الطقس أسوأ اليوم.

A careful MSA reading is:

aṭ-ṭaqsu aswaʾu al-yawma

In pause, many speakers would say something closer to:

aṭ-ṭaqs aswaʾ al-yawm

A few pronunciation notes:

• الطقس starts with الـ, but because ط is a sun letter, the l sound assimilates, so you hear aṭ-ṭaqs, not al-taqs.
• أسوأ is pronounced as-waʾ. The final ء is a glottal stop.
• اليوم is al-yawm.

Because Arabic normally leaves out the present-tense verb to be in simple statements.

So:

• الطقس أسوأ اليوم = The weather is worse today

This is called a nominal sentence:

• الطقس = the subject
• أسوأ = the predicate
• اليوم = a time expression, today

If you wanted past or future, you would use a verb:

• كان الطقس أسوأ = The weather was worse
• سيكون الطقس أسوأ = The weather will be worse

أسوأ means worse or worst, depending on context.

In this sentence, it most naturally means worse:

• الطقس أسوأ اليوم = The weather is worse today

It is the comparative/superlative form related to bad.

A useful contrast:

• سيئ = bad
• أسوأ = worse / worst

So أسوأ is not just describing bad weather; it is describing a higher degree of badness.

Because the meaning is different:

• الطقس سيئ اليوم = The weather is bad today
• الطقس أسوأ اليوم = The weather is worse today

So:

• سيئ gives a basic description: bad
• أسوأ gives a comparison: worse

If the English meaning shown to the learner is worse, then أسوأ is the correct choice.

Arabic can leave the standard of comparison understood from context.

So الطقس أسوأ اليوم can mean:

• the weather is worse today than yesterday
• the weather is worse today than usual
• the weather is worse today than expected

If you want to say the comparison explicitly, Arabic uses من for than:

• الطقس أسوأ من أمس = The weather is worse than yesterday
• الطقس أسوأ من الأسبوع الماضي = The weather is worse than last week

Yes, it can move. Arabic word order is fairly flexible, especially with time expressions.

All of these are possible:

• الطقس أسوأ اليوم
• اليوم الطقس أسوأ
• الطقس اليوم أسوأ

The differences are mostly about emphasis:

• الطقس أسوأ اليوم = neutral, very natural
• اليوم الطقس أسوأ = puts more focus on today
• الطقس اليوم أسوأ = also natural, with today attached closely to the weather

With full case endings, the sentence is:

الطقسُ أسوأُ اليومَ

Why?

• الطقسُ is the subject of the nominal sentence, so it is nominative
• أسوأُ is the predicate, so it is also nominative
• اليومَ is a time adverb (ẓarf), so it is accusative

In normal Arabic writing, these short vowels are usually not written, so you just see:

الطقس أسوأ اليوم

Because Arabic normally uses the definite form here, just as English says the weather.

So:

• طقس = weather / a weather condition, as a dictionary form
• الطقس = the weather

In an everyday sentence about current conditions, الطقس is the natural choice.

In careful grammar, أسوأ belongs to the أفعل pattern used for many comparative/superlative forms, and these forms are usually diptotes in standard Arabic.

That means they do not take tanwīn.

So in full reading you say:

• أسوأُ

not

• أسوأٌ

This is a grammar detail that many learners notice only later, but it is completely normal here.

Yes, الطقس أسوأ اليوم is a natural MSA sentence.

Another very common word you may hear is الجو:

• الجو أسوأ اليوم

Both are understandable, but:

• الطقس is very standard and neutral for weather
• الجو is also common and can feel a bit more everyday, depending on the context and variety of Arabic