like a cow on a flat rock

Adverb

US not-comparable of raining

(US, of raining, pissing, etc) Heavily, copiously, with a great deluge.

Japanese Meaning

猛烈に、豪快に、土砂降りのように（雨が降る、あるいは他の液体が勢いよく流れる様子を表す） / 大量に、溢れ出るほど（何かが行われるさまを形容する表現）

It was raining like a cow on a flat rock all afternoon, so the picnic was a disaster.

午後ずっと土砂降りで、ピクニックは台無しになった。

Sense（1）

(US, of raining, pissing, etc) Heavily, copiously, with a great deluge.

I better shake a leg before them clouds cut loose like a cow on a flat rock.