[#1152383] いきなりザアッと雨が激しく降ったかと思ったら、今はカラッと晴れている。
Does the "was about to" in this sentence indicate it was thought to rain, but it didn't? If so, this doesn't match.
降った = rained

Yes. It means that I thought it was going to rain (soon), but it didn't.

Could someone please propose a rewording in English?

If (heavy) rain is meant, "bucketing down" is fine:

https://www.collinsdictionary.c...sh/bucket-down

In this case, 思ったら doesn't suggest what the speaker thought, it implies the speaker's surprise.
https://nihongokyoshi-net.com/2...%81%AB%E6%9D%A5%E3%81%AA%E3%81%84%E3%80%82

I might be wrong, but I think it could be translated as "No sooner was sudden downpour, now the weather clears up".
I'm not sure it could convey the speaker's surprise, though.

So could this be translated as follows?

First it starts bucketing [it] down, then all of a sudden the weather clears up. OR
First it starts bucketing [it] down, then out of nowhere the weather clears up.

I added "[it]" because @tsp_2 has inserted it in the meantime. I hadn't heard of either, but Google Ngram Viewer has (though it says "bucketing down" is considerably more common than "bucketing it down").

> "bucketing down" is considerably more common than "bucketing it down"

It's usually "bucketing down" (no "it"), but 'chucking *it* down', which is another way of saying the same thing.