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Flash Flooding Along the Blue Ridge

26 June 2006

Patrick D. Moore
NOAA/National Weather Service
Greer, SC

Twelvemile Creek flooded the Pickens Jockey Lot on Monday, 26 June 2006
The Pickens Jockey Lot was under five feet of water when Twelvemile
Creek came out of its banks after heavy rain in the morning of Monday, 
26 June 2006.  Image provided by the Pickens Sentinel and used by 
permission.

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

1.  Introduction
Heavy rain during the early morning of Monday, 26 June 2006, resulted 
in flash flooding across parts of Pickens County and northern Greenville 
County in South Carolina, and in parts of Henderson, Polk, and Rutherford 
counties in North Carolina.  The most significant rainfall occurred
roughly in a band from the Lake Keowee area, northeast along the Blue 
Ridge, to Lake Lure, the Hickory Nut Gorge, and the upper reaches of the 
Rocky Broad River basin (Fig. 1).  The most significant flooding was 
observed in the basin of Twelvemile Creek in Pickens County, South 
Carolina, and in the basin of the Rocky Broad River in the southern 
Foothills of North Carolina.  During the course of the event, the National 
Weather Service office at Greer, South Carolina (GSP), issued 14 Flash 
Flood Warnings, along with numerous flood statements.
(Click here to view a summary of significant weather and flood 
reports for 26 June 2006.
(Click here to view a summary of rainfall reports for 25-27 June 2006.
48 hour rainfall accumulation for 25-26 June 2006
Figure 1.  Storm total rainfall for the period 25-27 June 2006.  
Click on image to enlarge.  
This event is notable in that the stream gage along Twelvemile Creek 
near Liberty in Pickens County, South Carolina, reached its highest 
level ever recorded (14.78 feet).  Farther north, a debris flow occurred 
in steep terrain inside Jones Gap State Park, which closed a popular 
hiking trail for at least six months afterward.  Additionally, the Rocky 
Broad River tied its record stage at Bat Cave, in extreme northeast 
Henderson County, North Carolina (11.0 feet), although the Rocky Broad 
River is known to have reached higher levels prior to the installation 
of the river gage, such as 4 September 1996 (Johnstone and Burrus 1998).   
2.  Synoptic Features and Pre-Storm Environment
Water vapor satellite imagery on the morning of 25 June (Fig. 2, left) 
showed a well-defined mid-level circulation over the upper Mississippi 
Valley and another off the east coast of Florida.  Both circulations
are readily apparent in the 700 mb analysis at 0000 UTC on 26 June 
(Fig. 3).  Over the next 24 hours, convergence at the edges of both 
circulations resulted in a deep plume of tropical moisture extending 
from the west coast of the Florida Peninsula, northward across the 
southern Appalachians, to the upper Ohio Valley (Fig. 2, right).  
Click here to view a 27 frame java loop of GOES-12 water vapor satellite 
imagery.
GOES-12 Water Vapor imagery 1145 UTC 25 June 2006GOES-12 Water Vapor imagery 1145 UTC 26 June 2006
Figure 2.  GOES-12 Water Vapor imagery at 1145 UTC 25 June (left) and 
1145 UTC 26 June (right).  Click on images to enlarge.
700 mb analysis at 0000 UTC 26 June 2006
Figure 3.  Storm Prediction Center (SPC) 700 mb objective analysis of 
geopotential height, temperature, and dew point at 0000 UTC on 26 June.  
Note the area of very high dew point stretching from the coast of Georgia 
and South Carolina to the northern edge of the Bahamas.  Click on image 
to enlarge.  
By 1200 UTC on 26 June, the environment aloft was favorable for deep 
convection across the western Carolinas.  The 500 mb analysis (Fig. 4) 
showed a short wave over north Georgia lifting northeast out of the base 
of the upper trough located over the Mississippi Valley.  On the 250 mb 
analysis (Fig. 5), the western Carolinas were under the right entrance 
region of a jet streak over the Ohio Valley and Great Lakes.  The 
approaching short wave and jet entrance region were both conducive to 
upward vertical motion on the morning of 26 June.
500 mb analysis at 1200 UTC 26 June 2006
Figure 4.  SPC 500 mb objective analysis of geopotential height, 
temperature, and wind at 1200 UTC on 26 June.  Click on image to enlarge.  
250 mb analysis at 1200 UTC 26 June 2006
Figure 5.  SPC 250 mb objective analysis of isotachs and streamlines 
at 1200 UTC on 26 June.  Click on image to enlarge.  
The upper air sounding taken at Peachtree City, Georgia (FFC), at 
1200 UTC showed a relatively modest amount of Convective Available 
Potential Energy (~ 1000 J/kg) for early Summer owing to a nearly moist 
adiabatic temperature profile (Fig. 6).  However, the precipitable 
water was very high (1.97 inch, which is approximately 150% of normal).
Upper air sounding at FFC at 1200 UTC 26 June 2006
Figure 6.  Skew-T log P diagram (upper left) and hodograph (upper right) 
for upper air sounding at FFC at 1200 UTC 26 June. The tables at the 
bottom summarize several objective parameters used by the SPC to determine 
severe weather potential.  Click on image to enlarge.
At the surface, a quasi-stationary frontal boundary (Fig. 7) provided the 
necessary low level convergence to focus the development of showers and 
thunderstorms over the western Carolinas during the morning of the 26 June.
The air mass to the east of the front was rich with low level moisture as
evidenced by dewpoints in the lower 70s.  The low level southeast wind
continued to supply the developing storms with Atlantic moisture.  The
transport of moisture can be inferred from the visible satellite imagery
by the bands of low level clouds oriented from southeast to northwest
across the Carolinas (Fig. 8)
Click here to view an 11 frame java loop of HPC Surface Fronts and 
Pressure Analyses from 1200 UTC 25 June to 1800 UTC 26 June.
Surface analysis at 1200 UTC 26 June 2006
Figure 7.  Hydrometeorological Prediction Center (HPC) surface pressure 
and fronts analysis at 1200 UTC 26 June.  Click on image to enlarge.
Visible satellite imagery at 1345 UTC 26 June
Figure 8.  Visible satellite image from GOES-12 at 1345 UTC 26 June.
3.  Radar observations
Between the early afternoon of 25 June and the early morning hours of 
26 June, at least three distinct waves of showers moved across the area 
of the Blue Ridge Escarpment from Lake Keowee to Lake Lure.  This early 
rainfall, especially between 0900 UTC and 1200 UTC on 26 June, served to 
saturate the soil across the area from Oconee County, South Carolina, to
Rutherford County, North Carolina.  New convective development over the 
western part of Upstate South Carolina around 1200 UTC (Fig. 9), moving 
northward toward the Blue Ridge, suggested an increased threat of runoff.
Click here to view a 21 frame java loop of the 0.5 degree reflectivity 
mosaic centered on the KGSP radar.
Radar reflectivity mosaic at 1200 UTC 26 June
Figure 9.  Radar reflectivity mosaic centered on KGSP radar at 1200 UTC 
26 June.  Click on image to enlarge.
The band of showers and thunderstorms that developed between Interstate 85 
and the Blue Ridge between 0800 and 0840 UTC was ultimately responsible 
for producing the flooding (Fig. 10).  This band of showers developed at
least partly in response to low level southeast upslope flow and low level 
convergence on the moist side of the stationary front seen in Figure 7. 
Click here to view a 46 frame java loop of the 1.3 degree reflectivity 
from the KGSP radar from 0840 UTC to 1220 UTC.
KGSP 1.3 deg reflectivity at 0840 UTC 26 June
Figure 10.  Radar reflectivity on 1.3 degree scan from the KGSP WSR88-D 
at 0840 UTC 26 June.  The radar is located just to the right of the 'e' 
in Greenville.  The point labelled 'Liberty' is the site of the stream 
gage on Twelvemile Creek.  The point labelled 'Bear' is the location of
the North Carolina ECOnet station on Bearwallow Mountain.  Jones Gap 
State Park and Bat Cave, North Carolina, are also shown for reference.
Click on image to enlarge.
By 1225 UTC, the convergence in the south-southeast upslope flow had 
organized the deep convection into distinct north-south oriented bands, 
running up the Blue Ridge Escarpment (Fig. 11).  The heaviest rain was 
occurring in the band that stretched from Bat Cave, North Carolina, 
southward into Greenville County, South Carolina, and in the band that 
stretched from near Jones Gap State Park southward across eastern Pickens 
County.  By this time, the initial band of precipitation seen in Figure 10
had moved over the North Carolina mountains and formed a large area of 
stratiform rain across Jackson, Haywood, and Buncombe counties.  Flash
flooding was imminent at both Jones Gap State Park and along the Rocky
Broad River at Bat Cave.
Click here to view a 59 frame java loop of the 0.8 degree reflectivity 
from the KGSP radar from 1225 UTC to 1631 UTC.
KGSP 0.8 deg reflectivity at 1225 UTC 26 June
Figure 11.  As in Figure 10, except for 0.8 degree scan at 1225 UTC.  
Click on image to enlarge.
The heavy rain persisted across the Blue Ridge near the border of North
Carolina and South Carolina through about 1600 UTC.  The primary rain 
bands finally moved north and east of Lake Lure, and shifted east of 
Pickens County, after 1630 UTC (Fig. 12).
Click here to view the entire 8 hour, 105 frame java loop of the 
0.8/1.3 degree reflectivity from the KGSP radar from 0840 UTC to 1631 UTC.
KGSP 0.8 deg reflectivity at 1631 UTC 26 June
Figure 12.  As in Figure 11, except for 1631 UTC.  Click on image to 
enlarge.
4.  Hydrologic Observations 
Flooding developed first across eastern Henderson County from Saluda to
the upper part of the Rocky Broad River above Bat Cave between 1200 UTC 
and 1330 UTC.  The heaviest rain fell across the Blue Ridge from Caesar's 
Head up to Bat Cave, where most locations measured in excess of 9 inches 
for the event.  Rain fell at the highest rate at the North Carolina ECOnet 
site on Bearwallow Mountain between 1200 UTC and 1300 UTC, when over one 
inch was recorded (Fig. 13).  It is likely that rain fell at an even 
greater rate closer to Bat Cave, as the band of highest reflectivity
was located just east of Bearwallow Mountain during this time frame (see
Figure 11 and the reflectivity loop from 0840 UTC to 1220 UTC).  In fact,
the automated recording station at Bat Cave received 1.46 inches of rain
in the 30 minute period from 1130 UTC to 1200 UTC (Fig. 14).  This period
of intense rainfall quickly sent the Rocky Broad River out of its banks
at Bat Cave.  The river gage showed a very rapid rise from less than 
4 feet at 1000 UTC to nearly 9.5 feet by 1400 UTC (Fig. 15).  Flood stage 
was exceeded probably around 1215 UTC, with a crest of 11 feet around 
1500 UTC.  The river fell below flood stage around 2000 UTC.
Rainfall at Bearwallow Mountain ECOnet site, 24-27 June
Figure 13.  Accumulated rainfall at the North Carolina ECOnet site at 
Bearwallow Mountain in northeast Henderson County.  The time axis is 
local time.  Click on image to enlarge.
Rainfall at Bat Cave gage, 0000 UTC 24 June - 0000 UTC 27 June
Figure 14.  Accumulated rainfall at the Army Corps of Engineers site at 
Bat Cave in extreme northeast Henderson County.  The time axis is in UTC.  
Click on image to enlarge.
River stage on the Rocky Broad River at Bat Cave gage, 0000 UTC 26 June - 0000 UTC 28 June
Figure 15.  River stage on Rocky Broad River at Bat Cave, North Carolina.  
The red line indicates the flood stage at 7.5 feet.  The purple line 
indicates the level above which major flooding can be expected, which 
is 9.5 feet.  The time axis is in UTC.  Click on image to enlarge.
Heavy rain fell across most of Pickens County in two main batches.  The 
first period of heavy rain fell from about 0900 UTC to 1100 UTC.  The 
second, more significant period of heavy rain fell from about 1300 UTC 
to 1600 UTC, as seen by the radar imagery in the second reflectivity 
loop.  The level of Twelvemile Creek came up at the rate of 3 feet per 
hour after 1400 UTC, eventually pushing it out of its banks near Pickens 
between 1430 UTC and 1500 UTC.  Although flooding was reported in low 
lying areas, the creek did not rise above flood stage at the gage site
near Liberty (13.5 feet) until much later, around 0200 UTC on 27 June 
(Fig. 16), as runoff moved down from the upper reaches of the basin 
north and east of Pickens.  A crest of 14.78 feet was reached around 
0500 UTC and the creek dropped below flood stage around dawn on June 27,
although flooding persisted at the Pickens Jockey Lot until late in the 
morning.
River stage on Twelvemile Creek near Liberty, 0000 UTC 26 June - 0000 UTC 29 June
Figure 16.  River stage on Twelvemile Creek near Liberty, South Carolina.  
Flood stage is 13.5 feet.  The time axis is in UTC.  Click on image to 
enlarge.
The axis of heaviest rainfall extended across the northern part of 
Greenville County, South Carolina, including Jones Gap State Park.  
Rainfall of greater than 9 inches was sufficient to cause a debris flow 
within Jones Gap State Park att some time prior to 1400 UTC, when the 
aftermath was discovered.  Numerous trees, boulders, and other debris
were carried several hundred yards down a steep slope, closing a main 
hiking trail (Fig. 17).
Aftermath of debris flow in Jones Gap State Park, 26 June 2006
Figure 17.  The aftermath of a debris flow in Jones Gap State Park 
during the morning of Monday, 26 June.  The large boulder in the 
middle of the image is several feet in diameter.
5.  Discussion 
Studies of previous devastating flash flood events in the area of the 
Hickory Nut Gorge area of western North Carolina have focused on a 
substantial low level easterly moist upslope flow relative to the 
unique terrain features of the gorge as a significant contributor to the 
production of heavy rain (Lee and Goodge 1984, Johnstone and Burrus 1998).  
In contrast, the atmosphere on the morning of 26 June 2006 had a deep 
unidirectional southerly flow, as seen in the upper air soundings at 
Peachtree City, Georgia (Fig. 7), and Greensboro, North Carolina.  
However, when viewed in hindsight from the perspective of an ingredients-
based methodology for forecasting flash floods (Doswell et al. 1996), 
the events of 26 June 2006 could have been anticipated just as easily.
Convective precipitation in the late evening hours of 25 June and the
early morning hours of 26 June provided the antecedent conditions for 
increased runoff.  The atmosphere on the morning of 26 June possessed 
copious amounts of moisture with little dry air aloft and enough available 
potential energy to support strong convective updrafts, which suggested 
that high precipitation rates were possible.  Several low level mechanisms, 
in the presence of a favorable synoptic environment for upward vertical 
motion, forced the development of numerous showers and thunderstorms.
A large stratiform rain region formed over the North Carolina mountains 
by 1300 UTC as the initial wave of deep convection moved up the eastern 
slopes of the Appalachians and over the quasi-stationary front (Fig. 18).  
It is hypothesized that the resulting rain-cooled air mass across the 
North Carolina mountains effectively relocated the surface front to a 
position very near the Blue Ridge by 1300 UTC.  Surface observations at 
1300 UTC (Fig. 19) show a light north wind at Asheville (AVL) and a 
south-southeast wind at reporting stations across upstate South Carolina 
(GSP, AND) and the southern Foothills of North Carolina (FQD).  
KGSP 0.8 deg reflectivity at 1254 UTC 26 June
Figure 18.  As in Figure 11, except at 1254 UTC.  The deep convection 
is shown by the higher reflectivity values extending in an arc across 
Oconee, Pickens, and northern Greenville counties to the area around
Bat Cave.  The stratiform rain region is denoted by the relatively low 
values of reflectivity across Jackson, Haywood, and Buncombe counties.  
Click on image to enlarge.
Regional surface plot at 1343 UTC 26 June
Figure 19.  Regional surface plot at 1343 UTC 26 June.  In spite of the
time stamp, most observations are valid at 1255 UTC. Observations are 
plotted according to the standard station model.  Click on image to 
enlarge.  
The south-southeasterly flow of unstable and very moist low level air 
up against the newly-relocated surface boundary, nearly coincident with 
the sharp terrain rise of the Blue Ridge, enhanced precipitation 
production.  New cells developing in the convergent low level flow over 
upstate South Carolina moved in a direction nearly parallel to the new 
boundary, which significantly increased the duration of heavy rain along 
the boundary.
6.  Summary 
The environment across the western Carolinas on the morning of 26 June 
had enough moisture, instability, and forcing for deep convection to 
develop.  Flash flooding across the basins of Twelvemile Creek and the 
Rocky Broad River resulted when the necessary ingredients were present 
in an area with favorable hydrologic conditions.  The repeated movement 
of convective cells with a high rainfall rate directed nearly parallel 
to a low level boundary resulted in heavy rain of sufficient duration 
to produce flooding.

Twelvemile Creek floods the Pickens Jockey LotTwelvemile Creek floods SC Hwy 183 near the Pickens Jockey Lot

Town Creek floods Jaycee Park in Pickens, SCTown Creek floods gas station in Pickens

More images from the flooding along Twelvemile Creek and Town Creek
near Pickens, South Carolina, on 26 June 2006.  Images courtesy of the 
Pickens Sentinel.  Used by permission.

Aftermath of debris flow in Jones Gap State ParkAftermath of debris flow in Jones Gap State Park

Aftermath of debris flow in Jones Gap State ParkAftermath of debris flow in Jones Gap State Park

Aftermath of debris flow in Jones Gap State ParkAftermath of debris flow in Jones Gap State Park

More images from the debris flow in Jones Gap State Park on 26 June 2006.  
The large boulders in the uppermost images are at least five or six feet 
in diameter.  The debris flow left a long and deep scar on the slope and
deposited a large amount of mud and tree trunks below.
Acknowledgements
Sandy Foster, editor of the Pickens Sentinel, provided the images of 
the flooding along Twelve Mile Creek and Town Creek near Pickens, South 
Carolina.  Greg Schoor (NWS) prepared the map of the storm total rainfall 
for the event.  The satellite imagery, radar mosaic imagery, and the
surface observations plot were obtained from the University Corporation 
for Atmospheric Research.  Upper air analyses and sounding plots were 
obtained from the Storm Prediction Center.  The surface analyses were 
obtained from the Hydrometeorological Prediction Center.  The local radar 
graphics were prepared with the Java NEXRAD Viewer, obtained from the 
National Climatic Data Center.  The rainfall data for Bearwallow Mountain 
was obtained from the State Climate Office for North Carolina.
References
Doswell, C. A., III, H. E. Brooks, and R. A. Maddox, 1996:  Flash flood 
     forecasting:  An ingredients-based methodology.  Wea. Forecasting., 
     11, 560-581.

Johnstone, T. P., and S. A. Burrus, 1998:  An analysis of the 4 September 
     1996 Hickory Nut Gorge Flash Flood in western North Carolina.  Preprints, 
     16th Conf. on Weather Analysis and Forecasting, Phoenix, AZ, Amer. Meteor. 
     Soc., 275-277.
	
Lee, L. G., and G. W. Goodge, 1984:  Meteorological analysis of an intense 
     "east-slope" rainstorm in the southern Appalachians.  Preprints, 10th 
     Conf. on Weather Analysis and Forecasting, Clearwater Beach, FL, Amer. 
     Meteor. Soc., 30-37.


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