It's going to rain big

[Silverobama]

I don't know how to describe this. The clouds made the sky dark and was going to storm soon. I told my friend who might not have an umbrella:

It's going to rain big.

I don't think it's natural but please tell me how native speakers would phrase this.

 

[teechar]

The clouds made the sky clouded up and got dark and was going to a storm was forming/imminent. soon. I told my friend who might not have an umbrella:

"Might" doesn't make sense there. Do you mean your friend did not have an umbrella?

It's going to rain big.

No. Try, for example:
It's going to pour down.
It's going to bucket down.

 

[emsr2d2]

Don't try to use "storm" as a verb.

 

[tedmc]

It's going to rain cats and dogs.

 

[Rover_KE]

It’s going to rain heavily.

 

[teechar]

For our learners:

Don't try to use "storm" as a verb.

I agree that in its literal sense (as in a weather phenomenon) "storm" cannot be used as a verb.

However, it does have another meaning as a verb, and it's common enough. See below.

storm

1. an extreme weather condition with very strong wind, heavy rain, and often…

dictionary.cambridge.org

 

[teechar]

It's going to rain cats and dogs.

That phrasing wouldn't be very common.
The usual is "It's/it was/it's been raining cats and dogs".

 

[emsr2d2]

For our learners:

I agree that in its literal sense (as in a weather phenomenon) "storm" cannot be used as a verb.

However, it does have another meaning as a verb, and it's common enough. See below.

storm

1. an extreme weather condition with very strong wind, heavy rain, and often…

dictionary.cambridge.org

Sorry, I should have been clearer that I was talking about the context of this thread. The use of the verb "storm" is indeed common and forms part of one of my favourite phrasal verbs, "storm off", to describe the way in which teenagers (and some people old enough to know better) walk away when angry or frustrated.

 

[BobK]

Another phrase you might come across (especially in older speakers, who knew what these things were) is 'Coming down in stair-rods'. But this is appropriate only when it's absolutely bucketing down - not in the 'looks like rain' context you describe.

 

[kadioguy]

I told my friend who might not have an umbrella:

It's going to rain big.

"Might" doesn't make sense there. Do you mean your friend did not have an umbrella?

As a Chinese speaker, I think I may know what Silverobama was trying to say, so I'd suggest two ways of rephrasing it:

a. I told my friend that it's going to rain heavily, in case they didn't have an umbrella.

b. I told my friend, who might not have an umbrella with them, that it's going to rain heavily.

 

[BobK]

'In case they didn;t <whatever>' is a good way of rendering this sort of 'might'.

 

[tedmc]

That phrasing wouldn't be very common.
The usual is "It's/it was/it's been raining cats and dogs".

I'm not sure about that, but it's been used with the future tense and with a modal:

https://fraze.it/n_search.jsp?q=rain+cats+and+dogs&l=0

 

[teechar]

I'm not sure about that,

I don't know what you mean by that.

but it's been used with the future tense and with a modal:

https://fraze.it/n_search.jsp?q=rain+cats+and+dogs&l=0

There's only one result involving the future in that link, and it does not involve using a modal.