It's actually pretty easy to predict a general 500mb pattern 8-12 days in advance most of the time.. The 12-15 is a bit more suspect but we do nail that well in advance sometimes.. The pattern that will lock into place mid next week for 7+ days was well forecast by ensembles starting at day 13-15 last week... Just because it doesn't snow doesn't mean that the forecasted 500mb pattern was wrong, sometimes we just get unlucky. In fact many torches are sniffed out well in advance usually by ensembles..