A 35 year old male presents to the Emergency department with a sudden, explosive headache around the back of his head. He denies any trauma but can now recall several similar but less severe headaches over the last 2 weeks. He has also felt dizzy intermittently over this time. On this occasion the headache started last night and since this morning he has also been vomiting.
Fundoscopy and neurological examination today is normal.
What is the most likely diagnosis?