Secret of the storm / Beth McMullen.

Enhanced descriptions from Syndetics:

Seekers of the Wild Realm meets My Diary from the Edge of the World in this poignant and "action-packed" ( School Library Journal ) story of a lonely girl who befriends a kitten that might be much more--the first in a new series from author of Mrs. Smith's Spy School for Girls , Beth McMullen!

Twelve-year-old Cassie King's father always told her the universe was on her side. All she had to do was work hard and things would go her way. But then Cassie's father died, her mom retreated into herself, and her best friend traded her in for the popular crowd at school. The only thing Cassie still has is the volunteer work she does at the local library, a place where she can leave her troubles behind. Unfortunately, classmate and school outcast Joe Robinson is always there doing the same thing.

One day, while Cassie and Joe are leaving the library, a bizarre storm hits, trapping them in a narrow alley. In the storm's aftermath, Cassie discovers a bedraggled little kitten abandoned in a smelly dumpster. Cassie feels an immediate connection to the kitten and takes him home.

But the kitten--who Cassie names Albert--is a little odd, with impossible strength and agility for a creature his size. At one point, Cassie swears she sees plumes of smoke rising from his water bowl, and one afternoon, while Albert is alone in her room, a strange symbol appears on the closet door. With new friend Joe's help, Cassie figures out the symbol is a map. But a map to what ?

The friends soon discover that Albert is much more than he appears and is in grave danger. He needs Cassie's help in ways she never could have imagined. Keeping him safe is the first thing Cassie has believed in for a long time. But is she strong enough to face down a sinister enemy moving ever closer and protect everything she loves?