The Pickens County F1 Tornado
Author's Note: The following report has not been subjected to the scientific peer review process.
Tornadoes have not been common over the western Carolinas and northeast Georgia the past year, and the few which have occurred have been in association with rather unusual radar signatures and parent thunderstorms. The April 3rd storm in Pickens County was no exception. Before we get into details, Figure 1 shows the damage path of the tornado as determined by our storm survey crew.
Figure 1. Damage path of the tornado east of Clemson, South Carolina, on 3 April 2000. Click on the image to get a larger picture.
2. Radar Observations
As for the storm itself, the radar operators that night issued a severe thunderstorm warning as the cell moved into our county warning and forecast area. The first warning was for Stephens County in northeast Georgia. As the storm raced northeast at nearly 50 mph, warnings were quickly issued into Pickens and Oconee counties in the early morning hours.The rather unusual structure of the storm can be seen in the radar picture in Figure 2, taken a little less than 30 minutes before the tornado touched down. The top two panels are 0.5 and 1.5 degree reflectivity cuts respectively and the bottom two are storm relative winds for the same cuts as seen by the WSR-88D radar at the Greenville-Spartanburg office. The storm of interest is the one nearly in the center of the frame. Notice the greens and yellows to the east (right) of the cell. It developed behind a previous line of storms. Typically the air behind a line of storms is cooler and less conducive for convective development. There is a sliver of almost no radar returns behind the storm. This is very dry air being pulled in behind the storm. The reason that the dry air is being entrained is that the cell itself is embedded in the circulation of a mesolow (a small area of low pressure only 100 or so miles across). When you look at the larger image, you can see that the storm relative winds are swirling into the storm. The red colors to the north of the cell are outbound from the radar (located about 50 miles to the east northeast of the picture), while the greens just to the south are inbound. If you look carefully you can see a smaller "red/green couplet" right at the very bottom of the storm. This very small circulation was apparently the circulation which eventually gave rise to the tornado. This is a very unusual position for a mesocyclone (rotating thunderstorm updraft), in a thunderstorm.
Figure 2. Base reflectivity (top) and storm relative motion (bottom) from the KGSP radar at 0.5 degrees (left) and 1.5 degrees (right) at 0830 UTC on 3 April 2000. Click on the image to enlarge.
The base reflectivity and storm relative motion from the KGSP radar at 0850 UTC are shown in Figure 3, shortly before the tornado touched down. It looks very similar to the image above, except that the dry air (blue reflectivities) has wrapped around to the south and east side of the cell. This "occlusion" process is possibly what contributed to the weak mesocyclone actually forming a tornado just minutes later. In looking at loop of these radar data for the duration of the storm (not provided here) it was obvious that the cell interacted with a boundary left by the earlier thunderstorms at about the time it produced the tornado. Such boundary interactions have been shown many times in the past to be associated with tornadogenesis.
Figure 3. As in Figure 2, except for 0850 UTC. Click on the image to enlarge.
Patrick Moore and Neil Dixon converted the html to the new template. Vince DiCarlo provided the damage survey.