Storm is a Half-Human Hybrid.
Specifically, her ancestral line is descended from the Faltine, the same other-dimensional race as Dormammu. Most of the women in Storm's family line possess her unique traits of white hair and blue eyes, and many have been skilled magic users — one of them was even Sorcerer Supreme, before the Ancient One (Doctor Strange's mentor and predecessor). In addition, Strange's wife Clea, who hails from the Faltine's native dimension, also possesses white hair and blue eyes. Lastly, when Storm gets sufficiently angry (as has only happened a small handful of times), her hair turns into a combination of fire and lightning, looking much like the Flames of Regency (the burning halo of flames around the head) that mark a Faltine. She still qualifies as a mutant, since her weather powers are not magical in origin, and Faltine ancestry is well back in her genealogy, but the signs are still there, especially since so many alternate universe versions of her are also sorceresses or tied to magic in some way.