An Updated U.S. Geographic Distribution of Concurrent, Collocated Tornado and Flash Flood Events and Look at those Observed during the First Year of VORTEX-SE

In recent research the, the authors have examined issues related to the threat posed by concurrent, collocated tornadoes and flash floods (referred to as TORFF events). These events pose many challenges from both a meteorological and communication standpoint.

This research presents an updated geographic distribution of TORFF events using flash flood local storm reports with varying spatial buffers over which an intersection with a tornado track is possible. Additionally, an in depth look at TORFF events that occurred during (or just outside) of the first year of the VORTEX-SE field project is presented.  Specific cases discussed with include the late December TORFFs of 2015 near Dallas/Fort Worth, TX and Birmingham, AL and another verified TORFF that occurred in Dermott, AR on March 31^st, 2016. During the Dermott event, an EF-1 tornado and widespread flash flooding were produced by a quasi-linear convective system in the overnight hours. A detailed look at the synoptic-to-mesoscale characteristics, radar patterns, precipitation accumulations, event timing, and the associated tornado and flood report details is presented. Care is given to classify each event in a similar manner to that presented for 68 previously verified TORFF events in Nielsen et al. 2015 (WAF).  Further, the meteorological characteristics of the confirmed TORFF events in the southeastern United States are compared to the general characteristics enumerated in Nielsen et al. 2015 and any differences identified.

The results show (as of this abstract writing) that 39 overlapping warnings have occurred in the southeastern U.S. in 2016, which make up ~25% of the total throughout the U.S. Additionally, the analysis of the confirmed TORFF events highlight very strong low-level shear, deep moisture, high surface relative humidity, and a stable nocturnal boundary layer as characteristics of TORFF events in the southeast. While these results are largely consistent with the bulk meteorological characteristics of TORFF events identified in Nielsen et al. 2015, slight, but important, differences do exist.