Rain water can contain contaminants from dust but in any situation short of a bad chemical plant fire or nuclear event one would think rain would be at least easy to sanitize. In a pinch I would try it before drinking urban flood water.

This is probably obvious enough that I almost shouldn't need to say it but the time to worry about water is before the flood! Water is so cheap that it's almost ridiculous, especially given the importance of clean water and how hard it is to come by in many parts of the world. It's sad how many people run thousands of gallons down the drain without in thought in normal times yet have nothing but what's in the toilet tank or water heater when trouble strikes.