On March 28, 1997, a major
severe weather outbreak occurred across central Kentucky and south-central
Indiana ahead of a cold front and upper-air system. Strong lift,
an unstable air mass, and strong vertical wind shear resulted
in development of various severe convective storm structures,
including supercells and multicells in an axis across central
Kentucky followed by a squall line and bow echo over south-central
Indiana and north-central Kentucky. Numerous reports of large
hail and wind damage occurred along with a few tornadoes in central
Kentucky. The following series of images from the NWS Louisville
WSR-88D Doppler radar system shows the structure of a few of the
storms during this event.
This reflectivity image shows numerous
strong to severe thunderstorms on March 28, 1997. One area of
storms was located from near Louisville south to Bowling Green.
Multicellular and a few supercellular storms were present in
this zone. The supercells were responsible for most of the large
hail and damaging tornadoes across central Kentucky. Farther
west, a squall line was located over southwestern Indiana, with
the leading edge of the line bowed out to the east. This signature
is called a "bow echo" or "bowing line segment"
and typically is associated with damaging straight-line winds
and possibly transient tornadoes along the leading edge of the
Close-up reflectivity image of a
supercell complex over southern Larue and northern Hart counties
in central Kentucky. The main inflow into the storm and the location
of a rotating updraft, i.e., mesocyclone was on the south side
over north-central Hart county. At this time, a tornado was on
the ground near Bonnieville in north-central Hart county. Just
north of the tornado, the red reflectivity colors indicated that
very heavy rain and hail were occurring over extreme northern
Hart and southern Larue counties.
This reflectivity image shows the
supercell a few minutes later over extreme northern Hart and
southern Larue counties. Particularly noteworthy is the well-defined
"hook echo" on the southwest flank of the storm with
a "weak echo region" located within the hook. This
represents the location of a mesocyclone where a tornado was
on the ground in extreme northern Hart county. Other storms developing
across northeastern Hart county eventually were associated with
a separate tornado over extreme northern Green county.
Corresponding storm-relative velocity
(SRM) image showing wind flow with respect to the supercell thunderstorm
and tornado over northern Hart county. Green (red) colors denote
winds directed toward (away from) the radar (located about 35-40
miles to the north). In this image, a tornado vortex signature
(TVS) was detected over north-central Hart county just north
of Bonnieville. The tight red-green couplet represented the strong,
cyclonically rotating circulation associated with the tornado.
Just southeast of this signature was a broader cyclonic circulation;
although non-tornadic at this time, it eventually produced another
tornado in extreme northern Green county. Also note the larger
area of light green colors southeast of the TVS; this represents
an area of strong storm-relative inflow into the supercell storm,
which is essential in maintaining a severe thunderstorm capable
of producing a tornado.
A reflectivity vertical cross-section
of large, mature severe thunderstorms over south-central Kentucky
(east of Bowling Green), but not the same storm as shown above.
The image reveals thunderstorms with echo tops of about 47000
feet, with high reflectivity values (red and pink colors) extending
up to between 35000 and 40000 feet. The significant area of pink
color (reflectivity values at least 65 dBZ) suspended aloft is
indicative of a very strong updraft within the storm and that
large hail likely may soon occur at the surface. Some storm tilt
also is apparent. Two separate storms/updrafts appear right next
to each other in this image. On the far left part of the cross-section,
a weakening thunderstorm is evident as highest reflectivity returns
are located in the lower portion of the storm.