Firsty, ignore MetGuess, the worst weather forecasting site in history.
It's as good as any other you're likely to find. They use the same weather modelling software that many other sites use. The trouble is people want to know what the weather's going to do in the next 24-48 hours very very locally to them. The sites will try to do this, but depends on where you live as to how accurate that info's going to be.
Long range weather forecasting is still tricky, and is basically trends based on historical data, with current weather patterns thrown into the mix. More than 48 hours ahead nationally is difficult, locally it's impossible, in the mountains or hills, you may as well use local weather lore.
There is a building El Nino, this has 'interesting' implications for summer in the southern UK, it will probably be hot…beyond that…think '06 rather than '07/08 if you want an opinion of a sad weather hobbiest…