A potentially useful and extensive list of words here for describing wet weather:
http://www.bbc.co.uk/cymrufyw/41684463
It’s all in Welsh but, if needed, switching on ‘Vocab’ on that page should help with many of the words
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I could be biased, but my favourite is “Glaw tinwyn Abertawe”, which the Vocab misses - something like Swansea white tailed rain.
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Bwrw glaw yn sobor iawn is a favourite of a friend of mine. It means something like seriously raining, and features in its own nursery rhyme.
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To be a bit “unsirious”, my most liked phrase about weather when I was in Wales was “bwrw cwrw”. For various reasons I just didn’t want to accept that it’s raining. Probably because back home there it was about 35 °C while in Wales only about 20. 
So “Mae hi’n bwrw cwrw.” it is …
Don’t mention “Bwrw eira”. I don’t like this part even in my own country.
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See Welsh idioms raining old ladies and sticks, bwrw hen wragedd a ffyn
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