It's definitely not the BBC!
I actually was bored one day a couple of months back and I put an official complaint in. Apparently they source all their data from the Met Office, and cc'd the Met Office in the reply to ask them to explain further about why the data is always so far off.
They didn't really explain except to say that due to variations they are only 65% correct by the fifth day (there are 5-day and 24-hour forecasts on the BBC). I was like this
when I read that, until I realised they probably base it on those 3 hour blocks (or maybe even more granular) - so if they get 1 block correct, they claim to be correct for the day.
It must be this cos they aren't even 65% correct for the 1st day
- for example, on Monday it said that Saturday was a 'full sun' symbol. And when I looked on Friday, it was a double rain, black cloud!
Here is the method the Met Office use to predict the weather:
