Examination of Recent TORFF Events in and around Houston, Texas

Concurrent, collocated tornadoes and flash flood events (referred to as TORFF events) pose many challenges from both a meteorological and communication standpoint. Recent research by the authors examining these events has shown the ability for rotation around a vertical axis, through rotationally induced dynamic accelerations, to enhance short-term rain rates. The precipitation enhancement and associated rotation strength were shown to increase with the magnitude of the low-level wind shear and can occur with or without the presence of a tornado. Further observational evidence has shown that TORFF events occur in many different storm modes, including hurricanes, supercells, MCSs, and transitional periods of upscale growth.

The upper Texas Coast, in and around the Houston, TX area, has experienced many intense TORFF events over the recent years. Examples include the damaging floods in April 2009, the Memorial Day flood of 2015, the Tax Day flood of 2016, and Hurricane Harvey in 2017. The first part of this research focuses on examining the environmental characteristics associated with high-profile TORFF events in and around the Houston area. Special attention was given to the environmental kinematic conditions and the strength and longevity of any rotation. Furthermore, radar and local mesonet rain gauge observations were used to examine the storm-scale characteristics to identify the locations and structures of extreme rain rate producing cells.

To supplement the observational based analysis above, a WRF-ARW simulation of the Tax Day flood in 2016 was examined. The simulation was initialized off the 0000 UTC 18 April 2016 real-time forecast of the experimental High-Resolution Rapid Refresh (HRRRx). The full three dimensional nature of the simulation allowed the examination of the dynamical and microphysical characteristics of the event and sub-event-scale features. Convective cells that produced the most intense short-term (i.e., sub-hourly to hourly) rainfall accumulations within the MCS were examined for the influence of any attendant rotation on both the dynamics and microphysics of the precipitation processes. Results show that the most intense accumulations, as in the observational based analysis, are often associated with rotating convective elements.