Weather is inherently unpredictable, because so many little variables can affect it to a great degree, especially when you try to forecast something accurately more than a day out. It's pretty easy to look at a satellite photo, notice a big system of warm moist air coming in off the ocean just a hundred miles off the coast and say with a certain degree of confidence that it'll rain tomorrow. Over the weekend? Now you're just guessing -- a cold gust from the north might blow the weather system down south of you before it rains on you, and the rain you predicted just doesn't happen. So the science of meteorology is part intuition, when you come right down to it -- you get a feeling based on what you know of historical trends for the area you're trying to forecast, given what you know of the conditions and the season of the year. Here in Seattle, autumn usually means gray, overcast skies, with rain at least three days out of any given week, so if you say that it will be cloudy, lows in the mid 50's, highs in the upper 60's, 30% chance of precipitation, you've pretty much covered all your bases -- if it rains, you can say there was always that chance, and if it doesn't, you can say, "Well, it was only a 30% chance, after all!"