When it rains, it pours describes how after there has been no rain for a long time, it suddenly rains a lot all at once. Example: "It hadn't rained for over two months. Now it has started raining and it has been raining for a week straight." Reply: "When it rains, it pours."

When it rains, it pours means that once something happens after a long pause, it happens in large amounts. Example: "It looks like everyone in our department is sick again, and all at the same time." Reply: "When it rains, it pours."

When there is a lot of rain all at once we say that it is pouring. Some people say, It never rains, but it pours; the meaning is the same as When it rains, it pours. Example: "Sometimes we have no customers for two or three hours, then suddenly we get 20 people all at once." Reply: "It never rains, but it pours."