What is the nature of Professor Snape's Patronus and why does it appear in the Forest of Dean?

Question

During Harry Potter and the Deathly Hallows: Part 1 the crew (now reduced to only Harry and Hermione) wander through the Forest of Dean when a Patronus in the shape of a doe leads Harry to the frozen lake where he finds the Sword of Godric Gryffindor. Later during the events of Part 2 we (and Harry) take a look at the late Prof. Snape's memories and see that it was actually Snape who cast a doe Patronus (supposedly to Harry's aid), much to Dumbledore's surprise, who therefrom deduces a relation to Lily Potter (who Snape was secretly in love with all the years, albeit only unilaterally):

Dumbledore: Lily, after all this time?
Snape: Always.

Which already makes me wonder what Dumbledore was actually surprised about there. Was he simply surprised that Snape still loved Lily or was there anything deeper suggested with this Patronus? Add to this that Harry was a bit confused, too, and asked Dumbledore (or at least his mind-image of him, however you take that scene) about the reason for the related Patronuses, which didn't clear any confusions for me, though:

Harry: Professor, my mother's patronus was a doe, wasn't it? That's the same as professor Snape's. That's curious, don't you think?
Dumbledore: Actually, if I think about it, it doesn't seem curious at all.

So my first question would be, is there any deeper reason why Prof. Snape and Lily Potter both have a doe as Patronus or is it just to express that they've always been what one would call soulmates? And why did the Patronus appear in the Forest of Dean at that moment? Snape's discussion with Dumbledore and his casting of the Patronus occurred way earlier than that (as Dumbledore obviously was still alive, so at maximum during the events of the Half-Blood Prince). If that was implied to have been the same Patronus that appeared at the Forest of Dean (as the montage implies), why/how/where did it wait for so long? Is this just one of those various moments of stuff "magically" appearing when needed (as the Gryffindor Sword and the Room of Requirements use to do) or is there a more elaborate reason the movie just didn't delve into (or am I just too stupid to get it)?

Answer 1

Was he simply surprised that Snape still loved Lily?

I believe so. This memory takes place at least 16 years after Lily and James' murder, and I believe it's likely to be the first time Snape shared his feelings with Dumbledore since the day he broke down in tears upon hearing of Lily's death. Considering that Dumbledore probably knows how Snape treats Harry, he may well be surprised to hear that Severus still cares so much for her, but so little for her son.

is there any deeper reason why Prof. Snape and Lily Potter both have a doe as Patronus or is it just to express that they've always been what one would call soulmates?

I don't think they were soulmates, Snape's infatuation with the dark arts and his prejudice against muggleborns were completely antithetical to Lily's ancestry, kindness and goodness. Snape simply loved her, and his patronus reflected this, as it does with several others:

• Harry's patronus forms itself into a stag, just like his father's. This happens before he finds out about his father being a stag in his animagi form, so it can't be intentional.

• In the book of The Half-Blood Prince, when Nymphadora Tonks casts a patronus to send a message, Snape comments on the fact that her patronus has changed (into a huge four-legged beast which Harry is unable to identify), towards the end of the book it is revealed that it has actually transformed into a Werewolf, as she has fallen in love with Remus Lupin.

• Aberforth Dumbledore's patronus is a goat, and he was once charged with performing "inappropriate charms on a goat"

It may even be the case that Lily's patronus is a doe because a doe is the female of a stag, and Lily loved James, though there's no hard evidence on this (that I'm aware of).

why did the Patronus appear at the Forest of Dean at that moment?

It doesn't. The scene in which Snape casts the patronus is unrelated to why (or how) Snape's patronus shows up in The Forest of Dean later, and the final memory Harry sees in the pensieve which explains this was removed from the film:

And now Snape stood again in the headmaster’s study as Phineas Nigellus came hurrying into his portrait.

“Headmaster! They are camping in the Forest of Dean! The Mudblood – ”

“Do not use that word!”

“ – the Granger girl, then, mentioned the place as she opened her bag and I heard her!”

“Good. Very good!” cried the portrait of Dumbledore behind the headmaster’s chair. “Now, Severus, the sword! Do not forget that it must be taken under conditions of need and valor – and he must not know that you give it! If Voldemort should read Harry’s mind and see you acting for him – ”

“I know,” said Snape curtly. He approached the portrait of Dumbledore and pulled at its side. It swung forward, revealing a hidden cavity behind it from which he took the sword of Gryffindor.

“And you still aren’t going to tell me why it’s so important to give Potter the sword?” said Snape as he swung a traveling cloak over his robes.

“No, I don’t think so,” said Dumbledore’s portrait. “He will know what to do with it. And Severus, be very careful, they may not take kindly to your appearance after George Weasley’s mishap – ”

Snape turned at the door.

“Don’t worry, Dumbledore,” he said coolly. “I have a plan…”

A lot of the context needed for this scene had already been removed, to understand this you need to remember (from the books only):

• All portraits of former Headmasters at Hogwarts will do whatever the current headmaster asks of them
• Phineas Nigellus has a portrait in Sirius' House as well as the headmaster's study at Hogwarts (he's Sirius' great great grandfather and a former headmaster)
• When Harry, Ron and Hermione read in the Daily Prophet that Snape has been named headmaster of Hogwarts, Hermione puts the portait in her extended handbag so Phineas can't spy on them and report back to Snape
• During their time in exile, they occasionally talk to the portrait to try and get information about what's happening at Hogwarts