A Comparison of WSR-88D Radar Velocity Signatures to Tornado Damage for the April 16, 1998 Tornado Outbreak in Middle Tennessee

The April 16, 1998 tornado outbreak across Middle Tennessee produced six significant tornado tracks and property damage in excess of 133 million dollars in Middle and East Tennessee. Remarkably, there were only four deaths and 105 injuries. Specifically, an extensive aerial and ground storm survey indicated that two tornadoes, one determined an F-3 on the Fujita scale and another F-2, moved through the immediate Nashville metropolitan area between 2030 and 2200 UTC that afternoon.

Due to the extensive amount of damage produced by the tornadoes in the immediate Nashville metropolitan area, this paper will document the damage produced by the two Nashville tornado tracks. WSR-88D Base Velocity, Storm Relative Map Velocity (SRM), and Base and Composite Reflectivity products were used in this study along with the traced damage paths produced from the completed storm surveys to determine a correlation between radar signatures and storm damage.

Examining the SRM indicated that when gate-to-gate rotational velocity values were around 50 knots, extensive tornado damage rated as F-3 occurred in the immediate Nashville metropolitan area. The close proximity of the two tornadoes to the WSR-88D NWSO Nashville also provided for optimal storm scale velocity analysis.

It was interesting to note that gate-to-gate rotational velocity values of the initial F-3 tornado which moved through the Nashville metropolitan area ranged from 45 knots initially at 2024 UTC to 50 knots at 2029 UTC and increased to 55 knots at 2034 and 2039 UTC. Shear values also increased from 0.016 s^-1 at 2024 UTC to 0.022 s^-1 at 2029 UTC, with a rapid increase to 0.077 s^-1 noted by 2034 UTC. By 2039 UTC, the shear value had only decreased to 0.051 s^-1.

Although this was only one severe weather event studied, it is hoped that results gained from this study will assist NWS forecasters in their interrogation of severe storms which are likely to produce tornadoes. The correlations determined in this study between the rotational velocities and shear values to F-scale tornado damage determined by the storm surveys may allow forecasters the ability to imply a certain tornadic intensity which may be present. Using the velocity values derived from this event should further aid forecasters in the development of a nowcasting scheme which could result in more strongly worded situation specific tornado warnings, severe weather statements, and short term forecasts in the southeast U.S. based upon these findings.