Jetstream Turbulence MET
A very common question with several variants. Turbulence is worst where there is a rapid change of wind in a short direction; this can be likened to the difference between slamming the brakes on in a fast moving car or letting it cruise to a stop; the amount of speed lost has not changed but the rapidness of change did. In a straight jetstream, there is fast moving air in the core - but as you leave it, this speed slows down. The direction where the speed slows down fastest (where the brakes are slammed on) is towards the cold air at the height of the core as here the speed change is compressed against the front. This is on the left looking downstream.
This turbulence will be worse if it is also changing direction - i.e with a curved jetstream.