During the early morning hours
of April 20, 1996, a linear line of thunderstorms (squall
raced east across east-central and south-central Kentucky. Straight-line
wind damage was common with the line, although a transformation
in storm structure occurred in a portion of the line that resulted
in enhanced wind damage and development of a transient tornado
in a few locations, including the town of Berea in southern Madison
county, KY. The following series of images from the NWS Louisville
WSR-88D shows the evolution of the line during this period.
At this time, the squall line was
moving east through Jessamine, Garrard, and Lincoln counties
in KY. A tight reflectivity gradient was present on the leading
edge of the storms, which was producing straight-line wind damage.
However, no tornadoes were occurring at this time as the line
basically was linear with no pronounced wave structure that can
be associated with transient tornado formation.
A short time later, a convective-scale
wave began developing in southern Garrard county indicating that
a local transformation in line structure was occurring. This
structure alerted forecasters to be particularly concerned about
wind damage, hail, and possible tornado development, given favorable
environmental conditions. Note the "inflow notch" (i.e.,
weak echo region) on the front forward flank of the storm; this
usually is the location of a strong updraft in the storm.
About 15 minutes later, the storm
structure evolved into a small-scale bow echo across southern
Madison county with a weak echo channel in the southwest tip
of the county just behind the leading line. This is an indicator
of strong straight-line winds along the leading line of storms;
indeed, wind damage occurred in Berea in southern Madison county.
Just north of the bow apex, the inflection point in the bowing
structure was coincident with a cyclonic circulation (rotating
updraft; not shown). This circulation was associated with a brief
transient tornado in Berea, embedded along the northern edge
of the straight-line wind damage.
This image from the NWS Jackson,
KY WSR-88D (located east of the squall line) shows the likely
reason why a local transformation occurred in this part of the
line. An "outflow boundary" (narrow zone of blue color
at left) from storms to the north was oriented from northeast
to southwest. Although not readily apparent, this boundary likely
extended westward and intersected the squall line as it moved
across Garrard and Madison counties. If the boundary/thunderstorm
intersection occurs in such a way, enhanced low-level convergence
and a stronger, possibly rotating updraft in the storm can occur.
As a result, the storm can intensify with tornado development
possible, as was the case here.