When Do Rain And Flood Become Plural?

I've been questioning myself this question for sometime now. I know rain is rain and flood is flood.

I just that I have no bloody idea when should I use the singular or plural form? Of course I should use the plural form if I'm referring of the rain occurred many times in a day; or flood in plural if I'm referring to several flooding that happened in the past.

But I've seen the plural form of the same rain and the same flood being used in some newsreports.

I guess, it's time to do some research.