I remember a guy in the Met Office telling me that beyond about 3-5 days the forecast is only accurate about 50% of the time. So at 20 days out it's probably nothing more than an educated guess and just what the computer models are saying at the moment. I wouldn't base any decisions on it until much closer to real time.
Is it on the plain?
This, a friend worked for Met Office, and said after 48hrs it's all 'best guess' ....and wouldn't rely on an accurate forecast for the same day....unless it says sunny in the Sahara.
A lot of weather that far out is often based on seasonal averages. 10-14 days out forecasts are more likely than not predictions based on real world data using high fidelity data modelling. Even then, not particularly accurate.
Shouldn't this be in the "Have you put your heating on yet" thread ?
