The 6 May 1999
NOAA/National Weather Service
A tornado damaged trees and vehicles in Asheville, North Carolina, on 6 May 1999.
Author's Note: The following report has not been subjected to the scientific peer review process.
A rare event occurred on the morning of 6 May 1999 as a weak
tornado cut a narrow path through the southern part of
Asheville, North Carolina. Mountain communities are not as
susceptible to tornadoes as those in the Piedmont and Coastal
Plain because the rugged topography tends to disrupt the low
level circulation needed to form a tornado. However, this
event serves as a reminder that no location is totally safe
from a tornado.
Click here to view a list of local storm reports from this event.
Severe thunderstorm warnings had been issued earlier for the
counties to the west of Buncombe County. No real-time reports
of damage were received, however, and as the line of low-topped
storms entered western Buncombe County, the warnings were allowed
to expire. This underscores the importance of communicating
damage reports as quickly as possible to the National Weather
Service. If you are not a spotter or a ham radio operator your
best course of action is to call your local sheriffs department
and have them relay the information to us.
2. Radar observations
The storms were undergoing some subtle changes as they entered
Buncombe County. The 0953 UTC (5:53 a.m.) reflectivity image
(what the radar "sees" as returned power from raindrops, hail,
grasshoppers, etc.), shows an S-shaped break in the line
(Fig. 1). There is some evidence that such a break in a line
segment might be a sign of a developing weak tornado, though
more research needs to be done. The Storm Relative Velocity
data from this time show very weak circulation in the line
(Fig. 2); weaker, in fact, than what the line had exhibited
for much of the previous hour.
Figure 1. KGSP radar reflectivity on lowest four elevation
scans at 0953 UTC 6 May 1999. The color scale shows the
intensity of precipitation. Click on image to enlarge.
Figure 2. KGSP storm relative motion on lowest two elevation
scans at 0953 UTC 6 May 1999. The color scale shows the relative
motion of the raindrops. Warmer red colors show motion away from
the radar and cooler green colors show motion away from the radar.
Click on image to enlarge.
The only images that showed significant rotation in the storm
occurred almost simultaneously with the first reports of
damage around 6 a.m. The Storm Relative Velocity image shows
strong rotation at the lowest elevation (Fig. 3). Notice the
couplet of bright green and dark red returns just on the
southwest side of Asheville at this time. The greens represent
motion toward the radar, and the reds motion away from the
radar. Notice also that the circulation is stronger on the
0.5 degree slice (the image on the left) than at the higher
1.5 degree slice. This was not a classic Supercell tornado,
typified by persistent, deep rotation. Instead it was a small
scale "spin-up" along the line. Such events are not p
articularly rare in the Southeastern United States, though
they are unusual over the mountains. The pronounced, broken
S-shaped signature in the reflectivity image was becoming
less organized even as the tornado began.
Figure 3. As in Fig. 2, except for 1003 UTC. Click on
image to enlarge.
Figure 4. As in Fig. 1, except for 1003 UTC 6 May 1999.
Click on image to enlarge.
Another interesting aspect of the storm was the location of
the damage. Almost all of it was on the south side of the
tornado path. Being a weak tornado, the rotational velocity
of the vortex was only on the order of 60 mph. The parent
storm was tracking east at around 50 mph. Thus, on the north
side of the track, the ground relative wind speeds were only
around 10 mph, while on the south side they were slightly in
excess of 100 mph! This explains why most of the damage was
confined to the south side of the track. This graphic shows
the various types of damage commonly associated with tornadoes
in the southeast (the arrows are vectors, with longer arrows
representing stronger winds). The top-most example most closely
resembles the Asheville damage.
Here are some of the reasons why the NWS survey team determined
the Asheville damage was the result of a weak tornado and not
- The damage path was of uniform width; straight-line winds
often have widening paths
- There was a damage gradient across the width of the damage
path, another sign of tornadic damage
- The damage was of the same intensity length-wise through
- Several trees were snapped-off in the middle. This
indicates rapidly increasing winds with height, another
- Some of the debris was blown slightly to the left of the
- The damage skipped from ridge to ridge, with no evidence
of largely descending air that would occur with straight
Make sure you give credit to any sources of images, such as SPC or HPC,
and especially sources outside the NWS.