It was raining heavily but now it has decreased so I can go out.

[tufguy]

It was raining heavily but now it has decreased so I can go out.

Is it correct to say "rain has decreased"?

 

[5jj]

No. Try:

It has been/was raining heavily but now it has eased* so I can go out.

* or eased up/eased off.

 

[Tarheel]

Or "let up".

I don't think the rain let up so you could go out. It's more like the other way around. You decided to go out because the rain eased up.