10% Chance of Flooding and Monsoons

I looked just yesterday and the forcast was a 10% chance of rain. This morning I checked with someone who's iphone said it would only be partly cloudy the rest of the day.
When five-o-clock rolled around there was a sudden loud clap of thunder and the sky got realy dark. It poured for nearly an hour making streams of water flowing in the streets.
At what point did 10% turn into a monsoon?
The up side is that I don't have to water the plants today. The cantaloupe really likes the rain water better anyway.
The down side is that it's now too wet to mow the rest of the yard.