Well the 530PM TWO is out, and it looks like we dont have Delta... just yet. I would guess that NHC are waiting for the system to generate persistent and deep convection. It has been generating convection during today, but most of this has been relatively shallow. Recent IR imagery would suggest that this may be changing though, with an area of intense cold topped convection having formed to the immediate west of the centre, and now wrapping around the southern and eastern quadrants. If this continues then it has a decent shot. I would also imagine that NHC are waiting for it to break free totally from the frontal system to its northeast. Given how it is moving away from the front, and the continually improving convective patterns, we could see this upgraded within the next 6 to 12 hours.

Regards