CAT near a jet stream
A jet stream is a narrow band of strong winds several kilometres above ground. The flow of air in a jet stream can be likened to a fast-flowing river. Where the fast-flowing river meets slow-moving water near the banks, waves and swirls are often generated. When these waves and swirls get big enough, the water there may become rather turbulent. The same applies to a jet stream. This explains the bumpiness experienced by aircraft when encountering CAT near a jet stream.