下大雨了。
Xià dà yǔ le
Big rain fell.
大 = big/heavy
外面下大雨了，最好别出门。
Wài miàn xià dà yǔ le, zuì hǎo bié chū mén
Outside heavy rain, best not to go out.
最好别 = best not to, common advice structure
外面倾盆大雨，根本没法出门！
Wài miàn qīng pén dà yǔ, gēn běn méi fǎ chū mén!
Outside torrential rain, absolutely no way to go out!
倾盆大雨 = classic idiom (pouring basin); 根本没法 = absolutely no way
More Chinese (Mandarin) Weather Phrases