Tim Bowen considers alternatives to this entertaining idiom.
Most learners of English will be familiar with the idiom It’s raining cats and dogs. Indeed, many people remember it because it’s such an odd expression and one which can conjure up quite entertaining images. There is, however, one small problem attached to this idiom: native speakers of English rarely use it and, if they do, it sounds rather old-fashioned. So, what do they say? Well, as with so many things, it depends on the context and who you are addressing your remarks to. In a polite, formal situation one might simply say It's pouring, with the optional addition of the slightly superfluous words with rain: It’s pouring with rain or It’s pouring down. Less formal alternatives are It’s chucking it down or It’s bucketing down, neither of which would be considered impolite in any way. If the rain is particularly heavy, the verb lash down can be used, as in 'It’s been lashing down for an hour now'. In British English one of the most common expressions is It's pissing down, although caution should be exercised when using this expression as some people may consider it impolite or even offensive. To be on the safe side it might be better to stick to It’s pouring down or the entertaining expression It’s pouring with rain out there, which suggests that it sometimes rains in here.