 On August 31 , a large tropical wave moved off the west coast of Africa . A tropical system along the wave axis contained a low pressure area as well as an impressive outflow pattern , though initially its convection was disorganized and limited . By September 1 a cyclonic circulation with a diameter of 690 mile ( 1115 km ) was evident on satellite imagery , well to the southeast of the Cape Verde Islands , and several hurricane forecast models anticipated development and strengthening . As it tracked quickly westward , the convection organized and developed into rainbands – bands of showers and thunderstorms that spiral cyclonically toward the storm center – and late on September 1 meteorologists began tracking the system using the Dvorak technique . Low amounts of wind shear and favorable outflow allowed vigorous deep convection to develop and persist near the center , and by 1800 UTC on September 2 the system developed into Tropical Depression Nine about 450 miles ( 730 km ) southwest of Praia , Cape Verde . 
