Αν δεν έχει συννεφιά και υγρασία αύριο, θα πάω βόλτα μετά τη δουλειά.

In Greek, a real or likely future condition is usually expressed with:

• αν
  • present/non-past verb in the if clause
• θα
  • verb in the main clause

So:

• Αν δεν έχει συννεφιά και υγρασία αύριο, θα πάω βόλτα... = If it isn’t cloudy and humid tomorrow, I’ll go for a walk...

You normally do not put θα right after αν in standard Greek here.
So αν δεν θα έχει... would sound nonstandard in this kind of sentence.

Here έχει is part of an impersonal weather expression. It does not really mean someone literally has something.

Greek often uses έχει for weather or atmosphere:

• Έχει κρύο = It’s cold
• Έχει ζέστη = It’s hot
• Έχει ήλιο = It’s sunny
• Έχει συννεφιά = It’s cloudy / overcast
• Έχει υγρασία = It’s humid

So in this sentence, έχει is best understood as there is / it is, not as a normal possession verb.

Greek very often describes weather with nouns in expressions like έχει + noun.

So instead of saying something literally like it is cloudy/humid, Greek can say:

• έχει συννεφιά = there is cloudiness / it’s cloudy
• έχει υγρασία = there is humidity / it’s humid

This is completely natural Greek.
A learner may expect adjective-based wording because English prefers adjectives, but Greek often uses noun-based weather expressions.

Because in these weather expressions, Greek often uses bare nouns.

So:

• έχει συννεφιά
• έχει υγρασία

sounds natural and general.

If you added articles, it would usually sound more specific or less neutral, as if you were referring to a particular humidity or a particular cloud cover already known from context.

In everyday weather talk, the article is usually omitted in this pattern.

Yes, in context it will usually be understood as meaning something like:

• if it isn’t cloudy and humid tomorrow
• or if there’s no cloudiness and no humidity tomorrow

But this is a good question, because under negation, και can feel slightly less explicit to an English speaker.

If you want to make neither... nor... very clear, Greek can say:

• Αν δεν έχει ούτε συννεφιά ούτε υγρασία αύριο...

That is more explicitly if there is neither cloudiness nor humidity tomorrow.

So the original sentence is natural, but ούτε... ούτε... is the clearer, stronger version.

Yes, αύριο is fairly flexible in Greek.

The original sentence says:

• Αν δεν έχει συννεφιά και υγρασία αύριο...

But you could also say:

• Αν αύριο δεν έχει συννεφιά και υγρασία...
• Αύριο, αν δεν έχει συννεφιά και υγρασία, θα πάω βόλτα...

These are all possible, with slightly different rhythm or emphasis.

Greek word order is often more flexible than English, especially with adverbs like αύριο.

By itself, πάω can indeed mean I go / I’m going in everyday Greek.

But with θα, it becomes future:

• θα πάω = I will go

This is the normal simple future form for a one-time action here.

Also, Greek distinguishes aspect:

• θα πάω = I will go once / on that occasion
• θα πηγαίνω = I will be going / I will go regularly

So in this sentence, θα πάω is exactly what you want, because it refers to one future event.

Πάω βόλτα is a very common expression meaning:

• I go for a walk
• I go out for a stroll
• sometimes more loosely I go out a bit

The noun βόλτα can mean a walk, stroll, outing, or little trip around.

In this expression, Greek usually does not use an article:

• πάω βόλτα
• βγαίνω βόλτα

So this is best learned as a fixed everyday phrase.

In Modern Greek, μετά normally takes the accusative in expressions like this.

So:

• μετά τη δουλειά = after work

Here τη is just the shorter form of την, which is very commonly used before a consonant in everyday language:

• την δουλειά → τη δουλειά

So:

• μετά τη δουλειά

is completely normal.

You may also hear:

• μετά από τη δουλειά

which is also natural.

Greek often drops subject pronouns because the verb ending already shows the person.

• θα πάω already means I will go

So εγώ is unnecessary unless you want emphasis or contrast:

• Εγώ θα πάω, αλλά ο αδερφός μου όχι. = I will go, but my brother won’t.

This omission of subject pronouns is very common in Greek and is something English speakers usually need time to get used to.

It is natural everyday Greek.

A Greek speaker could definitely say:

• Αν δεν έχει συννεφιά και υγρασία αύριο, θα πάω βόλτα μετά τη δουλειά.

Depending on personal style, you might also hear slightly different versions, such as:

• Αν αύριο δεν έχει συννεφιά και υγρασία, θα πάω βόλτα μετά τη δουλειά.
• Αν δεν έχει ούτε συννεφιά ούτε υγρασία αύριο, θα πάω βόλτα μετά τη δουλειά.

But the original sentence is perfectly normal and conversational.