WSR-88D reflectivity and velocity trends of a damaging squall line event on 20 April 1996 over south-central Indiana and central Kentucky

An intense, long-lived convective squall line moved east across south-central Indiana and central Kentucky during the early morning hours of 20 April 1996, containing several bowing segments. The squall line produced quarter-to-baseball size hail in some locations, wind gusts of 60-80 mph along much of its length, and a few F0-F2 tornadoes embedded within bowing segments. Widespread, major wind damage occurred with multiple downed trees reported and numerous buildings and structures damaged or destroyed. One community alone reported 840 homes and businesses damaged, and another 60 completely destroyed.

Dynamic forcing, moderate-to-high instability, and strong 0-3 km vertical wind shear characterized the convective environment. The symmetric "leading line-trailing stratiform" MCS exhibited classic signatures, including a tight reflectivity gradient along its leading edge, a distinct storm-scale frontal structure, significant storm-relative front-to-rear flow, and intense pulses of rear-to-front flow. Resultant bowing line segments correlated well with the locations of the greatest wind damage and tornadic development. Tornadogenesis occurred as transient shear zones, noted along and north of the apexes of bowing segments, developed rapidly and spun up into well-defined, deep-layered cyclonic circulations that often met mesocyclone rotational velocity (Vr) criteria. The squall line contained similar characteristics to other organized tornadic bowing line segments observed across the lower Ohio Valley in recent years.

Two distinct tornado-producing convective segments will be investigated in this paper. Of the two, the second evolved rapidly from an initially linear orientation that produced exclusively wind damage and hail to a bow echo which possessed HP supercell-like traits and enhanced pulses of rear inflow. This particular storm complex caused widespread wind damage and a cluster of tornadoes resulting in property damage around 15 million dollars. The sudden evolution appeared to result from a dynamic intersection between an east-west outflow boundary from weakening convection to the north and the north-south oriented portion of the squall line. This intersection promoted strong low-level cyclonic convergence which stretched rapidly into the convective updraft resulting in cyclic tornadic development.

This paper will examine key WSR-88D reflectivity and velocity severe weather signatures to help diagnose and better understand physical processes and observed changes in squall line structure. The essence of this case will aid forecasters in making informed decisions with respect to the short term warning process, and lead to enhanced knowledge of convective storm-scale frontal structure in squall lines and bow echoes.