Learn Swedish: Kommer det att blåsa mycket ikväll?

Just like with other verbs, you swap the subject and the verb. Instead of the statement det kommer (it will), you start with kommer det (will it). The word att and the infinitive verb blåsa stay right where they normally go.

Why does the sentence use blåsa (to blow) instead of the word for 'windy'?

In Swedish, it is much more common to describe this weather using the verb att blåsa. Instead of asking 'will it be very windy', a Swedish speaker literally asks 'will it blow much?'

What is the purpose of det here?

It acts exactly like the 'it' in English weather sentences (like 'it is raining'). Since weather doesn't have a real person or thing doing the action, det steps in as a placeholder subject.

Why use ikväll for 'tonight' instead of i natt?

ikväll means 'this evening', referring to the time before you go to bed. i natt refers to the deep night when people are asleep. For an active weather prediction you are experiencing, ikväll is usually what people mean.