Pissing on a flat rock 

A sudden and heavy downpour of anything

Would you look at the rain? Just like a frog pissing on a flat rock.

The bills are piling up like a mule pissing on a flat rock

Pissing on a flat rock by Larin Noel October 2, 2016

Cow pissing on a flat rock 

A particularly hard rain storm. In other words, it's coming down so hard, it's as if a cow is pissing on a flat rock.

"Wow, look at them clouds rolling in."
"Yep, it's getting set to rain like a cow pissing on a flat rock!"

Cow pissing on a flat rock by HAL 9000 February 26, 2008

raining like a double-cunted cow pissing on a flat rock 

An archaic "hick" term for heavy rain or snow fall.

"Sheeeeeet! It sure is cumin' down out there! Raining like a double-cunted cow pissing on a flat rock!"

raining like a double-cunted cow pissing on a flat rock by Carl J. Maltese April 28, 2007

bull pissing on a flat rock

Raining very hard

How bad is it raining ? Like a bull pissing on a flat rock

bull pissing on a flat rock by #1toohottie November 30, 2017