It is relatively easy, at least with my clone, to get imperialis to flower with lights in a grow tent but far more difficult to bloom it in December/January on a windowsill.  I believe what may have helped is the fact that we have had the warmest December on record, and the heat has not dried the air nearly as much as in a typical winter.  You can see though in the photo that one of the seams on one bud is starting to split.  In my experience, this condition is brought on from less than optimal humidity, but will not affect the flower.