Double Impact: When both Tornadoes and Flash Floods Threaten the Same Place at the Same Time

While both tornadoes and flash floods individually present public hazards, when the two threats are both concurrent and collocated (referred to here as TORFF events, short for “tornado and flash flood”), a unique set of concerns arises. In the case of these multi-threat scenarios, effective communication of the most pertinent threat to those in harm's way can be muddled due to differing instructions associated with each hazard. This is of particular concern for tornadoes and flash floods since the life saving actions for each hazard are contradictory; tornado safety protocol recommends taking shelter in a low-lying interior room, whereas flood safety protocol recommends retreating to high ground. Recent events (e.g., Tornadoes and Flash Flooding in Oklahoma in 2013 and 2015, Nashville Flood of 2010), underscore the exceptionally life-threatening situation that TORFFs can produce.

This study presents the climatological and meteorological characteristics associated with TORFF events over the continental United States. Two separate datasets, one based on overlapping tornado and flash flood warnings and the other based on observations, were used to arrive at estimations of the instances when a TORFF event was deemed actionable and verified to have occurred, respectively. This data was then used to discern the geographical and meteorological characteristics of recent TORFF events. During 2008-2014, TORFF events were found to be publicly communicated via overlapping warnings on average 400 times per year, with a maximum frequency occurring in the lower Mississippi Valley. Additionally, 68 verified TORFF events between 2008-2013 were identified and subsequently classified based on synoptic characteristics and radar observations. In general, synoptic conditions associated with TORFF events were found to exhibit similar characteristics of typical tornadic environments, but the TORFF environment tended to be moister and have stronger synoptic scale forcing for ascent.

These results indicate that TORFF events occur with appreciable frequency, in complex meteorological scenarios. Furthermore, despite these identified differences, TORFF scenarios are not always easily distinguishable from tornadic events that fail to produce collocated flash flooding, and present difficult challenges both from the perspective of forecasting and public communication. This work represents a first step toward an integrated line of research that aims to understand the meteorological, hydrological, societal, and communication processes at work in situations with multiple natural hazards.