Hurricane Stan originated from a tropical wave that was first identified by the National Hurricane Center (NHC) off the west coast of Africa on September 17, 2005. Traveling westward, the wave remained weak, barely being notable. By September 22 convection began to increase; however, wind shear in the region prevented this development from continuing. The system entered the Caribbean Sea several days later, a region with more favorable conditions for cyclonic development. Intermittent convection formed around the wave as it moved west-northwestward before the development maintained itself. On October 1, the system had become sufficiently organized for the NHC to classify it as Tropical Depression Twenty, with the center of circulation situated roughly 135 mi (215 km) southeast of Cozumel, Mexico.