pouring
v.v. flowing quickly and in large amounts. You use this to describe heavy rain or when you move a lot of liquid from one container to another.
v. flowing rapidly and steadily in a continuous stream. Often used intransitively to describe heavy precipitation or transitively when transferring liquids between vessels.
It is pouring with rain outside today.
The waiter was busy pouring water for the guests as they sat down at the table.
While the rain was pouring down in sheets against the window, the chef continued pouring the thick sauce over the pasta with practiced precision.
When describing weather, it is often used with 'with' (pouring with rain) or as a standalone verb (it is pouring).
It is pouring rainIt is pouring with rainIn British English, the expression for heavy rain usually requires the preposition 'with', though 'it is pouring' alone is also correct.