The July 8th, 2001, Mesoscale Convective
System in the Western Carolinas
Bryan P. McAvoy
NOAA/National Weather Service
Author's Note: The following report has not been subjected to the scientific peer review process.
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.
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
Click here for a list of severe weather reports for this event.
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.
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
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.
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
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.
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.
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).
Figure 6. KGSP Velocity Azimuth Display wind profile ending
2345 UTC 8 July 2001. Click on image to enlarge.
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