The storm that eventually became Hurricane Ismael started off the southern coast of Guatemala on September 9. As it grew, it moved to the west-northwest. After a few days with no activity, a low pressure area developed off the coast of Mexico. Late on September 12, it formed into Tropical Depression Ten. At this time, the storm was about south-southwest of Manzanillo, Colima. The depression moved to the northwest, and strengthened to a tropical storm early on September 13.