Kamo ćete ako se do večeri otopli?

Kamo asks about destination or direction: where to? / where are you going? / where will you go?

• Kamo ćete...? = Where will you go...?
• Gdje ste? = Where are you?
• Gdje ćete biti? = Where will you be?

A simple way to remember it:

• gdje = location
• kamo = movement toward a place

So kamo is the natural choice here because the sentence is about going somewhere.

Because ćete is a clitic form, and Croatian clitics usually go in second position in the clause.

Here:

• Kamo = first element
• ćete = clitic in second position

So Kamo ćete...? is the normal word order.

This same pattern appears very often:

• Kada ćete doći? = When will you come?
• Zašto ste otišli? = Why did you leave?

The question word takes first position, and the clitic follows it.

Yes, the full version could be:

Kamo ćete ići ako se do večeri otopli?

But in Croatian, especially in everyday speech, the verb of motion can be left out when it is obvious from kamo.

So:

• Kamo ćete?
  naturally means
• Kamo ćete ići?

This omission is very common and sounds natural, not incomplete.

It can be either:

• you all / you plural
• you singular, but in the formal/polite sense

So without context, ćete is ambiguous in exactly the same way that many European languages have a formal you.

Examples:

• speaking to several friends: Where will you all go...?
• speaking politely to one person: Where will you go, sir/ma’am...?

Because in Croatian, after ako (if), you normally do not use the future auxiliary će for this kind of future condition.

Croatian usually uses the present form, and very often a perfective present, to talk about a future event in if-clauses.

So Croatian says:

• ako se do večeri otopli

not usually:

• ako će se do večeri otopliti

This is a very important pattern:

• Ako dođe, razgovarat ćemo. = If he comes, we’ll talk.
• Ako prestane kiša, idemo van. = If the rain stops, we’re going out.

So the Croatian structure is different from English, but completely normal.

Otopli is a 3rd person singular present form, but here it refers to a future result because it is used after ako.

In this sentence it means something like:

• if it gets warmer
• if there is a thaw / warm-up by evening

The subject is not a specific person or thing. It is an impersonal weather expression, similar to English if it gets warmer.

Here se is part of the verb expression. It does not literally mean itself in a way you need to translate word-for-word.

The phrase otopliti se / otopli se means:

• to get warmer
• to warm up
• sometimes to thaw

In weather expressions, Croatian often uses verbs with se where English just uses a plain verb.

So it is best to learn se otopli as a whole expression in this kind of sentence.

Do večeri means by evening or until evening.

The preposition do requires the genitive case, so večer changes to večeri.

• nominative: večer = evening
• genitive: večeri

So:

• do večeri = by evening / until evening

This is a very common pattern:

• do jutra = by morning
• do sutra = by tomorrow
• do kraja tjedna = by the end of the week

In this sentence, the best interpretation is by evening: the warming happens at some point before evening arrives.

So ako se do večeri otopli means:

• if it gets warmer by evening

In other contexts, do can sometimes feel closer to until, but here by evening is the most natural English match.

Yes. Croatian word order is flexible, and a version like this is also possible:

Ako se do večeri otopli, kamo ćete?

That means the same thing. The difference is mainly focus and rhythm:

• Kamo ćete ako se do večeri otopli?
  focuses first on the destination question
• Ako se do večeri otopli, kamo ćete?
  sets up the condition first

Both are grammatical.

It is basically neutral Croatian.

Nothing in it is slangy. The only thing to notice is that omitting ići after ćete is very natural in everyday speech, so the sentence sounds normal and idiomatic rather than overly formal.

If you wanted a fuller version, you could say:

Kamo ćete ići ako se do večeri otopli?

That is also correct, just slightly more explicit.