I think we should remember that it's still more than possible for the
MO Winter forecast to be correct. It's only Jan 16th at the moment;
Dec was around average, January might well have a cold last week and
Feburary has shown some interesting synoptic patterns in recent years
(noticeably last year). If the pacific jet weakens as we head towards
the end of Jan (quite possible, if not probable) then we could very
well see that negative NAO pattern kick-in (Sea temp anomlies in the
Atlantic would seem to support it at present).

So in answer to the question of when I would say the last week of Jan
IMO - but that's a guess based on the current model output, and the
large cold pool that's developing to the East at the moment. Of course
it may not happen in which case the forecast may be wrong, but as they
said it was only a 66% probability.