It's not a common expression at all. Because it's so amusing, it's always at the top of people's lists of 'strange things that English speakers' say. Both native and non-native speakers invariably refer to this phrase as a typical example of a funny English idiom. But the fact is that it's very rarely used, and never among younger people. As you point out, we tend to say something much simpler, such as 'It's pouring.'