Also, lots of sources saying that rain is unlikely during the race. Like at Silverstone, it'll probably bucket it down just after the race has finished.
I remember I was at the Canadian Grand Prix three years ago (when McLaren got the one-two, with Alonso third), and they were predicting storms all weekend. As it turned out, Saturday and Sunday were glorious sunshine, really hot and summery weather, and everyone was wondering how they could have got it so wrong - they said there'd be rain and there wasn't, nothing like it.
What most people don't see, of course, is the fact that on Monday it bucketed it down all fecking day. Miserable day. So it wasn't that the teams got the wrong forecast in that they totally fecked it up and couldn't have predicted it more inaccurately, but that they were out by twenty hours or so. The rain was coming, just later than they said.
The same thing happened at Silverstone (when it rained after the race had finished), and probably last week too.