Those temperatures are predictions and also max temps. They were saying the same for last week and I don't think they reached over 8 degrees so my advise is don't panic. If you look at the forecasts they also change everyday.
The day the forecasters get it spot on is the day they will start giving out the winning lottery numbers the week before. There will be plenty of snow next week.
Right now the weather is fine in Bansko. Sun is shining tomorrow, I believe, but in upper levels there is both minus degrees and lots of snow. And you can get back in blue ones, if you want to.
I also read the plus 17 degree forecast before we got here and was a little bit worried when we drove up from Sophia. But between Bansko city and the highest peak there can be 10 degree difference, so don't worry.
You'll never know, but I think you're gonna get nice week here.