How can you write a decent cover letter if you're relocating but haven't yet arrived at your new location?

It’s a good idea to state in your cover letter that you are relocating and will soon be situated in City, State, and are looking for work in that area.
It’s short and to the point, and it informs companies that your move is already underway, so they don’t have to worry about adding a relocation package to the cost of filling their available position.