What caused the 2013 Oklahoma tornado?

There were numerous tornadoes in Oklahoma in 2013. However, the most significant tornado was the EF5 that hit Moore on May 20. A low pressure system was slowly making its way eastward across the Great Plains. The system led to the convergence of a warm, moist air mass from the Gulf of Mexico, a cooler air mass from Canada, and a dry air mass from the Rockies. This caused powerful thunderstorms to form over the plains. Strong wind shear set these storms rotating, turning them into supercells. One of these supercells was especially powerful, and spawned the violent tornado that hit Moore.