 A tropical wave emerged into the Atlantic from the west coast of Africa on September 13 . The wave later interacted with a Kelvin wave , the latter of which contributed to the formation of Tropical Depression Nine . Moving westward with a large area of convection , the tropical wave and the Kelvin developed into a well @-@ defined low pressure area around midday on September 15 , according to satellite imagery . However , disorganization prevented its classification as a tropical depression until 06 : 00 UTC on September 18 , while located about 750 mi ( 1 @,@ 210 km ) south of the southernmost Cape Verde Islands . The depression moved west @-@ northwestward due to a subtropical ridge to the north and intensified into Tropical Storm Ida early the following day . Westerly wind shear exposed the storm 's low @-@ level circulation , causing Ida to strengthen only slightly . 
