Avui plou, i hi ha molt vent al carrer.

Because Catalan does not use a dummy subject like English does in weather expressions.

• Plou = it is raining / it rains
• English needs it
• Catalan does not

So plou is a complete sentence by itself. The same happens with other weather verbs too.

Hi ha means there is or there are.

So:

• hi ha molt vent = there is a lot of wind

It is a very common Catalan structure, and it is best learned as a fixed expression.

Examples:

• Hi ha un cotxe. = There is a car.
• Hi ha cotxes. = There are cars.

Notice that Catalan normally still uses hi ha for both singular and plural.

In hi ha, hi is a locative particle. Historically, it has a sense a bit like there, but in modern Catalan you usually do not translate it word by word.

The important thing for learners is:

• learn hi ha together
• treat it as the normal way to say there is / there are

So in practice, hi is part of the standard existential expression.

Because vent is a masculine singular noun.

When molt means much / a lot of and comes before a noun, it agrees with that noun:

• molt vent = a lot of wind
• molta pluja = a lot of rain
• molts núvols = many clouds
• moltes fulles = many leaves

So here molt is masculine singular to match vent.

A useful contrast:

• plou molt = it rains a lot
  Here molt is an adverb, so it does not change.
• molt vent = a lot of wind
  Here molt is modifying a noun, so it does agree.

Al is the contraction of a + el.

• a = to / at / in
• el = the
• a + el = al

So al carrer literally means in the street or on the street.

Depending on context, English might also translate it more naturally as:

• outside
• out in the street

So hi ha molt vent al carrer can mean there is a lot of wind in the street / outside.

Yes. Fa vent is a very common way to say it is windy.

There is a small difference in feel:

• fa vent = it’s windy
• hi ha molt vent = there is a lot of wind

So hi ha molt vent sounds a bit more literal and focuses on the amount of wind, while fa vent is the more general weather expression.

Both are natural, depending on what you want to emphasize.

Usually, in a short sentence like this, the comma is not necessary.

A more neutral version would often be:

• Avui plou i hi ha molt vent al carrer.

The comma in your sentence is not impossible, but it adds a slight pause. In everyday writing, many people would leave it out.

Yes, it could go somewhere else, but putting avui first is very natural because it sets the time right away.

• Avui plou... = Today, it’s raining...
• Plou avui... is possible, but less neutral and more marked
• Hi ha molt vent al carrer avui is also possible, but sounds less straightforward

So Avui at the start is a very normal word order when you want to establish today as the topic.

Not always. Literally, yes, it means in/on the street, but in real use it can also mean something closer to outside, especially when talking about weather or what conditions are like outdoors.

So in this sentence, a learner should understand both ideas:

• literal: in the street
• natural English sense: outside

The exact nuance depends on context.