"Probability of appreciable rainfall" * "fraction of specified area which will receive it" is 0.7.

Or, I guess more properly it should be an integral over possible rainfall patterns. But "70% of London will definitely see lots of rain, and 30% will see none" and "we have 70% credence that all of London will see lots of rain, and 30% credence that no rain will fall in London" would both be reported as 70% chance of rain in London.

https://en.wikipedia.org/wiki/Probability_of_precipitation