Tremorsense is described as:
A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance.
Tremorsense can't be used to detect flying or incorporeal creatures. Many burrowing creatures, such as ankhegs, have this special sense.
Assuming that tremorsense allows a monster to "see" other creatures that are touching the ground as is covered by option 1 of this answer and also assuming that the monster is unable to see the character except via tremorsense, does jumping cause a creature to be unseen by the monster?
As an example Raflannan the thief is facing a fearsome Ankheg in an area covered by heavy fog. Raflannan runs and jumps firing his crossbow at the Ankheg while midair. Is Raflannan's attack rolled at disadvantage due to not seeing his target or is it a regular roll because the advantage of being unseen cancels out disadvantage?
Best Answer
It is up to the GM
As far as I can tell, there aren't clear-cut rules on this; what we do know is that tremorsense doesn't work against flying creatures and that jumping is not flying.[citation needed] Does this mean that tremorsense can detect jumping creatures? I don't know.
We also know the following from the "Unseen Attackers and Targets" section:
Whether tremorsense literally counts as "seeing" isn't entirely clear. I personally would say that it does as otherwise creatures with nothing but tremorsense would always grant advantage to their attackers and that hardly seems fair.
But can they "see" you while you're jumping? I have found no rule describing this, so I believe it is up to the GM.
I could see a GM argue that tremorsense can detect a creature before they jump and thus a creature could accurately predict how they will move through the air.
I could see a GM argue that tremorsense does not detect jumping creatures for exactly the same reasons that it does not detect flying creatures.
A small frame challenge I want to point out is the scenario happening at all; this would require the attacker knowing that the enemy has tremorsense, being in a Heavily Obscured area, and then successfully attacking while moving in mid-air. At least for me, that is no easy task. I have not personally dealt with mid-jump attacks so I'm unsure how I'd actually rule mid-session, but I imagine that I would require some sort of roll to determine whether the character can even pull this stunt off.