The 6 May 1999
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.
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 straight-line winds: