The Unusual Early Morning Tornado in Ciudad Acuna, Coahuila, Mexico, on 25 May 2015

This study analyzes the synoptic-and mesoscale conditions present during initiation and intensification of the supercell thunderstorm that produced a tornado in Ciudad Acuna, a community located in the state of Coahuila, Mexico, 10 km southwest of the U.S. border. Early morning convective activity, first detected by radar at 0628 UTC 25 May 2015, developed into an intense and well-defined supercell thunderstorm that produced a tornado between approximately 1045 and 1130 UTC. Hourly analyses from the Rapid Refresh model indicated an upslope component to surface flow in the region of convection initiation over the Serranias del Burro (SdB). Along the storm's trajectory, dewpoint temperatures increased from 15 degrees to 22 degrees C, convective available potential energy increased from 1500 to near 4000 J kg(-1), and convective inhibition changed from- 150 J kg(-1) at the time of convection initiation to near zero in Ciudad Acuna. Simulations from the Weather Research and Forecasting Model confirmed the sensitivity of both convection initiation and storm intensification to the topography of the SdB. In the control simulation and two simulations in which topography was reduced in elevation, a cluster of storms formed and intensified over the central mountains. However, when topography was further reduced and the SdB region became a large flat plain, little convective activity was seen, forming only along the dryline without intensifying or propagating to the east as was observed.