Link to NWS Louisville Science and Technology Homepage WSR-88D Images of the April 20, 1996 Squall Line Event
Green Line Separater

During the early morning hours of April 20, 1996, a linear line of thunderstorms (squall line) 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.
WSR-88D Image 1 of April 20, 1996 Severe Weather Event 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.
WSR-88D Image 2 of April 16, 1998 Severe Weather Event 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.
WSR-88D Image 3 of April 16, 1998 Severe Weather Event 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.
WSR-88D Image 4 of April 16, 1998 Severe Weather Event 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.

Green Line Separater

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