From Randal Olson's blog, based on data from Nate Silver's book The Signal and the Noise.
"The Weather Service is pretty accurate for the most part, and that’s because they consistently try to provide the most accurate forecasts possible... Weather.com often forecasts that there’s a higher probability of raining than there really is.
This phenomenon is commonly known as a wet bias, where weather forecasters will err toward predicting more rain than there really is. After all, we all take notice when forecasters say there won’t be rain and it ends up raining but when they predict rain and it ends up not raining, we’ll shrug it off and count ourselves lucky.
The worst part of this graph is the performance of local TV meteorologists. These guys consistently over-predict rain so much that it’s difficult to place much confidence in their forecasts at all."