A rain sensor won't help when it rains right after you water the grass. I'm looking for the same thing John is mentioning. If you could get an accurate forecast about the probability of rain, that could be used to prevent watering on the same day it rains, even if the rain happens after your scheduled watering time. Weather Underground has a huge amount of info for your area on their website. Unfortunately, only a small amount of that is available from the weather Thing. Even if you could just get the current weather condition, not just the forecast, that would help. As far as I can tell, right now you can only get the current temperature and UV index.