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?
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.