A655: Looking at the PHB definition of the skill, the word 'ground' never appears. Short of using the dictionary definition of the word jump to imply the ground, I do not see any evidence that Jump checks require that you be on the ground. In fact, I don't see anything preventing you from jumping in mid-jump.
Well, that's it. Look what you've gone and done, Emy. Now Thri-Keen can fly.