Wàimiàn xià zhe piáopō dàyǔ.

外面下着瓢泼大雨。

It's raining cats and dogs outside.

Notes:

外面(wàimiàn): out door

下着(xià zhe): be raining

瓢泼大雨(piáopō dàyǔ): extreamly heavy rain

Example:

A：外面下着瓢泼大雨。

     Wàimiàn xià zhe piáopō dàyǔ. 

     It's raining cats and dogs outside.

B：我有伞。

     Wǒ yǒu sǎn.

     I have an umbrella.