Simultaneous and collocated tornado and flash flood warnings associated with tropical cyclones in the contiguous United States

Simultaneous and collocated tornado and flash flood (TORFF) warnings are a dangerous hazard because the recommended protective action for the two threats are opposite, leaving residents unsure if they should shelter below or seek higher ground. Tropical cyclones (TCs) cause both tornadoes and flash flooding and are thus favourable environments for TORFF warnings. In this study, we provide a unique examination of TORFF warnings in 32 TCs that made landfall in the contiguous United States between 2008 and 2018. We identify TC TORFF warning characteristics including duration, area, distance from coastline, geographic location, and location relative to TC centre, and we compare these results to established findings on TC tornadoes. We found that TORFF warnings were geographically most common in the states of Texas, Louisiana, and Mississippi, and within 200 km of the coastline. TORFF warnings occurred almost exclusively east of TC centre. When compared to TC tornadoes, TORFF warnings were relatively more frequent nearer to the coastline and in the right-back quadrant of the TC. Over half (59%) of the 32 TCs we studied produced at least one TORFF warning. Using logistic regression, we determined that TC intensity effectively determines how likely a TC is to produce at least one TORFF warning, while TC translational velocity determines how likely a TC is to produce many TORFF warnings. Thus, intense TCs were likely to produce at least a few TORFF warnings, while intense and slow-moving TCs were likely to produce many TORFF warnings. These findings establish a knowledge base on the climatological characteristics of this unique and dangerous hazard.

Automated summary

The study focused on simultaneous tornado and flash flood (TORFF) warnings in 32 tropical cyclones that made landfall in the contiguous United States between 2008 and 2018. The research identified TORFF warning characteristics such as location and frequency, with TORFF warnings being more common in Texas, Louisiana, and Mississippi, and mostly occurring east of TC center. It was found that TC intensity and translational velocity determine the likelihood of TORFF warnings, with intense TCs producing more warnings and slow-moving TCs generating multiple warnings.