已经下雨

The rain came down heavily all night.

大雨下了一整夜。

The rain poured steadily down.

大雨不停地倾盆而下。

It rained pitchforks last night.

昨晚开始下倾盆大雨。

正在下雨

It's raining hard/heavily outside.

外面正在下大雨。

The rain is pelting down.

下大雨了。

It's raining like old boots.

大雨倾盆。

It's raining dogs and cats.

正在下大雨。

将要有雨
There must be a heavy rain soon.

马上要下大雨了。

A downfall is expected, so take an umbrella.

估计要下大雨,把伞带上。

It's going to pour with rain.

要下大雨了。