Karl originated in a strong tropical wave that moved off the coast of Africa on September 13. The wave gradually became better organized, and it was declared Tropical Depression Twelve about 670 miles (1,080 km) west-southwest of the Cape Verde islands on the morning of September 16, as it headed westward in the open tropical Atlantic following the periphery of the subtropical ridge. That afternoon, the depression continued to rapidly organize and was upgraded to Tropical Storm Karl. Due to healthy outflow around an upper-level anticyclone over Karl and a favorable environment with the warmest sea surface temperatures of the year, rapid deepening began on the evening of September 17, with the storm developing a small eye and being upgraded to Hurricane Karl.