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.

1.  Introduction

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.

4.  Discussion 

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 
straight-line winds:

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 
the track
Several trees were snapped-off in the middle.  This 
indicates rapidly increasing winds with height, another 
tornado indicator
Some of the debris was blown slightly to the left of the 
track
The damage skipped from ridge to ridge, with no evidence 
of largely descending air that would occur with straight 
line winds.

Make sure you give credit to any sources of images, such as SPC or HPC,
and especially sources outside the NWS.