In my case, I started the conversation very early, almost a year in advance. "I just married someone who might make me move." Even though we didn't dwell on the discussion, the fact that I brought it up regularly made sure that it wasn't a surprise when I announced that, "I am moving and will need to work remotely."
If you've already discussed how much you like working remotely with your manager, then it shouldn't come as a surprise that you're asking to remain remote.
In your case, maybe consider stating that you will be in the office day a week? Then, when "life happens," just don't show up for 2-3 weeks and see what happens.
When I moved, my employer set up an office for me with a group that I was physically close to. I showed up once a week, then due to a bad winter, I just didn't go in for 6 weeks. No one noticed. (Heck, no one noticed when I was there.) Then, the office situation changed and I asked to be remote full time.
If you get a firm "no," then everything depends on how much leverage you have. If there is some major project due in the near future, point out that the job market is very healthy for remote employees and imply that you might quit without finishing your project. See if you can get some kind of severance package or retention bonus if you stay through the project as a remote employee. Otherwise, just quit when you have a new job.