Slightly confused by one or two posts, as I understand it, any arctic blast (as described in the beeb forecast), is still 5/6 days away, even they don't know what happens beyond that. Not only that, I was under the impression that the jet stream is also modeled, so it's forecast position on x date could well be different tomorrow to what it is today, and even then any forecast can be wrong. I've also been climbing/mountaineering many places, but never once looked at a jetstream forecast before setting out...