المطر شديد اليوم.

In Modern Standard Arabic, the verb to be is usually not expressed in the present tense.

So instead of saying The rain is heavy today, Arabic simply says:

• المطر شديد اليوم

This is called a nominal sentence. It consists of:

• المطر = the subject
• شديد = the predicate
• اليوم = a time expression, today

If you wanted was or will be, Arabic would use a verb:

• كان المطر شديدًا = The rain was heavy
• سيكون المطر شديدًا = The rain will be heavy

المطر includes الـ because Arabic often uses the definite form for natural phenomena and general categories in a way that sounds normal and idiomatic.

So المطر literally looks like the rain, but in context it can simply mean rain in a general sense.

This is very common in Arabic. You should not always expect definiteness to match English word-for-word.

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

In Arabic nominal sentences, a common pattern is:

• definite subject + indefinite predicate

So:

• المطر = definite subject
• شديد = indefinite predicate

That is why شديد does not take الـ here.

Compare:

• المطر شديد = The rain is heavy
• المطر الشديد = the heavy rain

In المطر الشديد, الشديد is an adjective describing المطر inside a noun phrase, so it must match it in definiteness.

Yes, but more specifically it is a predicate adjective.

It describes the subject المطر, but it is not part of the same noun phrase. Instead, it is telling you something about the subject.

So the structure is not:

• heavy rain

It is:

• the rain [is] heavy

That distinction explains why شديد is indefinite here.

Because المطر is a masculine singular noun, and شديد agrees with it.

In Arabic, adjectives and predicate adjectives usually agree with the noun in:

• gender
• number

Since المطر is masculine singular, the form is:

• شديد

If the noun were feminine, you would usually expect:

• شديدة

اليوم is a time expression meaning today, and placing it at the end is a very natural word order in Arabic.

So:

• المطر شديد اليوم = The rain is heavy today

But Arabic word order is flexible, so you may also see:

• اليوم المطر شديد
• المطر اليوم شديد

These are all possible, though they may differ slightly in emphasis or style.

The version with اليوم at the end is simple and very natural.

In fully vocalized formal Arabic, it would be:

• المطرُ شديدٌ اليومَ
• al-maṭaru shadīdun al-yawma

Here is why:

• المطرُ is the subject (mubtadaʾ) and gets ḍamma
• شديدٌ is the predicate (khabar) and also gets ḍamma
• اليومَ is an adverbial time expression and commonly takes fatḥa

In normal pause pronunciation, especially in everyday reading, people often drop the final short vowels:

• al-maṭar shadīd al-yawm

Because اليوم is the normal Arabic word for today, and it is commonly used with الـ.

Historically and grammatically, it comes from the day, but as a fixed expression it means today.

So learners should usually just memorize:

• اليوم = today

Yes. That would also be correct, but it has a slightly different nuance.

• شديد means strong / intense / severe
• غزير often means abundant / heavy and is very commonly used for rain

So both can work, but they are not always identical in feel. Arabic often has several natural ways to describe weather.

Yes, absolutely.

• المطر شديد اليوم
• المطر اليوم شديد

Both are grammatical.

The difference is mainly one of focus and rhythm, not basic meaning.
With المطر اليوم شديد, the word today is brought closer to the rain, which can make the timing feel slightly more prominent.

It is a nominal sentence (جملة اسمية).

Its main parts are:

• المطر = mubtadaʾ (the topic / subject)
• شديد = khabar (the comment / predicate)
• اليوم = adverbial expression of time

This is one of the most important basic sentence patterns in Arabic:

• Noun + predicate

For example:

• الجو جميل = The weather is nice
• الباب مفتوح = The door is open
• المطر شديد اليوم = The rain is heavy today

Because predicates do not have to match the subject in definiteness the way attributive adjectives do.

There are two different situations:

1. Predicate adjective

   • المطر شديد
   • The rain is heavy

2. Attributive adjective

   • المطر الشديد
   • the heavy rain

In the second case, the adjective is part of the noun phrase and must match in definiteness.
In the first case, شديد is the predicate, so being indefinite is normal.

Yes. It is a fully complete and natural sentence in Arabic.

Arabic does not need an explicit present-tense is, so:

• المطر شديد اليوم

already gives a complete statement.

Nothing is missing grammatically.