On August 13 , while located near the northwestern Bahamas , an increase in convection made the upper-level low building downwards to the middle levels of the troposphere , which caused the development of an upper level anticyclone . On August 13 , while located near the northwestern Bahamas , a substantial increase in convection resulted in the upper-level low building downwards to the middle levels of the troposphere , coinciding with the development of an upper level anticyclone .