Hannah Zemaitis, from King's Road, was last seen by her family at around 7.30pm on Saturday, March 10.
Nicola Cecil, missing person co-ordinator, said: "Due to the length of time that Hannah has been missing, there are growing concerns for her welfare.
"One line of enquiry is that she may be in London, in particular north London, possibly in the Enfield or Haringey areas.
"However, we are keeping an open mind and urge anyone who sees Hannah or knows of her whereabouts to get in touch as soon as possible."
Hannah is white, 5ft 6ins with short brown hair. She was last seen wearing a grey and silver cardigan, leopard print leggings, black pumps and a white top.
Police do not think she took a change of clothing with her. Call 101 or Crimestoppers anonymously on 0800 555 111.