The jet stream is exactly where clear-air turbulence (CAT) is shown on aviation meteorological charts (sigwxh). Smooth air is least likely under these conditions (outside of thunderstorms). As others have noted, this would not be the time for messing around with cruise speeds and MMO.
BTW, all the media reports seem to be focused on ground speed, nothing from onboard the aircraft, or passenger comfort. Imagine alternative headlines: "It was so bumpy I thought we were going to die". Unless it was freakishly smooth.
Originally Posted by
nonsense
"And you might notice something suspect about the 1289 kmph reading - it's above the speed of sound (1234 kmph). Commercial aircraft ordinarily can't break the sound barrier, because they're not designed to handle the sudden increase in drag and other aerodynamic effects associated with those speeds. Despite a ground speed that high, the plane didn't come close to reaching that threshold because it was embedded in the swiftly moving air." (OK, better...)
Edit: The speed of sound varies with temperature (altitude), at cruising level it would be1062 km/h, which is very different from sea level. Not that the average journalist would know this. This would emphasise that the numbers are actually more impressive than the article states.