any weather attempting to predict more than a couple of days ahead is probably gonna turn out to be a load of horseshit:
atmospheric molecules obey the laws of chaos theory - small changes get magnified quickly over time (the film 'the butterfly effect' is based on this idea, just applying it to every day life).
when they (try to) predict the weather, they do so by using satellites to split the atmosphere up into a grid, and they analyse what is going on in each of these areas, and put that into a computer which works out what all of the different areas will do and how they will interact with each other. from this, they aim to come up with a picture of what the weather will do. the problem is, that depending on who is trying to predict the weather, they can generally only analyse chunks of atmosphere that are kilometres across, so it is quite inaccurate. then, in accordance with chaos theory, these inaccuracies are quickly magnified, so it is often the case that the forecasts given become inaccurate very quickly.
if, as it seems with you, the forecast is wrong pretty much straight away, then the equipment the whichever weather company is using to analyse the atmosphere above you with must be pretty poor!