The only thing they can say is it is past due, and they THINK it may be a big one. But we all know that mother nature does exactly as she wants, and it may be a low level eruption despite all of the horror story's.
Besides, the temperature of the earth will take quite a long time to cool, it wont be a flash freeze. And nobody knows how long the ash will stay in the atmosphere, so mankind could be here for a while after the event, maybe forever, nobody really knows.
I am preparing for realistic possible events, but I'm just saying that this one is in the top 10, based on it's past eruption intervals.