In reply to Webster:
Two years ago we had a solid cold snap in late November / early December and I did Idwal Stream in reasonable conditions on Sunday 28 November. There was even enough ice to place the occasional screw for a laugh and there were lots of people on it as usual. I don't recall any of the other Idwal routes being in but they were building up. I have found an email that I sent the previous Sunday saying "The BBC, Metoffice and MWIS forecasts all agree that we’re in for a chill, with freezing levels down to a few hundred metres from Wednesday." I can't remember what I would have meant by 'a few hundred metres' but it must have been under 500m (the base of the Idwal ice routes is around 550m). So about 4 days was just about enough for the Stream but not quite enough for the other routes in Idwal.
It stayed cold througout the following week and by the following weekend Idwal was awesome - almost all the ice routes were in, with the notable exception of the Appendix. Then there was a short, sharp thaw and there was hardly anything left anywhere in Snowdonia by the following weekend!
Looking at the latest forecast, I don't think it's going to be quite cold enough for the Idwal routes to be in by the weekend as MWIS is forecasting the freezing level to still be as high as 650 on Thursday (though presumably a lot lower overnight, in fact with clear skies forecast the BBC is suggesting around zero overnight even outside the mountains).
I suspect the longer term build up is also important, i.e. how cold is it in the week before the sub-zero temperatures kick in. Going by MWIS forecasts, the freezing level has been hanging around the summits for the past few days, which bodes well for the higher up crags.
Of course Idwal isn't the only place! Still, I'd be betting on some mixed climbing somewhere high up rather than ice.
I'm planning to be in Snowdonia on Friday as have a random meeting with the NT in Ogwen in the morning, so might go for a wander into Idwal and Cneifion and report back. Will be there over the weekend as well on a club meet - mildly optimistic about doing a route somewhere but we shall see! For this time of year just a wintry scramble would be nice.