At the moment, we are getting heavier rainfall in the 98 double-ought 3 than is usual.
So it may be worth noting that, when attempting to use colorful colloquial expressions, one should use less than prefect grammar.
The phrase is "Rainin' like a cow pissin' on a flat rock". Note the vulgarism and the dropped "g's".
Saying "Precipitating like a bovine urinating on a planar stone" will win you no street cred at all.