National Weather Service United States Department of Commerce

Tornadoes and Flooding Associated With the

Remnants of Tropical Storm Fay

Justin Lane, Christopher Horne, and Patrick D. Moore
NOAA/National Weather Service
Greer, SC

Clemson tornado, 26 August 2008.  Image taken by Rob Harrison, SC Dept. of Natural Resources

This tornado moved through the south side of the Clemson University campus on 26 August 2008. It was one of three confirmed tornadoes that affected the Upstate of South Carolina. The photo was taken from the south side of Lake Hartwell by Rob Harrison, South Carolina Department of Natural Resources.

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

1. Introduction

Tropical Storm Fay and its remnants affected the southeastern United States for over one week in late August of 2008. After making landfall over southwest Florida early in the morning of 19 August (Fig. 1), the storm dumped significant rainfall as it moved across the Florida Peninsula. The storm moved off the east coast of Florida and then slowly northward for about 36 hours beginning on 20 August. A surface high pressure ridge strengthened to the north of the storm on 22 August which steered Fay westward for about three days, producing extreme rainfall amounts over parts of north Florida and south Georgia. Fay weakened to a Tropical Depression on 24 August over the western Florida Panhandle and south Alabama and then stalled over southern Mississippi on 25 August. Eventually, the remnants of Fay moved northeast and merged with a frontal boundary over eastern Tennessee, but not before it spawned tornadoes across parts of Alabama and parts of Georgia.

Click here to view an 81 frame Java loop of HPC surface fronts and pressure analysis from 0000 UTC on 18 August to 0000 UTC on 28 August.

Best track positions for Fay, 19-28 August 2008

Figure 1. Best track positions for Fay, 15-28 August 2008, as determined by the National Hurricane Center. Click on image to enlarge.

The passage of the remnants of Fay brought much-needed rain to an area suffering from the effects of a significant drought. Unfortunately, in spite of the drought, the heavy rain caused significant flooding across parts of the western Carolinas and northeast Georgia, especially the southern Piedmont of North Carolina and the Charlotte metropolitan area. The remnants of Fay also provided a favorable environment for the development of numerous thunderstorms, including several supercells, on the afternoon of 26 August. These supercells spawned a series of tornadoes across portions of Upstate South Carolina and northeast Georgia. Two tornadoes affected the Clemson, South Carolina area, with one tornado causing damage on the main campus of Clemson University. A third tornado touched down to the east of U.S. Highway 25 in the southwest part of Greenville County. Several reports of wind damage were received from the western part of Upstate South Carolina as well (Fig. 2). The remnants of Fay also produced significant rain across the Midlands of South Carolina and tornadoes across central North Carolina.

(Click here to view a list of event total rainfall reports from across the region)

Severe thunderstorm and tornado reports for 26 August 2008

Figure 2. Large hail, damaging wind, and tornado reports compiled by the Storm Prediction Center for the 24 hour period ending 1200 UTC 27 August 2008. Click on image to enlarge.

The events of 25-27 August 2008 were well anticipated by the National Weather Service (NWS) Weather Forecast Office (WFO) at the Greenville - Spartanburg Airport (GSP) in Greer, South Carolina. The forecast staff at WFO GSP are well versed in the effects of tropical rainfall across the southern Appalachians were able to recognize the threat when tropical cyclone remnants encroached upon the area. As early as 17 August, as Tropical Storm Fay was still off the southern coast of Cuba, the pre-dawn issuance of the Hazardous Weather Outlook (HWO) highlighted the threat of "heavy rains and flooding" across GSP's county warning and forecast area (CWFA). Although unanticipated slowing of the movement of Fay over Florida during the following days would eventually delay the onset of excessive rainfall across the CWFA from initial expectations, confidence increased that Fay's remnants would affect the CWFA. Again, the pre-dawn issuance of the HWO on 21 August stated "a chance for a significant rain event...next Tuesday and Wednesday." This outlook information would turn out to be very perceptive.

A detailed HWO was issued during the evening of 24 August which stated "total rainfall amounts could reach 8 inches" over western North and South Carolina and extreme northeast Georgia. The most interesting part of the outlook information was a reference to the ongoing "drought and record low stream levels." The long-standing practice of never issuing a Flash Flood Watch during a drought would be tested over the coming days, although it often has been said that droughts end with a flood.

The threat of severe thunderstorms and tornadoes loomed larger through the day on 25 August as it appeared that some instability might accompany the strong wind shear as the remnant circulation of Fay passed by to the west on 26 August. The Storm Prediction Center placed most of the western Carolinas in a Slight Risk of severe thunderstorms in the initial Day 1 Severe Weather Outlook for 26 August. The SPC expanded the Slight Risk farther east across the Charlotte metro area on the morning update issued at 1254 UTC on 26 August.

Note: All times in this report are referenced to Universal Time Coordinated (UTC), which is Eastern Daylight Time plus four hours.

2. Synoptic Overview and Antecedent Conditions

Prior to the arrival of the remnants of Fay, most of the western Carolinas and northeast Georgia struggled with long term drought. The U.S. Drought Monitor issued on 19 August (Fig. 3) highlighted most of the region as "Exceptional," the highest of the five drought categories. Many streams ran at record low levels.

U.S. Drought Monitor for Southeast Region issued 19 August 2008

Figure 3. U.S. Drought Monitor for the Southeast Region, issued 19 August. Intensity of drought is given by the color scale on the left. Click on image to enlarge.

Even with the center of then downgraded Tropical Depression (TD) Fay over southern Mississippi during the night of 24-25 August, the air mass over the southern Appalachians was assuming tropical characteristics. As evidenced by the 25 August 1200 UTC upper air sounding at Peachtree City, Georgia (FFC), precipitable water (PWAT) had increased to 1.87 inches, which is 160% of normal (Fig. 4). The mean flow from the surface to the mid-layers of the atmosphere had a deep south southeast orientation. Aided by terrain lift, widespread heavy showers developed around 0800 UTC on 25 August across the mountains of northeast Georgia, southwest North Carolina and western Upstate South Carolina (Fig. 5). With the favorable flow orientation and increasingly tropical airmass, a swath of 3-5 inches of rain fell during the pre-dawn hours, prompting the issuance of the first Flash Flood Warnings for the event. Due to underestimation, the radar estimated rainfall (Fig. 6) should be multiplied by 1.5 to achieve representative totals.

Upper air sounding from Peachtree City, GA, at 1200 UTC 25 August 2008

Figure 4. Skew T - log P diagram (upper left) and hodograph (upper right) of upper air sounding taken at FFC at 1200 UTC on 25 August 2008. A table of convective parameters and indices is shown at the bottom. Click on image to enlarge.

Regional mosaic of radar reflectivity at 0758 UTC 25 August 2008

Figure 5. Regional radar reflectivity mosaic at 0758 UTC 25 August 2008. The intensity of precipitation is given by the color scale at the bottom. Click on image to enlarge.

KGSP storm total precipitation at 1358 UTC 25 August 2008

Figure 6. KGSP Storm Total Precipitation estimate ending at 1358 UTC on 25 August. Rainfall estimate is given by the color table at the lower right. Click on image to enlarge.

3. Preliminary Flooding

Several creeks were reported out of their banks in Habersham County, Georgia, on the morning of Monday, 25 August. To the WFO forecasters on duty that morning, this was an ominous sign that even with record low streamflow conditions, the high rainfall rate from tropical precipitation would result in flooding. Given the expectation that the CWFA would be under the threat of additional rounds of tropical rainfall for the next 48 hours, a Flood Watch was issued later that morning for northeast Georgia, the northwest Upstate, and the central and southern North Carolina Mountains through Wednesday morning.

Numerous if not widespread showers continued to develop across the balance of the CWFA throughout the day and into the evening hours on 25 August. A localized rainfall maximum (Fig. 7) along the urbanized Interstate 85 corridor, from northeast Charlotte to Concord, prompted more Flash Flood Warnings and caused limited flooding. More importantly, heavy rain moistened antecedent conditions and raised stream and river level flows, setting the stage for more serious flooding across this area over the next 36 hours.

KGSP storm total precipitation at 0400 UTC 26 August 2008

Figure 7. As in Fig. 6, except ending at 0400 UTC on 26 August. Click on image to enlarge.

Meanwhile, the center of TD Fay assumed a northeastward motion, bringing her into west central Alabama during the night of 25-26 August. As evidenced by the upper air sounding taken at FFC at 0000 UTC on 26 August (Fig. 8), the airmass across the CWFA became increasingly favorable to support heavy tropical showers. The sounding was saturated up to near 400 mb, the warm air advection flow had strengthened, and PWATs had risen to 2.11 inches, or about 170 percent of normal. Even though rainfall was relatively light north of Interstate 40 during 25 August, the perceptive midnight shift forecast team expanded the Flood Watch northward to encompass the North Carolina Foothills and northern North Carolina Mountains.

Upper air sounding from Peachtree City, GA, at 0000 UTC 26 August 2008

Figure 8. As in Fig. 4, except at 0000 UTC on 25 August 2008. Click on image to enlarge.

After a period of relatively limited, but isolated heavy shower activity during the pre-dawn hours of 26 August, widespread heavy showers increased rapidly across the Charlotte metro area shortly after daybreak (Fig. 9). This prompted another round of Flash Flood Warnings for Mecklenburg and Cabarrus counties in North Carolina and York County in South Carolina, which were issued at 1318 UTC and then extended until 1730 UTC. During this same timeframe, widespread heavy showers developed northward across the Mountains and Foothills, as shown by the increase in the coverage and magnitude of total rainfall across the western CWFA and from York, South Carolina, to Salisbury, North Carolina (Fig. 10).

Click here to view a 19 frame Java loop of regional radar mosaic imagery from 2356 UTC on 25 August to 1756 UTC on 26 August.

Regional mosaic of radar reflectivity at 1159 UTC 26 August 2008

Figure 9. As in Fig. 5, except for 1159 UTC 26 August 2008. Click on image to enlarge.

KGSP storm total precipitation at 1558 UTC 26 August 2008

Figure 10. As in Fig. 6, except ending at 1558 UTC on 26 August. Click on image to enlarge.

4. Synoptic Characteristics on 26 August

The surface analysis at 1200 UTC on 26 August (Fig. 11) indicated the center of TD Fay located over north central Alabama. A quasi-stationary boundary extended from the cyclone across middle Tennessee into extreme southern Kentucky and Virginia. The upper air sounding from FFC at 1200 UTC on 26 August (Fig. 12) revealed an atmospheric profile typical of the northeast quadrant of a remnant tropical cyclone (McCaul 1991). The sounding possessed high moisture content (precipitable water of 5.76 cm), with lapse rates tending toward moist adiabatic through a deep layer. However, due to the very warm and moist boundary layer, there was potential instability in the sounding, with Convective Available Potential Energy (CAPE) calculated at 675 J kg-1. A strongly veering wind profile was noted, with a deep layer of 40 to 50 kt winds in the lowest 6 km. This resulted in strong shear parameters, with Storm Relative Helicity (SRH) of 411 m2 s-2 and 365 m2 s-2 in the 0-3 km and 0-1 km layers, respectively.

HPC surface analysis at 1200 UTC 26 August

Figure 11. National surface analysis of fronts and pressure at 1200 UTC on 26 August 2008. Click on image to enlarge.

Upper air sounding taken at FFC at 1200 UTC on 26 August

Figure 12. Observed upper air sounding from FFC at 1200 UTC on 26 August 2008. Click on image to enlarge.

Although widespread cloud cover north of the frontal boundary inhibited surface heating for much of the day, the visible satellite image in Fig. 13 indicated some clearing had occurred south of the boundary over the South Carolina Midlands by early afternoon. A regional surface analysis at 1800 UTC (Fig. 14) on 26 August revealed the evolution of the lower atmosphere during the late morning and early afternoon. As the strong high pressure system over the northern part of the United States (Fig. 11) continued to move to the southeast, the frontal boundary pushed south across the western Carolinas. The location of the boundary can be inferred from Fig. 14 by the shift from southerly to east and southeast winds. There was also a change in air mass across the front, with lower-to- middle 80s air temperatures south of the boundary, and lower- to-middle 70s temperatures to the north. This temperature gradient was enhanced by the clearing skies in the warm sector.

Click here to view a 43 frame Java loop of GOES-12 water vapor satellite imagery from 1145 UTC to 2345 UTC on 26 August 2008.

Click here to view a 12 frame Java loop of GOES-12 visible satellite imagery from 2345 UTC on 24 August to 2245 UTC on 26 August 2008.

GOES-12 visible image at 1745 UTC 26 August 2008

Figure 13. GOES-12 visible satellite image from 1745 UTC on 26 August 2008.

Regional surface observations at 1800 UTC on 26 August

Figure 14. Regional surface observations plot at 1800 UTC on 26 August 2008. The plots follow the traditional station model. Click on image to enlarge.

It is well established that baroclinic boundaries can play a key role in enhancing the potential for tornadogenesis in severe weather environments, especially on the "cool" side of boundaries (Markowski et al. 1998). This occurs due to baroclinic generation of horizontal vorticity that results from gradients in buoyancy. If there is positive surface-based CAPE on the "cool" side of the boundary, convective updrafts can reorient this vorticity vertically, resulting in cyclonically rotating updrafts and greater potential for tornadogenesis. Moreover, it is thought that weak east-west oriented boundaries may have played an important role in previous tornado events associated with the remnants of tropical cyclones across the Carolinas and northeast Georgia (Schneider and Sharp 2007; Lane 2005; McCaul et al. 2004).

5. Convective evolution

A sequence of regional radar images (Fig. 15) showed the effect that destabilization had on the character of precipitation. While the morning hours mainly saw widespread stratiform rain, convective activity, including discrete thunderstorms, became more common by mid-afternoon in the vicinity of the front. Of particular interest was the band of showers and embedded thunderstorms moving across northeast Georgia at 1800 UTC on 26 August.

Radar reflectivity mosaic at 1459 UTC on 26 August Radar reflectivity mosaic at 1756 UTC on 26 August

Figure 15. Regional radar mosaic from 1459 UTC (left) and 1756 UTC (right) on 26 August 2008. Click on images to enlarge.

a. The Reed Creek - Clemson Tornado

Figure 16a is a reflectivity image at 0.5 degree elevation from the Greer, South Carolina (KGSP) Weather Service Radar (WSR-88D) at 1803 UTC. Although the cell appeared rather innocuous, there was a cyclonic curvature in the reflectivity pattern on the southern flank of the cell. The storm relative velocity (SRV) images at 1803 UTC in Fig. 16b-e were more revealing. The images show a shallow, but significant circulation associated with the convective updraft near Hartwell, Georgia. The rotational shear associated with this vortex was calculated at 17.1 x 10-3 s-1. This value of shear was within the "Tornado Possible" range of the rotational shear nomogram (Falk and Parker 1998). Contrast this with the rotational velocity (Vr) of the vortex, calculated at 25.3 knots at a distance of 47 km from KGSP. According to the mesocyclone detection nomogram (Andra 1997) this value suggests only a minimal mesocyclone. It is important for forecasters to remember that the mesocyclone detection nomogram is based upon studies of classic supercells. Therefore, it has little, if any utility in assisting with warning decisions in environments that are unsupportive of classic supercells (i.e., weakly buoyant and highly sheared). This is because the diameter of the circulation is critical in assessing the strength of the vortex. Weakly unstable environments will result in small updrafts, and therefore compact vortices. A rotational velocity over a distance of 1 km is more significant than the same value of rotational velocity over a distance of 5 km.

KGSP base reflectivity on 0.5 degree scan at 1803 UTC on 26 August KGSP Storm Relative Motion 4-panel at 1803 UTC 26 August

Figure 16. Base reflectivity at the 0.5 degree scan (a) and storm relative velocity at the 0.5 degree (b), 1.5 degree (c), 2.4 degree (d), and 3.4 degree (e) elevation angles from KGSP at 1803 UTC on 26 August 2008. Click on images to enlarge.

According to a study by Schneider and Sharp (2007), most of the Carolinas tornadoes that occurred in association with tropical cyclones during the active 2004 season occurred in association with vortices possessing WSR-88D derived rotational velocities of 20 kt or more over an average distance of less than 3 km. Based largely upon this study, a tornado warning was issued for portions of Hart, Anderson, and Oconee counties at 1801 UTC.

An EF1 tornado touched down near the community of Reed Creek, Georgia, at around 1818 UTC (Fig. 17). Once again, the reflectivity from the KGSP radar at 1820 UTC was rather non-descript (Fig. 18). However, the series of SRV images from this time showed the circulation evident in Fig. 16 had intensified significantly and deepened slightly. Rotational velocity at 0.5 degrees was 38.4 kt. Meanwhile, rotational shear was 58.0 x 10-3 s-1, an increase of 70% from 1803 UTC. This is well within the "Tornado Likely" category of the rotational shear nomogram.

Track of Reed Creek/Clemson tornado on 26 August 2009

Figure 17. Map indicating the track of the Reed Creek - Clemson tornado. Movement of the tornado was from the lower part of the image to the upper part (southwest to northeast). Click on image to enlarge.

KGSP base reflectivity on 0.5 degree scan at 1820 UTC on 26 August KGSP Storm Relative Motion 4-panel at 1820 UTC 26 August

Figure 18. As in Fig. 16, except for 1820 UTC. Click on images to enlarge.

The Reed Creek tornado proceeded to cross Lake Hartwell into South Carolina, skipping along an intermittent path across western Anderson County and extreme southeast Oconee County before affecting the Clemson area. The 0.5 degree SRV image at 1850 UTC (Fig. 19b) indicated a gate-to-gate rotational shear couplet calculated at 65.3 x 10-3 s-1. This value is "off the chart" according to the rotational shear nomogram.

KGSP base reflectivity on 0.5 degree scan at 1850 UTC on 26 August KGSP Storm Relative Motion 4-panel at 1850 UTC 26 August

Figure 19. As in Fig. 16, except for 1850 UTC. Click on images to enlarge.

A time series of the rotational velocity and rotational shear values associated with the convective cell that produced the Reed Creek/Clemson tornado showed that both parameters were highly variable during this two-hour window (Fig. 20). However, rotational shear during the lifetime of the tornado was generally contained within the range of 15 x 10-3 s-1 and 30 x 10-3 s-1. The two "spikes" in shear values coincided with the most significant damage near Reed Creek and Clemson.

Time series of rotational shear and rotational velocity for Reed Creek storm

Figure 20. Time series of rotational shear (red line) and rotational velocity (blue line) from the KGSP radar for the Reed Creek/Clemson storm. Times of significant damage are indicated as are times of Tornado Warning issuances (TOR 042 and TOR 043). Click on images to enlarge.

b.  The Pendleton Tornado

 

Another tornadic cell developed across northwest Anderson County, South Carolina, shortly after 1930 UTC. At 1920 UTC, the rotational shear observed from KGSP was quite weak (Fig. 21). However, the reflectivity image revealed a rather pronounced cyclonic curvature on the southern flank of the storm. A Tornado Warning was issued at 1927 UTC for northwest Anderson and southern Pickens counties, based mainly on the Vr of 25.5 kt and the cyclonic curvature in the reflectivity field. By 1937 UTC (Fig. 22), shear increased to 16.1 x 10-3 s-1. Shear and Vr increased dramatically after 1937 UTC, peaking at 75.3 x 10-3 s-1 at 1941 UTC. This rapid increase coincided with the development of a tornado between Pendleton and Townville, which tracked into extreme southwest Pickens County before dissipating (Fig. 23).

KGSP base reflectivity on 0.5 degree scan at 1920 UTC on 26 August KGSP Storm Relative Motion 4-panel at 1920 UTC 26 August

Figure 21. As in Fig. 16, except for 1920 UTC. Click on images to enlarge.

KGSP base reflectivity on 0.5 degree scan at 1937 UTC on 26 August KGSP Storm Relative Motion 4-panel at 1937 UTC 26 August

Figure 22. As in Fig. 16, except for 1937 UTC. Click on images to enlarge.

Track of the Pendleton tornado on 26 August 2009

Figure 23. Map indicating the track of the Pendleton tornado. Movement of the tornado was from the lower part of the image to the upper part (south-southeast to north-northwest). Click on image to enlarge.

The time series of rotational velocity and rotational shear for the Pendleton tornadic storm (Fig. 24) illustrated the rapidity at which tornadogenesis occurred. In just one volume scan, the strength of the vortex as sampled by KGSP increased from the lower bound of the "tornado possible" category (10.4 x 10-3 s-1) at 1933 UTC to "tornado likely" (70.0 x 10-3 s-1) at 1937 UTC. This was an intensification of 70% in four minutes. This illustrated the importance to warning forecasters of thoroughly evaluating each radar volume scan and taking quick, decisive action in making warning decisions during tropical cyclone events.

Time series of rotational shear and rotational velocity for Pendleton storm

Figure 24. As in Fig. 20, except for the Pendleton tornadic storm. Click on image to enlarge.

c.  The Piedmont Tornado

 

The third tornadic storm on 26 August 2008 developed over southern Greenville County shortly after 2000 UTC. The radar reflectivity and SRV images from KGSP at 1954 UTC (approximately 10 minutes prior to tornado occurrence) revealed weak rotational shear (6.8 x 10-3 s-1) and a cyclonic curvature in the reflectivity field (Fig. 25). However, Vr at this time was 25.0 kt, within the range suggested by Schneider and Sharp (2007) as a tornado warning threshold. Images from KGSP at 2003 UTC (Fig. 26) revealed a rapid intensification of this circulation from 1954 UTC. An EF1 tornado touched down just east of the Pelzer community at 2005 UTC (Fig. 27). Rotational shear increased to 24.5 x 10-3 s-1, which was an increase of over 70% from the previous volume scan.

KGSP base reflectivity on 0.5 degree scan at 1954 UTC on 26 August KGSP Storm Relative Motion 4-panel at 1954 UTC 26 August

Figure 25. As in Fig. 16, except for 1954 UTC. Click on images to enlarge.

KGSP base reflectivity on 0.5 degree scan at 2003 UTC on 26 August KGSP Storm Relative Motion 4-panel at 2003 UTC 26 August

Figure 26. As in Fig. 16, except for 2003 UTC. Click on images to enlarge.

Track of the Piedmont tornado on 26 August 2009

Figure 27. Map indicating the track of the Piedmont tornado. Movement of the tornado was from the lower part of the image to the upper part (south to north). Click on image to enlarge.

Time series of rotational shear and rotational velocity for Piedmont storm

Figure 28. As in Fig. 20, except for the Piedmont tornadic storm. Click on image to enlarge.

Due to the unexpected, rapid increase in the strength of the vortex, a warning was issued with negative lead time for southern Greenville County (at 2007 UTC). Using the Vr guidelines suggested by Schneider and Sharp (2007) would have provided much greater opportunity for a warning with positive lead time than rotational shear analysis, as suggested by Figure 28. While Vr was at or above 25 kt (with vortex diameter of less than 3 km) for 32 minutes prior to tornado occurrence, rotational shear did not "spike" to the "Tornado Likely" category until two minutes before the tornado.

6. Heavy Rain and Flooding

Rain totals within the region continued to increase throughout the afternoon (Fig. 29). It was at this juncture that accumulated rainfall and excessive runoff contributed to several streams and creeks approaching and exceeding flood stage across York, Mecklenburg and Cabarrus counties. For example, Figs. 30 and 31 show these effects on river levels at two locations, Mallard Creek in northeast Mecklenburg County, which is a tributary of the Rocky River, and the Rocky River gage in southern Cabarrus county, just above Irish Buffalo Creek. Both of these gages exceeded flood stage during the afternoon of 26 August, and river levels continued to rise into the 27th, cresting at record levels.

KGSP storm total precipitation at 2203 UTC 26 August 2008

Figure 29. As in Fig. 6, except ending at 2203 UTC on 26 August. Click on image to enlarge.

Mallard Creek stream gage height 24-31 August 2008

Figure 30. Time series of stream gage height for Mallard Creek below Stony Creek near Harrisburg, North Carolina, for the period 24-31 August. Flood stage is shown by the yellow line. Click on image to enlarge.

Rocky River stream gage height 24-31 August 2008

Figure 31. Time series of stream gage height for Rocky River above Irish Buffalo Creek near Rocky River, North Carolina, for the period 24-31 August. Flood stage is shown by the yellow line. Click on image to enlarge.

Excessive rainfall also increasingly became a concern across the western CWFA during the afternoon of 26 August as Flash Flood Warnings were issued for the North Carolina counties of McDowell, Rutherford, Mitchell and Yancey, where various roads were closed due to flooding and mudslides into the evening hours. With the expanding bullseye of rainfall across northeast Georgia into the southern escarpment of the North Carolina Mountains, additional Flash Flood Warnings were issued for the Georgia counties of Rabun and Habersham, the North Carolina counties of Macon, Jackson, Haywood and Transylvania, and Oconee County, South Carolina, into the evening.

During the evening hours of 26 August, as the center of Fay degenerated into a remnant low over north Alabama and moved into southeast Tennessee, less favorable air was beginning to encroach from northeast Georgia. This was exhibited on the upper air sounding at FFC at 0000 UTC on 27 August (Fig. 32), as notable drying aloft occurred, and the mean lower and mid level flow veered to west-southwest.

Upper air sounding from Peachtree City, GA, at 0000 UTC 27 August 2008

Figure 32. As in Fig. 4, except at 0000 UTC on 27 August 2008. Click on image to enlarge.

Rainfall trends throughout the evening featured diminishing activity across the central and southern North Carolina Mountains, and then across northeast Georgia and western Upstate South Carolina (Fig. 33). The combination of lingering rainfall and antecedent conditions prompted additional Flash Flood Warnings for the northern North Carolina Mountains and Foothills during the latter half of the evening, with flooding occurring on both sides of the Blue Ridge (Figs. 34 and 35).

KGSP storm total precipitation at 0359 UTC 27 August 2008

Figure 33. Storm Total Preciptitation estimate from KGSP for the 48 hour period ending 0359 UTC 27 August 2008. Click on image to enlarge.

South Toe River near Celo stream gage height 24-31 August 2008

Figure 34. Time series of stream gage height for the South Toe River near Celo, North Carolina, for the period 24-31 August. Flood stage is shown by the yellow line. Click on image to enlarge.

Johns River at Arney's Store stream gage height 24-31 August 2008

Figure 35. Time series of stream gage height for the Johns River at Arneys Store, North Carolina, for the period 24-31 August. Flood stage is shown by the yellow line. Click on image to enlarge.

Widespread and locally heavy rain gradually diminished from the southwest across the Foothills and Piedmont during the overnight hours, but not before several more Flash Flood Warnings were issued for much of the North Carolina Piedmont through early Wednesday morning.

Click here to view a 19 frame Java loop of regional radar mosaic imagery from 1856 UTC on 26 August to 1156 UTC on 27 August.

Event total rainfall was expectedly impressive, and the magnitude and orientation of the amounts were well forecast across the western CWFA, although a minimum was noted in the Smokies and lower French Broad Valley. Radar rainfall totals (Fig. 36) underestimated reality by a rough factor of 1.5 to 1.75 in comparison to the observed raingage values (Fig. 37). The underestimation of the secondary maximum across the Interstate 85 corridor from Rock Hill, South Carolina, to Charlotte to Salisbury, North Carolina, was notable, especially given that there was record flooding in spots. No main stem river flooding occurred, although the French Broad River at Rosman, North Carolina, did just reach flood stage, and the downstream gage at Blantyre crested just below flood stage, probably due to the low flows at the onset of the event. The main Piedmont precipitation axis was aligned downstream of the forecast point at Lowell, North Carolina, and there was no main stem flooding of the Catawba River downstream either.

KGSP storm total precipitation at 1200 UTC 27 August 2008

Figure 36. Storm Total Preciptitation estimate from KGSP for entire event ending at 1200 UTC 27 August 2008. Click on image to enlarge.

Event total rainfall analysis, Remnants of Tropical Storm Fay, 25-27 August 2008

Figure 37. Total observed rainfall for the period 25-27 August 2008. Click on image to enlarge.

7. Summary and Conclusions

Several tornadoes occurred on 26 August 2008 across Upstate South Carolina and northeast Georgia in association with the remnants of Tropical Storm Fay. Examination of environmental data indicated the synoptic pattern was very similar to previous tropical cyclone- associated tornado events across the region. Low-level helicity was very high, while CAPE was adequate for strong convection. In addition, a weak baroclinic boundary was analyzed across the region, with surface-based potential instability noted even on the cool side of the boundary. The three tornadoes mentioned in this study occurred within the cool air 30 to 40 km north of the boundary.

The tornadoes that occurred during the afternoon of 26 August 2008 were associated with vortices characterized by rotational shear values of 25 x 10-3 s-1 or greater, all within the "tornado probable" or "tornado likely" portion of the rotational shear nomogram. Values of rotational velocity were generally between 25 and 40 kt. This is consistent with the findings of Schneider and Sharp (2007) for tropical cyclone tornadoes. However, it should be noted that there were a number of rotating updrafts that produced Vr values of greater than 25 kt that were apparently not associated with tornadoes. Rotational shear seemed to be a better discriminator between tornadic and non-tornadic vortices on 26 August 2008. However, from a warning decision-making perspective, sole reliance on the rotational shear nomogram over the warning guidelines suggested by Schneider and Sharp (2007) would have resulted in short, and in some cases, negative lead time. It is recommended that forecasters use both of these tools in conjunction with reflectivity analysis in making tornado warning decisions in tropical cyclone environments.

The most significant flash flooding occurred across the heavily urbanized area around Charlotte, North Carolina, late on 26 August and early on 27 August. Some smaller streams exceeded flood stage, most notably Mallard Creek and the Rocky River tributary of Irish Buffalo Creek. However, none of the main stem rivers across the Foothills and Piedmont of the Carolinas reached flood stage. The same was true for the mountains, with the exception of the headwaters of the French Broad River. Most likely, this was due to very dry antecedent conditions with most streams and rivers at all-time record low flows prior to the arrival of the rain associated with Fay. Although heavy rain was widespread across most of the region, not enough fell to mitigate the ongoing drought across the western Carolinas. Most of Upstate South Carolina remained in the exceptional category into early September after Fay's rains, but the North Carolina Piedmont saw some relief.

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     22, 278-286.
Acknowledgements

The authors wish to thank Mr. Rob Harrison of the South Carolina Department of Natural Resources for providing the image of the Clemson Tornado near Lake Hartwell. The track plot of Tropical Cyclone Fay was obtained from the National Hurricane Center. Surface analysis graphics were obtained from the Hydrometeorological Prediction Center. Satellite imagery, radar mosaics and surface observation plots were obtained from the University Corporation for Atmospheric Research. Time series graphics for rotational shear and velocity were created using Microsoft Excel. Radar images used in the tornado study were created using the GR2Analyst software. The upper air sounding image in figure ii was created using the RaOB (Radiosonde Observation Program) version 5.8 for Windows. Tornado damage path graphics were prepared using Delorme Street Atlas USA 2006 Plus. Storm Total Precipitation estimate graphics were created using the Java NEXRAD viewer obtained from the National Climatic Data Center. Stream gage plots were obtained from the United States Geologic Survey. Blair Holloway created the rainfall map.