 A tropical wave exited the west coast of Africa and entered the Atlantic on October 11 . Four days later , a mid- to upper @-@ level trough increased convection along the northern portion of the wave . The southern portion of the wave continued westward , while the northern portion moved slowly west @-@ northwestward and developed into a surface trough . Eventually , the system transitioned into a broad surface low . Despite southwesterly vertical shear , deep convection began to increase by late on October 20 . After becoming better defined , the system developed into a tropical depression at 06 : 00 UTC on October 21 while located about 620 mi ( 1 @,@ 000 km ) east @-@ southeast of Bermuda . Six hours later , the depression strengthened into Tropical Storm Lorenzo . 
