الطقس سيئ اليوم.

In the present tense, Arabic usually does not use a separate word for is/am/are.

So:

الطقس سيئ اليوم
literally looks like: the weather bad today
but it means: The weather is bad today.

This is called a nominal sentence in Arabic: a subject plus a predicate, with no present-tense to be.

If you wanted was, Arabic would use a verb, for example:

كان الطقس سيئًا اليوم = The weather was bad today

الـ is the Arabic definite article, meaning the.

So:

• طقس = weather / a weather condition
• الطقس = the weather

In this sentence, الطقس is the subject: the weather.

Because ط is a sun letter.

When الـ comes before a sun letter, the l sound of الـ is not pronounced, and the next consonant is doubled.

So:

• written: الطقس
• pronounced: aṭ-ṭaqs

This is a very common feature of Arabic pronunciation.

By contrast, in اليوم, the l is pronounced, because ي is not a sun letter:

• اليوم = al-yawm

Because سيئ here is the predicate of the sentence, not an adjective directly attached to the noun.

In الطقس سيئ اليوم:

• الطقس = the subject
• سيئ = the predicate, meaning bad

In Arabic, the predicate in this kind of sentence is often indefinite, so سيئ appears without الـ.

This difference is important:

• الطقس سيئ = The weather is bad
• الطقس السيئ = the bad weather

So adding الـ to سيئ changes the structure and meaning.

It is a word that means bad, so in meaning it behaves like an adjective. But in this sentence, grammatically, it is functioning as the predicate (خبر) of the sentence.

So:

• الطقس السيئ = the bad weather
  Here السيئ is an adjective modifying الطقس.
• الطقس سيئ = the weather is bad
  Here سيئ is the predicate.

English learners often confuse these two patterns, so this is a very useful distinction to notice.

It is pronounced approximately sayyiʾ.

A few things to notice:

• the ي is effectively doubled
• the final ء is a hamza, a glottal stop
• the word does not end smoothly like English sayy; it ends with a little stop: say-yiʾ

In fully vowelled Arabic, it is usually written:

سَيِّئ

Because Arabic uses اليوم literally as the day, but in usage it commonly means today.

So in many sentences:

• اليوم = today

This is just the normal Arabic way to express that idea.

In this sentence, اليوم gives the time: today.

In fully vowelled formal Arabic, the sentence would be:

الطَّقْسُ سَيِّئٌ اليَوْمَ

The endings are:

• الطقسُ — nominative, because it is the subject
• سيئٌ — nominative, because it is the predicate
• اليومَ — accusative, because it functions as an adverbial expression of time

In normal modern writing, these short vowels are usually omitted, so you normally see:

الطقس سيئ اليوم

Yes, Arabic has some flexibility in word order.

The most straightforward version is:

الطقس سيئ اليوم = The weather is bad today

But you may also see:

• اليوم الطقس سيئ = Today, the weather is bad
• الطقس اليوم سيئ = The weather today is bad

These are all understandable, but they can shift the emphasis slightly.

الطقس is treated as masculine singular in Arabic.

That is why the predicate appears in the masculine singular form:

• سيئ

If the subject were feminine singular, the predicate would normally be feminine too. For example:

• السماء صافية = The sky is clear

So yes, grammatical gender matters here.

A common MSA version is:

الطقس ليس سيئًا اليوم

Here ليس is used to negate a present-tense nominal sentence.

Notice that سيئًا now takes a different case ending in fully vowelled Arabic because of ليس.

In normal unvowelled writing, you will usually just see:

الطقس ليس سيئًا اليوم or الطقس ليس سيئا اليوم

Both are commonly encountered in everyday text.