 A tropical wave moved off the coast of Africa on September 1 . An area of low pressure associated with the wave moved slowly westward , and its convection initially appeared to become better organized . On September 3 , as it passed to the south of the Cape Verde islands , organization within the system degraded , though convection increased the next day . The system gradually became better organized , and Dvorak classifications began early on September 5 . Based on the development of a closed surface circulation , it is estimated the system developed into Tropical Depression Thirteen early on September 6 . Hours later , it intensified into Tropical Storm Isabel , though operationally the National Hurricane Center did not begin issuing advisories until 13 hours after it first developed . 
