[thejeff]

Short answer, you'll never catch him.

You're misunderstanding the movement process.
All moves are done first, then battles resolved. The only time randomness comes into it is when paths cross.

Example:
Jarl in Province A
Your Party in Province B
Jarl attacks C
You attack A.
The Jarl will always be gone before you get there.

If instead, the Jarl attacks B, while you attack A from B, the game will randomly decide whether the battle happens in A or B, with a small chance of the armies missing each other and battles happening in both A and B.
Since he's flying and probably jumping over provinces, this will be hard to do.

The only guaranteed solution is to teleport on top of him. Otherwise you'll have to guess where he's going and have a force waiting for him.

Good Luck