Is it self-plagiarism if I am up-front about similarities? I am writing a paper which expounds on a portion of a previous paper that I got published.  

If I did not even mention the previous paper in my new one, I'm fairly certain this would be seen as trying to pass off old work as new (unethical).  

However, in the introduction of the new paper, the beginning of the relevant section, and throughout the relevant section, I am very up-front about the similarities to the old paper, with many citations. Also, there is no copy/paste from the old paper.  Some phrasing may be similar in places, but only because I happened to write both papers.

Am I safe submitting the new paper to a journal?
As long as the new paper extends the prior one, presents new data or analysis, and you cite properly, you are fine. It is best to pretend you did not write the prior work and cite accordingly, because you assume no special status even though you authored it. This would only become problematic if you attempt to pass off the same work (even if edited) as a distinct novel work.