I want to leave my tenure track position before fall. I have great prospects but no new position "locked up": when should I break the news? I am a relatively new tenure track faculty member at my school. For many reasons, I am certain I want to leave my current position. All of my reasons are professional reasons; there are no relocation issues or anything like that. 

I do not have a new position lined up but I am being actively recruited and feel that having a new position by the fall is a near-lock. Regardless, my finances are strong and, outside of good personal relationships with a couple colleagues, I have absolutely no hesitation about leaving this position. 

One issue for my department is that some of my future classes are "important" (required classes that, right now, only I am qualified to teach). I do not want to put them in an unnecessarily difficult position. Therefore, my main question is: 


when should I break this news? Given my certainty about this decision, should I tell them ASAP? 
Or, should I follow the general logic that one should never leave a position without a new job lined up? What if this means waiting two more months, REALLY leaving them in a tough spot for covering my "important" fall classes? 


As a secondary question: 


any advice about how to break this news? Some of my reasons are related to the way the program is run and the behavior of some of my colleagues. Should I go into this, or should I simply say that the position turned out the be a poor fit and that I must move on?
Giving notice in 6 weeks time gives them the entire summer to replace you. Wait as long as you can to give your potential next department the time to make you a formal offer. As soon as you have accepted it, tell your current department chair. Don't do it before you have the offer in hand and have accepted. Unless you outright resign effective tomorrow, your current department may assume you are fishing for a counter offer, a raise, or early tenure. Without waiting until you have accepted the potential offer, you may end up burning even more bridges through this process even if you deny that you are trying to force your current department to upgrade you. If you don't walk out the door immediately, then you will have to spend the next several weeks being around your current colleagues. Wait until the semester is over and you have completed your obligations for the spring at the very least.