I don't think 30% chance means it will rain 30% of the day. I'm pretty sure it means 30% of the viewing area will get rain.
It means that given the exact same temperatures and conditions it rained 30% of the previous days with matching temperatures and conditions. In other words, they're just guessing based on previous patterns.
We started to leave, but took a look at the gridlock going on around our neighborhood and decided we were better off inside of our brick house than inside of a car.