On May 28, 1996, a severe thunderstorm
developed over south-central Indiana, then "split" into
two separate severe storms. The northern storm headed northeast
as a "left-moving" storm and produced some wind damage
and hail before weakening. The southern storm turned east-southeast
and evolved into a classic "right-moving" supercell
that produced wind damage, large hail, and at least 4 tornadoes
along its path through north-central and east-central Kentucky.
The strongest tornado was estimated on the ground for about 30
miles and was categorized as an F3 with isolated F4 damage over
parts of northeast Bullitt and Spencer counties in north-central
Kentucky (just south and southeast of Louisville). This tornado
produced severe structural damage to many homes in the area; some
homes were destroyed. The following series of Doppler radar images
from the NWS Louisville (KLVX) WSR-88D show the overall evolution
of this storm.
Base reflectivity data (lowest radar
elevation) showing a severe storm in progress across southern
Dubois county in south-central Indiana (northwest of Louisville).
The storm was moving east-northeast producing strong winds and
hail. The heaviest rain and hail were located in the western
quadrant of the storm (red colors) with lighter rain to the east.
Ten to twenty minutes later, the
storm was undergoing a split with two distinct cells apparent.
Note that the tightest reflectivity gradient was on the south
side of the southern storm and the north side of the northern
storm. This is typical in "mirror-image" splitting
storms and represents the most likely location within these type
of storms for severe weather phenomena.
About 10 minutes later, the splitting
process continued. Note that the red colored areas were now completely
separate with lighter reflectivity returns between the 2 storms.
This likely was a downdraft area that was promoting the split.
The southern storm started to take on some curvature on its southern
flank indicating an increase in severe weather activity.
The split was complete 10 to 20
minutes later with an almost mirror effect to the 2 storms, albeit
a stronger southern storm. The "left mover" (northern
storm) had a tight reflectivity gradient on its northern side
where strong winds and hail occurred. This storm then weakened
after this time. The southern "right moving" storm
was evolving into a classic supercell with a hook echo on its
southwest flank and a "V notch" (note the red colors),
i.e, blocking flow pattern aloft, just downwind in reflectivity
data. The first tornado (F2) touched down around this time with
the southern cell.
The mature right-moving supercell
continued to move east-southeast. While the first tornado dissipated,
golfball size hail fell in southwest Jefferson county, KY (near
Louisville). At left, the storm is shown over northeast Bullitt,
southeast Jefferson, and Spencer counties in north-central KY.
A definitive hook echo is present over Bullitt county indicative
of a strong, cyclonically-rotating updraft, i.e., mesocyclone
wrapping precipitation around the circulation. A second strong
tornado (maximum strength F3-F4) developed and was on the ground
within the hook area. Just north of the tornado, large hail and
very heavy rain were occurring (dark red and pink colors). Downwind,
moderate and heavy rain fanned out in a "V notch" signature.
This is a low-level storm-relative
velocity (SRM) image at the same time as shown in the reflectivity
image above. Green colors are storm-relative radial winds directed
toward the radar (located to the west of the image); red is flow
away from the radar. A tight tornado vortex signature (TVS) is
evident near the town of Mt. Washington in northeast Bullitt
county (20-25 miles east of the radar), which was coincident
with the hook echo in the corresponding reflectivity image above.
The TVS was associated with a strong tornado on the ground. Just
northeast of the TVS, light green colors denoted strong storm-relative
flow into the parent mesocyclone of the supercell.
The storm continued east into Spencer
county, KY where a mature hook echo still was evident on the
supercell's southwest quadrant. The tornado was still on the
ground at this time producing structural damage to homes and
uprooting and debarking numerous trees. Heavy rain and hail continued
just north and east of the hook.
Corresponding storm-relative velocity
(SRM) data for the reflectivity image shown above. The radar
is located to the west (left) of the image. A tight circulation
again is evident over west-central Spencer county, coincident
with the hook in the reflectivity image above. The associated
tornado eventually weakened and lifted over eastern Spencer county,
although 2 additional, but weaker (F0-F2) tornadoes occurred
along the storm's path over east-central KY. The storm then gradually
weakened and was overtaken by an approaching squall line (not