Firstly, I do not mean this thread to be a dig in any way whatsoever, I am genuinely interested. Second, I apologise if it is in the wrong place. What I am wondering is; what makes weather forecasts go through periods of accuracy and huge errors. I understand that storms and snow are difficult to forecast, but that aside, I have noticed many sites going through phases of continually getting it wrong. For example at the moment the BBC has been accurate only 20%* of the time in the last month, compared to being pretty much spot on the previous month. That's not in regards to unusual weather but things such as rain, temperature etc. I know that many sites use the same models and such to get their forecasts, so my question is what causes phases of inaccuracy in forecasts, even with 'normal' weather? * I measured that in terms of if the weather was doing what it said at that time range (give or take an hour) and if the temperature was what it said to within 4-5C, for example.