There were temperatures of below -15'C in both bansko and borovets in the last couple weeks at the height of the top ski lifts. However, these temperatures have taken into account the wind chill. There has also already been quite a few days where snow has fallen on the tops but not enough, as yet, to stay for very long.
We will see a white winter such that we have not seen for quite a while. All you have to do is believe!!