Veðurspáin segir að það verði skúrir í kvöld, en sólskin á morgun.

Veðurspáin is the definite form of veðurspá, which means weather forecast.

It breaks down like this:

• veður = weather
• spá = forecast, prediction

Together they form the compound noun veðurspá = weather forecast.

The ending -in is the definite article attached to the noun, so:

• veðurspá = a weather forecast
• veðurspáin = the weather forecast

That attached article is very normal in Icelandic.

Segir að means says that.

• segir = says
• að = that

So:

• Veðurspáin segir að ... = The weather forecast says that ...

In Icelandic, að is commonly used to introduce a subordinate clause after verbs like say, think, know, and similar verbs.

This is one of the most important grammar points in the sentence.

Verði is the subjunctive form of verða.

• verður = indicative
• verði = subjunctive

After segir að, Icelandic often uses the subjunctive when the speaker is reporting something predicted, uncertain, or not presented as a plain fact from the speaker’s own point of view. Weather forecasts are a very common place to see this.

So:

• Veðurspáin segir að það verði skúrir = The forecast says that there will be showers

If you made it a direct statement instead, you would more naturally get:

• Það verða skúrir í kvöld. = There will be showers tonight.

For a learner, the key takeaway is: after segir að, seeing a subjunctive like verði is normal.

Yes, það here works a lot like English it in weather expressions or impersonal statements.

In það verði skúrir, the það is a kind of dummy subject. It does not refer to a specific thing. It is just there because Icelandic, like English, often wants a subject in the clause.

Compare English:

• It will rain
• There will be showers

Icelandic often uses það in similar kinds of expressions:

• það rignir = it is raining
• það verði skúrir = there will be showers

So yes, it is translated loosely as it, but grammatically it is more of a structural subject than a meaningful noun.

Because skúrir is the plural of skúr in the weather sense of shower.

• skúr = a shower
• skúrir = showers

In weather forecasts, the plural is very common because it suggests scattered or repeated showers rather than one single shower event.

So:

• það verði skúrir í kvöld = there will be showers tonight

This sounds very natural in a forecast context.

The verb is being left out because it is understood.

The first part says:

• að það verði skúrir í kvöld = that there will be showers tonight

Then the second part says:

• en sólskin á morgun = but sunshine tomorrow

A fuller version would be something like:

• ... en það verði sólskin á morgun

But Icelandic often omits repeated words when the meaning is clear, especially in short forecast-style sentences.

So the second half is basically an ellipsis: but [there will be] sunshine tomorrow.

Because these are fixed time expressions, and you usually learn them as whole phrases.

• í kvöld = tonight / this evening
• á morgun = tomorrow

The prepositions do not match English word-for-word, so it is best not to translate them literally.

A good strategy is to memorize them as chunks:

• í dag = today
• í kvöld = tonight
• á morgun = tomorrow

That is much easier than trying to force English preposition logic onto Icelandic.

Sólskin means sunshine, and that is often the more natural weather word here.

• sól = sun
• sólskin = sunshine

In a forecast, sólskin suggests bright, sunny conditions. So:

• sólskin á morgun = sunshine tomorrow

That sounds more like a weather prediction than simply saying sun.

Because sólskin is being used in a general, uncountable sense, like sunshine in English.

Compare English:

• There will be sunshine tomorrow not usually
• There will be the sunshine tomorrow

Icelandic works similarly here:

• sólskin = sunshine, in general
• no definite article is needed

So sólskin á morgun means sunshine tomorrow, not the sunshine tomorrow.

Yes.

In a main clause, Icelandic typically follows the verb-second rule. But after að, you are in a subordinate clause, and the word order is different.

Here you get:

• að það verði skúrir í kvöld

Notice that það comes before verði.

That is normal subordinate-clause order. So a learner should notice this contrast:

• main clause: Veðurspáin segir ...
• subordinate clause: að það verði skúrir ...

You do not need to overanalyze it at first; just know that after að, Icelandic does not keep the usual main-clause verb-second pattern.

Yes. A fuller version could be:

• Veðurspáin segir að það verði skúrir í kvöld en að það verði sólskin á morgun.
• or more naturally:
• Veðurspáin segir að það verði skúrir í kvöld en sólskin á morgun.

The version you were given is more natural and economical. Weather language often leaves out repeated material when it is obvious.

So the original sentence is not strange or incomplete; it is simply concise, which is very typical of forecast style.