig. 6) and all satellite imagery were obtained from the University Corporation for Atmospheric Research. Radar images were created using the Java NEXRAD Viewer and Data Exporter, obtained from the National Climatic Data Center. The background map in Figure 19 was obtained from the U.S. Geological Survey.