My favorite statistic is that 24.5 trillion gallons of water is 75 million acre-feet, or slightly more water than all California agriculture uses in 2 years.
If you have trouble visualizing this (as I do), the height of the Empire State Building is 0.2 miles. So imagine a sphere of water 20 times taller than the Empire State Building falling on New York.
Of course, the area and rate it falls on is crucial, but as a way to think about the scale of it I found this useful.
Edit: just saw this was done nicely by the article dreamcompiler linked to!
Will be targeting the English market
An acre-foot is the amount of water required to cover a one acre area in one foot of water, or about 1.2 million liters.
For water resources and agricultural uses, it's (given that you are using US Customary units for length and land area to start with) a reasonably sensible unit.