The 16-year-old was last seen on November 14.
Kirah, who lives in Sompting, has links to the Croydon area.
Detective Inspector Dave Payne said: “Kirah has been missing before and although she has been in intermittent phone contact with her family, given her age, we are concerned for her wellbeing.
“Kirah is medium build and 5ft 4 tall. She has shoulder-length hair with blonde highlights. I would appeal to anyone who recognises her or knows of her whereabouts to contact Sussex Police on 101, email email@example.com or the independent charity Crimestoppers anonymously on 0800 555 111.”