On August 22 , as it passed 140 miles ( 225 km ) miles to south of the Cape Verde islands , deep convection developed over the center of circulation , and early on August 23 the depression strengthened into Tropical Storm Debby about 300 miles southwest of Cape Verde . On August 22 , as it passed 140 mi ( 225 km ) to south of the Cape Verde islands , deep convection developed over the center of circulation , and early on August 23 the depression intensified into Tropical Storm Debby about 300 mi ( 485 km ) southwest of Cape Verde .