Leaving soft-money job for tenure track position that doesn't start until August: when to tell current advisor? A verbal offer has recently been extended to me for a tenure track position that I'm 95% sure I want to accept, although I have not officially (verbally or otherwise) done so yet. This position is slated to start at the beginning of the next academic year. 

Right now, I am working as a very junior-level "soft money" research faculty at a nearby university (with a considerably more renowned research reputation than the university where the TT job is, if that matters). I have a primary mentor there who I obviously have to notify about my change of position but the question is: when? This mentor is a reasonable person and we get along fairly well but I'm not sure how s/he would react to this news. 

Obviously, there is "work in progress" at my current position that I am fully committed to finishing and, in a perfect world, I would maintain my current research collaborations in my new position. I would even accept some minimal responsibilities (not necessarily requesting pay) after I leave to make the transition smoother and to ensure that these collaborations remain strong and I do not leave them "high and dry". 

Any advice from the managerial end or from someone who has been in a similar position would be greatly appreciated.
DO NOT say anything at all until your TT offer is signed, sealed and delivered. Hopefully this can be expedited. 

Once that's done, you should tell your mentor the information as soon as possible just so that s/he doesn't get the news from elsewhere. 


  The overall message you want to convey is: "I've enjoyed working here,
  and am excited to move on to the next phase of my career. I value the
  collaborations I've had with you so far and want to make this
  transition as easy for everyone as possible"


Don't worry about their reaction though: that's not something you can control, and for the most part a mentor will be happy for you (as well disappointed). Just make sure above all else that you are straightforward and open, and don't try to hide/shade/evade the truth for fear of "offending" anyone. 

Note: As regarding financial commitments, it might be possible to carry over some of the money to your new position, at least for travel/equipment/students. You should make a list of potential carryovers, and bring that up for discussion when you talk to your mentor.