How to describe something that is very likely happening immediately?

Solution 1:

Certainly. You can say "It is about to rain" like you said. Depending on the context, you could also say:

It's probably going to rain soon.

Look at those clouds, it looks like rain.

It's going to rain.

(However, this question is a little vague. More detail about context would help.)

A single word would be :

Imminent : about to happen, occur, or take place very soon

For example:

His death is imminent

It is also correct the way you stated it, "It is about to..."

Just to make a distinction, "It is about to..." is best used for verbs, i.e. "rain", or "snow" or "melt". "It is about to melt."

"Imminent" is used with nouns e.g. "death", or "decision", or "arrival". "Uncle Fred's arrival is imminent."