What do you mean by extended time intervals - the weather in the UK is near impossible to predict more than a few days ahead which is why most weather forecast will not give more than a 5 day forecast and that is classed as long range.
There are sites that give predictions further into the future but they are always so in accurate that in my opinion it is pointless.
MetCheck is one of them - if you don't believe me try it for yourself - for a week note down the predicted weather for 14 days ahead and then check back daily to see how the forecast changes each day as they feed more real info into it - it wont be until the last 3 days that it starts to stabilise and can be relied on.
Most weather data is gathered and calculated by the met office anyway - as far as i know there is not another organisation collecting data on anywhere near the same scale as those guys. Most weather sites just add their own colour scheme to the same black and white charts that met office dish out.
Metcheck have a couple of good articles on how the GFS (Global Forecasting System) computer calculates the weather this one shows how the computer comes up with 10 possible outcomes for just 5 days ahead - just see how much difference there is even in the 10 possible predictions by the computer to get an idea of how hard and inaccurate weather prediction still is.
http://www.metcheck.com/V40/UK/FREE/5day_ensemble.asp
