I'm not sure if posting external links is alright so I'll just let you know that Omniglot has an interesting collection of idioms for heavy rain in many different languages. It seems that it's raining chair legs in Greece, it kills the mice in Serbia... and tractors are falling in Slovakia? I wonder if anyone could confirm if this is true?
In Czech(ia) (and Slovakia also I'm pretty sure) it's not tractors [cz: traktor] but wheelbarrow [cz: trakař]. It not just sounds very similar in czech, but also has similar purpose in agriculture. One is modern and one oldfashioned vehicle (but still used in the coutryside). Nevertheless, both are pretty painful when raining and hitting people's faces :)
I thought "cats and dogs" was pretty much a UK exclusive expression.. but maybe including Australia and New Zealand. We also talk of rain coming down "like stair rods," but stair rods are devices not now much used... so that term may be increasingly obscure, even to us Brits!