Adults of P. melanoleucus are large, growing to 48–90 in (120–230 cm) in total length (including tail) and are powerfully built. The head is small and somewhat pointed with an enlarged rostral scale that extends upward between the internasal scales. Usually, four prefrontal scales are seen. At midbody are 27-37 rows of keeled dorsal scales. The anal plate is single. The color pattern consists of a light ground color overlaid with black, brown, or reddish-brown blotches.