The Pickens County F1 Tornado
of 3 April 2000
NOAA/National Weather Service
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
The public information statement provides information about the
path as well.
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 converted the html to the new template. Vince
DiCarlo provided the damage survey.