What species of snake found in central Alabama, US?
Definitely looks like some species of rat snake (genus Pantherophis). Based on the size, location and coloration, I'd say it's a dark variant of a Gray rat snake (Pantherophis spiloides).


It might also be a cross between a black and grey rat snake (i.e., Pantherophis spiloides x obsoletus).




 Source: Todd Pierson 

 

 Sources: Alan Brumbeloe | Bill Funderburk 

Description: Adults usually 99-183 cm long; dark to light gray with darker gray or brown blotches 


  Unlike other Pantherophis, whose conspicuous juvenile pattern fades into adulthood, gray ratsnakes do not undergo drastic ontogenetic changes in color or markings. These snakes retain the juvenile pattern of dark elongate dorsal blotches separated by four, or more, pale gray body scales, a light gray crown with dark striping that forms an anteriorly facing spearpoint, and a solid band which covers the eyes and extends rearward to the posterior upper labial scales. The venter is usually off-white or pale gray with darker irregular blotches, and a double row of black spots behind the divided anal plate of the vent. The dorsal scale rows around midbody are usually weakly keeled. Because the gray ratsnake shares its range with other members of its genus, hybrids of midlands x eastern ratsnakes are not uncommon. [Source].



Note, According to here one could have a difficult time IDing juveniles between species of rat snakes:


  The juveniles of all subspecies [of rat snakes] resemble the gray rat.



However, your specimen's size indicates it's likely an adult, and therefore its strong patterns preclude the black rat snake.



Distribution:  commonly found in the forests of eastern and central United States, including throughout Alabama:



 Source: Trey Dunn 

Behavior: Gray ratsnakes are agile climbers and can be found in tree tops as well as barns/sheds/buildings close to forests


  The gray ratsnake will defend itself by raising its head and bluffing a strike. If handled, these snakes will musk a victim by releasing the foul-smelling contents of their cloaca, and will bite if necessary. [Source].