Cincinnati, Ohio Tornadic Outbreak 9 April 1999—A Case Study

During the pre-dawn hours of 9 April 1999 a tornado outbreak unfolded over southeast Indiana and southwest Ohio. Between 0600 and 1000 UTC, a sub-severe line of showers and thunderstorms rapidly intensified as it moved into the vicinity of southwest Ohio. The simultaneous coupling of enhanced jet dynamics with increasing low level instability resulted in this rapid intensification. Substantial wind shear, as noted by storm-relative heliciy and positive shear estimates, pointed to the possibility of strong to violent tornadoes.

Within a three hour period seven tornadoes were observed. Two of the seven tornadoes were apparently spawned by supercells. The others were associated with storms which exhibited less familiar characteristics than that of this more classic supercell. The strongest tornado, which occurred in the northeast suburbs of Cincinnati, was classified as F4 on the Fujita Scale. This tornado caused four deaths, as well as numerous injuries. The other six tornadoes ranged in classification between F0 and F3.

From a forecasting perspective, however, this was not a clear-cut case depicting the type, as well as extent, of severe weather the Ohio Valley area would experience. The fact that this line of convection had weakened considerably between 0000 and 0600 UTC on the 9th, as well as the latest model guidance (0000 UTC) indicating continued diminishing of low-level instability to just marginally weak levels, seemed to add a bit of uncertainty to the forecast process.

This paper will investigate not only the synoptic and mesoscale environment for which these storms occurred, but also that of the storm scale for which WSR-88D data were examined. We will show from the radar perspective how effective the storm-relative rotation was as a precursor to tornadic development. This rotation, when coupled with corresponding reflectivity imagery, did provide advanced warning for the majority of the tornadoes which occurred this day. Other WSR-88D parameters, such as cell-based VIL, echo tops, and height of the maximum reflectivity, were also investigated, but only in association with the three strongest storms on this day. Their role alone as precursors did not show much promise.