Seinna verður aðeins skýjað, en veðrið helst samt næstum alveg logn allan daginn.

Here seinna means later or later on. It introduces a later stage in the forecast, so the idea is later in the day rather than simply late.

It is related to the comparative form of seint (late), but in this sentence it works as an adverb.

English weather sentences normally need a dummy subject: it will be cloudy.

Icelandic often does not need to say an explicit dummy subject in weather expressions. So a clause like verður skýjað is natural even without það.

That is why the sentence can begin directly with Seinna verður...

verður is the 3rd person singular of verða.

In weather forecasts, verða often means become, turn, or simply be in a future sense. So:

• verður skýjað = it will become cloudy
• or more naturally in a forecast, it will be cloudy

So Seinna verður aðeins skýjað suggests a change or later condition.

aðeins can mean only, just, or slightly, depending on context.

In a weather sentence like this, it softens the description. So aðeins skýjað does not usually mean a strong nothing but clouds idea. It is closer to:

• a little cloudy
• somewhat cloudy
• slightly cloudy
• sometimes partly cloudy, depending on how the forecast is being phrased

Weather expressions in Icelandic are very often built with a neuter singular descriptive form.

So skýjað is the neuter form used in an impersonal weather construction, similar to:

• Það er skýjað = It is cloudy

Even when no subject is stated, the neuter singular form is the normal one for this kind of expression.

It is the same form, but not the same function.

Here helst is a verb, not an adverb. It means remains, stays, or keeps.

In this sentence:

• veðrið helst logn = the weather stays calm

So here helst comes from the verb haldast, not from the adverb meaning preferably.

They do different jobs:

• en = but
• samt = still / nevertheless / all the same

So the sentence is not repeating the same word twice. Instead, en links the two clauses, while samt adds the idea of contrast:

It gets a bit cloudier, but the weather still remains calm.

That combination is very natural in Icelandic.

Usually, logn is a noun, meaning calm, stillness, or specifically no wind.

It is very common in weather language:

• Það er logn = It is calm
• veðrið helst logn = the weather stays calm

So even though it may look like the kind of word English would express with an adjective, Icelandic often uses the noun logn in this way.

Literally, it means almost completely calm.

Word by word:

• næstum = almost
• alveg = completely / entirely
• logn = calm, no wind

Together, the phrase emphasizes that there will be very little wind.

allan daginn means all day or throughout the day.

This uses the accusative case because Icelandic often puts expressions of duration of time in the accusative.

Breakdown:

• dagurinn = the day (nominative)
• daginn = the day (accusative)
• allan agrees with daginn in masculine accusative singular

So allan daginn is the normal way to say all day long.

Because Icelandic is a verb-second language in main clauses.

That means if something other than the subject comes first, the finite verb usually still comes second. So:

• Seinna verður aðeins skýjað

Here:

1. Seinna comes first
2. verður comes second

This word order is one of the most important patterns in Icelandic main clauses.

The first clause is an impersonal weather expression, so no explicit subject is needed.

The second clause is built more like a normal clause with a subject:

• veðrið = the weather
• helst = remains
• logn = calm

So the sentence switches from an impersonal weather statement to a regular subject-based clause. Both patterns are completely natural in Icelandic.