 A well @-@ defined tropical wave emerged into the Atlantic from the west coast of Africa on September 20 . The cloud pattern quickly became better @-@ defined , and developed into Tropical Depression Thirteen on September 21 while centered south of Cape Verde . The depression quickly intensified due to low wind shear and became Tropical Storm Isaac early on September 22 as it headed west @-@ northwestward . Isaac continued to strengthen and was upgraded to a hurricane on September 23 . After reaching a preliminary peak with winds of 120 mph ( 200 km / h ) , the structure of the storm became less organized , possibly due to slightly colder ocean temperatures and increased wind shear . By 1200 UTC on September 26 , Isaac ceased weakening after winds dropped to 85 mph ( 140 km / h ) . The storm began to re @-@ strengthen on September 27 after wind shear decreased . 
