For some reason I didn't expect much from the summary. Man, was I ever wrong!
First off, your descriptions of New Orleans are... incredibly graphic. Surely you must have witnessed some of the devastation, to write so eloquently.
But what I like even more is the way you have written Snape. I'm a total slut for Snape being in character, and you hit the nail on the head. He's perfect! How did you ever come up with sentences like: "He is an arrogant, self-centred, self-righteous little pillock with the intelligence of a mentally impaired flobberworm and the social skills of a cabbage."
**flails and squees**
How when they first got together it WAS because Severus wanted Lily - not many writers let that option into their Snarry. Including it makes the entire story more realistic. And Harry's reason for leaving... that is absolutely, 100% perfect. As is his reason for seeking Severus out again after all those years.