I got curious about how much water a storm cloud can hold. Turns out to be a LOT. I decided to go with an inch of rain over a square mile: (5280x12)2 cubic inches. That's about 17,378,725.5 gallons! Some of the tropical systems we got in the last month brought a lot more rain than that, and over a much wider area — easily multiple billions of gallons.
Those big dark clouds hold a mind-boggling amount of water.