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Local forecast by
"City, St"

The July 8th, 2001, Mesoscale Convective

System in the Western Carolinas

Bryan P. McAvoy
NOAA/National Weather Service
Greer, SC

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

1.  Introduction
On the evening of 8 July 2001, a classic MCS (Mesoscale 
Convective System) affected the North Carolina Mountains 
and a small part of the South Carolina Upstate with damaging 
winds of 60 to 80 mph.  Locally, the winds may have been 
higher in some mountain valleys which served to funnel the 
strong winds.  The MCS entered the northern mountains of 
North Carolina shortly before 4:45 p.m. (Fig. 1), and moved
between 50 and 60 mph for the next two hours.  In other words, 
in about one hour and 45 minutes, the MCS went from the 
northernmost tip of Avery County, to the North Carolina - 
South Carolina border.
KGSP 0.5 degree base reflectivity scan at 2049 UTC 8 July 2001
Figure 1.  KGSP radar base reflectivity on the 0.5 degree 
scan from 2049 UTC (4:49 p.m.) to 2255 UTC (6:55 p.m.).  
Click on image to view loop.
The MCS started in the late morning over western Indiana.  The 
system tracked east southeast in excess of 60 mph. During this 
time it was a classic derecho system producing winds of 70 to 
80 mph over a path length of about 300 miles.  The Storm 
Prediction Center (SPC) storm reports from 8 July show where 
the MCS tracked (Fig. 2).  However, there was actually a second 
system that formed later in the day, and took nearly the same 
track.  In fact, some wind damage was reported after midnight 
in the North Carolina Mountains as this weaker MCS moved in.  
For the purposes of this review, however, we will focus on the 
first MCS.
Click here for a list of severe weather reports for this event.
Severe thunderstorm and tornado reports for 8 July 2001
Figure 2.  Hail, damaging wind, and tornado reports received 
by the Storm Prediction Center for the 24 hour period ending 
1200 UTC 9 July 2001.
2.  Synoptic Features and Pre-Storm Environment
The reason for the convective system's intensity and rapid 
movement can be inferred from the 1200 UTC upper air sounding 
from Wilmington, Ohio (Fig. 3).  This was a classic derecho 
sounding with very high Convective Available Potential Energy
(CAPE), and strong, unidirectional wind flow.  Modified for a 
temperature of 89 degrees Fahrenheit and a dewpoint of 74 deg F
(rather conservative numbers based on some of the reports ahead 
of the MCS), the CAPE was 4700 J/kg, and wind speeds changed 
40 knots from the surface to 3 km, which was very strong shear.
Wilmingon Ohio, modified sounding 1200 UTC 8 July 2001
Figure 3.  Skew-T, log P diagram of upper air sounding observed
at Wilmington, Ohio (ILN), at 1200 UTC 8 July 2001.  A table of 
selected convective parameters is shown below.  Click on image 
to enlarge.
Interestingly, the numerical models were completely unaware of 
this system's existence as it formed in association with no 
minor upper-air troughs.  Nor was it in an area of strong warm 
air advection.  The dynamics were not as good by early evening
once the MCS begin to move into an area of more northerly 
shear, as it rounded the periphery of a strong, warm ridge 
which was centered over the southern Plains.  At this time, 
the MCS began to move on a more southerly course, approaching 
the Greenville - Spartanburg (GSP) County Warning Area (CWA).  
The upper air sounding from Blacksburg, Virginia (RNK), is the 
closest, non-convectively contaminated sounding available 
(Fig. 4).  In fact, it probably represents very well the air 
mass that the MCS was in as it entered the GSP CWA in the 
evening of 8 July.  At 2345 UTC, there was still a pronounced 
700 mb wind maximum in the vicinity of the MCS.  However, 
shear decreased in the mid-levels of the atmosphere.  Despite 
the weakening shear aloft, the system maintained a strong 
cold pool and new convective development occurred at the 
leading edge of the outflow boundary all the way into 
South Carolina.  By the time the MCS made it into South 
Carolina, it had weakened considerably.  Available CAPE of 
2000 J/kg and slightly lower dewpoints, coupled with weakening 
shear as the system was getting south of the mid level ridge 
axis, lead its gradual dissipation.
Blacksburg, Virginia,  unmodified sounding at 0000 UTC 9 July 2001
Figure 4.  Skew-T, log P diagram of upper air sounding observed
at Blacksburg, Virginia (RNK), at 0000 UTC 9 July 2001.  A table 
of selected convective parameters is shown below.  Click on image 
to enlarge.
3.  Radar observations
A gap can be seen in the SPC damage reports just upstream of 
the GSP CWA as the strong derecho in southern Kentucky lost 
almost all of its associated leading edge cells a little 
after 2000 UTC (4 pm).  However, new cells began to form over 
northeast Tennessee a little ahead of the outflow boundary 
associated with the convection over southern Kentucky.  These 
cells were not very strong with a Vertically Integrated Liquid 
only up to around 45 (not shown).  There was one strong cell 
which formed ahead of the line in Watauga County, North 
Carolina but it formed east of where the MCS tracked.  In 
fact, a leading area of convective cells is often thought of 
as a location where an MCS will frequently bow out.  This MCS 
tracked well west of these leading line cells.  However, the 
new cells did exhibit some mid altitude radial convergence
(MARC) as seen from the KGSP radar (Fig. 5).  There was about 
50 knots of convergence in the mid-levels of the leading edge 
of the line when it was still north of Mitchell County, North 
Carolina.  It is a little difficult to see in Figure 5, but 
the bright red on the left most panel is about 35 knots of 
outbound velocity, and the darker green is 15 knots of inbound 
velocities.  Studies have shown that a MARC signature of this 
strength is sometimes associated with damaging winds.
Mid-Altitude Radial Convergence signature
Figure 5.  KGSP storm relative motion (left) and composite 
reflectivity (right) at 2059 UTC 8 July 2001.  Values are 
given by color tables on the right side of each image.  
Click on image to enlarge.
4.  Discussion 
The active ham radio network in northeast Tennessee was 
probably the primary reason that the National Weather Service
at GSP issued the warnings with substantial lead time.  
Otherwise, forecasters may have been tempted to wait and see 
if the line produced any damage as it was not clear if the 
rear inflow jet would translate to the ground in the mountains. 
The local storm reports from our office show just how much 
wind did translate down to the ground.  Interestingly, once 
the MCS exited the mountains, damage reports all but stopped. 
A plot of wind speeds aloft as the line crossed WFO GSP shows 
that the system still had severe criteria winds a few thousand 
feet off the ground, but they were not making it to the 
surface. In fact, the 35 knot gust at GSP matched very well 
what the VAD wind profile showed at the surface (Fig. 6).
GSP VAD wind profile
Figure 6.  KGSP Velocity Azimuth Display wind profile ending 
2345 UTC 8 July 2001.  Click on image to enlarge.
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