Angel, aged 17, was last seen by police in Hatfield on Tuesday 15 July, with further sightings on Wednesday 16 and Friday 18 July.
She is described as being approximately 5ft 6ins tall and of very slim build with pink hair and blue eyes. However, her hair colour may now be different if she has not had access to hair-colouring products.
Officers understand her appearance has changed since the attached photos were taken.
When Angel went missing, she was described as wearing an oversized puffer jacket, black McKenzie jogging bottoms with white writing and black Fila trainers.
Angel is known to frequent the Parkhouse Court area of Hatfield. She may have also travelled to Bedfordshire or London, although may now be further afield.
Angel has links to Luton in Bedfordshire, Harlow in Essex and Southampton in Hampshire.
Pete Frost, Welwyn Hatfield Chief Inspector, said: “Angel hasn’t been seen in almost two weeks, and we are growing increasingly worried for her welfare. Our understanding is that she has no phone and no access to banking, and we need to make sure she is safe.
“Our officers are carrying out numerous enquiries to find her, which is our top priority.
“If you have seen Angel, or have any information that may help our enquiries, please make you sure you contact the police immediately.
“I’d also like to make a direct appeal to Angel: You are not in any trouble, we just want to make sure you’re okay and safe.”
If you have seen Angel since she was reported missing or have information about where she has been, report information online, speak to an operator in our Force Communications Room via our online web chat or call the non-emergency number 101.
If you believe you are with Angel now or have seen her in the last few moments, please call 999 immediately.