大雨です。
Ōame desu
It is heavy rain.
大雨 = heavy rain
大雨で外に出られません。
Ōame de soto ni deraremasen
It's heavy rain, can't go outside.
出られない = cannot go out, natural
土砂降りで傘も役に立たないくらいですよ！
Doshaburi de kasa mo yaku ni tatanai kurai desu yo!
It's a downpour so bad even an umbrella is useless!
役に立たない = useless; くらい = to the degree that
More Japanese Weather Phrases