I just want to add a little and make a small correction to what Les has said here.
Seeing can still be good even when a jet stream is fast moving over head. Fast seeing is really where the interaction between the layers is either at cross angles or diving or rising rapidly into each layer. Careful studies via observation has shown both myself and Anthony convincing evidence that seeing is more likely to be affected by the interaction between the jet stream level and the level of atmosphere directly underneath it. It all levels are moving at the same speed and not interacting the seeing can still be great. My images of
10 May Jupiter are evidence of this. The jet stream was moving over the top on that night, yet the seeing was probably some of the best I have seen at my Blackwood home.
Typically though the presence of a high pressure system over the top of your location and the absence of either the Jet or localised winds will likely result in good seeing to varying degress. As Les stated it is the stillness of the atmosphere which leads to statements of good seeing.
Hope that helps.