One way to say this is 要下雨了(yào xiàyǔ le).

The 要(yào)…了(le) construction can be used to talk about actions or events that “are going to” happen soon.

Examples:

要周末了，你打算做什么(Yào zhōumò le, nǐ dǎsuàn zuò shénme)?
It’s going to be the weekend soon, what are you planning to do?

已经一点了，我们要迟到了(Yǐjīng yī diǎn le, wǒmen yào chídào le).
It’s already one o’clock, we’re going to be late.

飞机要起飞了，请大家坐好(Fēijī yào qǐfēi le, qǐng dàjiā zuò hǎo).
The plane is going to take off soon, everyone please be seated.