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. Another separate damaging area of supercell thunderstorms produced tornadoes ranging from F-3 to an F-5 rating on the Fujita scale across southern middle Tennessee shortly after 2200 UTC on 16 April. This second area continued to produce extensive damage in the F-3 to F-5 range through 2330 UTC.

Due to the extensive amount of damage produced by the tornadoes in southern middle Tennessee and the fact that this was the first tornado to hit the immediate Nashville metropolitan area since 1933, this paper will document the damage produced by the six tornado tracks. WSR-88D Doppler radar 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 88-D Velocity product (SRM), indicated that when gate-to-gate rotational velocity values were around 50 knots, extensive tornado damage rated as F-3 and above occurred in Wayne, Lawrence, and Giles counties in southern middle Tennessee. It was interesting to note that the gate-to-gate maximum rotational velocity value of 55 knots at 2310 UTC correlated well with the F-4 tornado damage determined by an extensive aerial and ground survey completed in Giles county, Tennessee. During the time of the tornado track through southern middle Tennessee from 2205 through 2330 UTC, rotational shear values ranged from around 20 x 1000 (per second) to a maximum of around 30 x 1000 (per second). These values also correlated well with the F-4 tornado damage in Giles county, Tennessee around 2310 UTC.

Although this was only one large 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.