Rhinoceros

I quote this from >>OffBroadWay

Make a helix with the right number of turns – less than one in this case I guess, and of larger diameter then the surface. Next make a line up the center of the surface – actually the center of the helix, really. Loft the helix to the line and get the curve of intersection between the lofted surface and the original surface (a revolve in the attached image)

I quote this from Off BroadWay

1. Add one curve vertically between the lower branches (white).
2. Loft both branches together with this curve (red surface)
3. Trim this with a plane.
4. Loft the upper curves.
5. MatchSrf for tangency the upper to the trimmed loft.

I added a knot horizontally in the lower span of the blue surface before matching to limit the area distorted by Matchsrf.