Prema prognozi, popodne će se naoblačiti, ali bi se navečer moglo razvedriti.

Here prema means according to.

So prema prognozi = according to the forecast.

The form prognozi is used because prema normally requires the dative case in Croatian. The basic form is prognoza; the dative singular is prognozi.

• prognoza = forecast
• prema prognozi = according to the forecast

This is a very common pattern:

• prema planu = according to the plan
• prema zakonu = according to the law

Because Prema prognozi is an introductory phrase.

In English, we also often write a comma after phrases like According to the forecast, ... Croatian punctuation works similarly here. The comma helps separate the introductory frame from the main statement.

So the comma is natural and standard.

Here popodne is functioning as an adverb of time, meaning in the afternoon.

Croatian often uses words like this without a preposition where English would use in the:

• popodne = in the afternoon
• ujutro = in the morning
• navečer = in the evening

As a standalone word, popodne can also be a noun meaning afternoon, depending on context. In this sentence, though, it is clearly adverbial: it tells you when the weather change will happen.

Navečer is a fixed adverb meaning in the evening or toward evening / later in the evening, depending on context.

It is written as one word because it functions as a lexicalized adverb, not as a normal preposition + noun combination.

So in this sentence:

• popodne = in the afternoon
• navečer = in the evening

English learners sometimes expect something like u večer, but navečer is the normal adverb here.

Because both verbs are used in their reflexive / impersonal weather form:

• naoblačiti se = to become cloudy / to cloud over
• razvedriti se = to clear up

In weather expressions, Croatian very often uses se where English does not. So you should think of se here as part of the verb pattern, not as something separately translated word-for-word.

You generally learn these as whole expressions:

• Naoblačuje se. = It is getting cloudy.
• Razvedrilo se. = It cleared up.

Usually, no—not in this weather meaning.

In this sentence, se is part of the standard verbal expression:

• naoblačiti se
• razvedriti se

If you remove se, the sentence will usually sound wrong or will no longer express the same weather meaning naturally.

So it is best to memorize these as full units, not as bare verbs plus an optional se.

Because Croatian clitics—short unstressed words like će, bi, and se—have placement rules.

Here, će and se come near the beginning of the clause, after the first stressed element:

• popodne će se naoblačiti

That is normal Croatian word order.

Also, with the future tense, Croatian has two common patterns:

1. će + infinitive

   • će se naoblačiti

2. infinitive + će

   • naoblačit će se

In the second pattern, the final -i of the infinitive is usually dropped in writing:

• naoblačiti → naoblačit će

So both of these are possible in principle:

• Popodne će se naoblačiti.
• Popodne će se naoblačit is not the standard way to write it.
• Naoblačit će se popodne. is possible.

In your sentence, će se naoblačiti is perfectly normal.

Because it expresses a less certain prediction.

Compare:

• će se razvedriti = it will clear up
  → stronger, more definite prediction

• bi se moglo razvedriti = it might clear up / it could clear up
  → more cautious, less certain

So the sentence contrasts:

• a firmer expectation for the afternoon: će se naoblačiti
• a more tentative possibility for the evening: bi se moglo razvedriti

This is very natural in weather forecasts, where speakers often avoid sounding too certain.

Because this is an impersonal construction.

There is no real grammatical subject like on or ona here. Croatian often uses neuter singular in subjectless or impersonal expressions, especially with weather and general possibility.

So:

• moglo bi se razvedriti = it might clear up

The -lo ending is the neuter singular form, and that is the default choice here.

You can think of it as similar to English it might clear up, where it does not refer to a real object.

They are common weather-change verbs.

• naoblačiti se = to become cloudy, to cloud over
• razvedriti se = to clear up, to become fair/clear

These are not always translated word-for-word. It is better to learn them as weather expressions.

Examples:

• Naoblačuje se. = It’s getting cloudy.
• Poslije kiše se razvedrilo. = It cleared up after the rain.

Ali means but and gives a clear contrast:

• it will get cloudy in the afternoon,
• but it might clear up in the evening.

You might also see a in some contrasts, but ali is stronger and more clearly adversative. In this sentence, ali fits very well because the second part goes against the expectation created by the first part.

So ali is the most natural choice if you want a clear but.

Yes, Croatian word order is fairly flexible, but not completely free.

For example, you could also say:

• Prema prognozi, popodne će se naoblačiti, ali navečer bi se moglo razvedriti.

That version is also natural. The main difference is emphasis.

However, you still have to respect clitic placement rules for words like će, bi, and se. So not every rearrangement is possible.

A useful rule of thumb is:

• content words move more freely
• clitics have stricter positions

So learners should focus not only on meaning, but also on where short words like će, bi, and se are allowed to go.