In English, if it is raining very heavily, you could say that it is 'raining cats and dogs'.
Is there an equivalent in Spanish?
Does Spanish use 'cats and dogs? If not why something different?
a cántaros 1. loc. adv. En abundancia, con mucha fuerza. Llover, caer, echar a cántaros.
being cántaro a (large) pitcher.
caer chuzos, o caer chuzos de punta
1. locs. verbs. coloqs. Llover, granizar o nevar con mucha fuerza o ímpetu.
llover chuzos, o llover chuzos de punta
1. locs. verbs. coloqs. Llover con mucha fuerza o ímpetu.
being chuzo a spiked stick.
- intr. impers. Llover copiosamente.
a más y mejor 1. loc. adv. Denota intensidad o plenitud de acción. Llover a más y mejor.
Besides the examples given above, two observations:
There seems to be a common theme of things falling from the sky point first (de punta) as a simile for heavy rain. In Spain the word chuzo is found, which the meaning of "stick with a metallic point". In Argentina the same word can be used with this pattern, though it's not so common and it means "small jury-rigged knife". The Portuguese cognate chuço means "spear" or "pike". Coincidentally, Brazilian Portuguese employs the simile chover canivetes, where canivete means "penknife" (this word exists also in Spanish, though I doubt it's in wide use.)
On top of the above, there is in Argentina a tendency to replace chuzos with obscene or escatological references. On top of that there has been an interesting development: since the pattern caer 𝒙 de punta expects 𝒙 to be a reference to some disagreable object, people in social media are now replacing 𝒙 by names of public people they dislike (typically politicians). Googling for this I just found an Argentinian Twitter user who, on the occasion of some observations of Podemos politician Íñigo Errejón about Argentina's government, tweeted: "Están cayendo errejones de punta".
It will be better if you use expressions like "está lloviendo a cántaros", or "está diluviando", since those are more common expressions than "llover a más y mejor" or "caer chuzos de punta" or "llueve a mares". In fact, if you say something like that in Argentina, you will look like a crazy dude, and with the "chuzo" expression... well, the word doesn't even exist here.
So, stay with "lloviendo a cántaros" or "está diluviando" (or simply "está lloviendo mucho"). It's far more neutral.