The farther out the forecast, the greater the likely error, especially of the timing of weather systems and fronts.
Best thing is to learn more about met and learn to make your own interpretation of the charts.
Irv Lee's columns in Flyer (and his seminars ref.
http://www.higherplane.flyer.co.uk/seminars.htm) are very good - covers weather and more general post-PPL stuff.
I would also recommend attending a weather-focussed seminar such as those run occasionally for sailors and airmen by either the Met Office or
http://www.weatherweb.net/aviation.htm for example.
No-one has the right answer! Hope this helps,
Andy
