I know a company that will give you a forecast that's 5% accurate provided you send them 20 quid. PM me for the e-mail
I used the BBC website, but nowhere will give you a month in advance forecast these days it seems. Last year I found outthe day before it was a swamp (and there is always the Wacken webcam to confirm this!) and that was enough to take the right kind of clothing.
don't trust the german tv text. Last year it said clear, warm and sunny. I walked around in a friggin wintercoat with a sweater underneath, thats how warm it was!