If I remember the 2010 event correctly, in swindon it was supposed to be a snow to rain event as there was supposed to be a mild sector mucking things up, but it never happened.
Then I remember that 3 days later there was supposed to be a mild breakdown, and that didn't happen either! Instead it was just more snow which then stuck around for several days before a very slow and gradual warming up.
So even at T0, things can be considerably different from what's expected.