I think the key criteria is whether it has a well-defined center of circ (it has a center of circ), because when I looked this morning, winds at buoy 42056 (centered at about 19.9N 85.0W) peaked at 25 kts when 99L slid by to the south last night, going from ENE to E to ESE. It looks like it is going past the buoy to the NW because winds are at 100deg (and winds have been increasing a little over the past hour, right now up to 23 kts) and have stayed a little higher than when they were going ENE. So the winds are certainly good enough. It looks like this was the deciding factor for the NHC this morning:

"SATELLITE IMAGES INDICATE THAT THE THUNDERSTORM ACTIVY REMAINS POORLY ORGANIZED"

Edit -- so it seems like NRL calls them a little earlier than NHC, but I haven't been doing this for very long, only a couple months.

So it if keeps going like this it is going to go right from a TD to a TS.

This is what I thought was going to happen, but like four days ago! But going over the warm Carribean water, I figured it would get to a TS sooner or later.

NHC TPC, at 10am, just put TD 20 up on their home page; also saw announcement on TWC at 10am.