MinuteEarth explains why meteorologists are often wrong about rain. I think that frames the question as if we expect weather forecasters to be right all the time. Speaking for myself, I find them to be astonishingly accurate compared to how the weather forecast was treated, say, 50 years ago. Back then, a forecast of rain meant there was a possibility, but you may as well flip a coin. Now I can pull up a website and see where it's raining on a map, and the forecast for the next few days is usually pretty spot on. In other words, we can't expect weather forecasters to be 100% right all the time. Please adjust your expectations.
Okay, that being said, MinuteEarth explains why some types of rain are harder to predict than others. This video is only three minutes long; the rest is an ad. -via Geeks Are Sexy