Apparently, the answer is simple and quite unsurprising:
JK Rowling's chemistry teacher.
From this article about John Nettleship, who died aged 71:
"The first I knew was when a reporter from a national newspaper knocked on the door seven years ago and said: 'You're Professor Snape aren't you'," said the former head of science.
"I suppose I was quite strict as a teacher, but I said to my wife, 'she thinks I'm Professor Snape'. She said 'of course you are, but I didn't want to tell you'.
"Fortunately for me, quite a lot of people like Alan Rickman, who plays Snape in the films."
And he does look a bit like Alan Rickman, minus the curtain of dark hair:
