First an overview of the large-scale setup. Here's the GFS forecast (from HOOT) for this Wednesday afternoon at 300mb, right before this cyclone is forecast to really get going:
Further south, we have a strengthening subtropical jet stream, most visible over the eastern Pacific into Baja California as a strengthening jet streak there. However, extending east a little further we can see that the height gradient associated with the subtropical jet, while weaker, is showing a trough over the southeastern US and Florida. A trough there means that there must be a pocket of slightly colder air over the southeast, with some kind of gradient in temperature to the south. We know that extratropical cyclones get their energy from temperature gradients, and we usually see those develop in association with a trough aloft. So, seeing this setup, one might expect an extratropical cyclone to develop off the southeast coast as that trough moves east-northeastward.
Well, a cyclone is indeed forecast to develop there. Let's zoom in now and take a look at what the deterministic ECMWF forecast (courtesy of Wunderground) from last night is saying about this developing cyclone. Let's start with the sea-level pressure (blue contours), precipitation (colored shading), and 1000-500mb thickness (yellow contours) for 21Z Thursday:
But let's look at the thickness contours (the yellow lines). In the core of that storm, there's actually a small solid contour of thickness, at 5700m. This is a positive thickness anomaly in the center of the storm. What does that mean? Higher thicknesses mean that the column of air being measured is warmer on average. So, a positive thickness anomaly would point more towards this storm being a warm-core cyclone. Tropical cyclones are warm-core cyclones; whereas extratropical cyclones are cold-core cyclones (they tend to be colder in the center). So, this warm-core characteristic seems more indicative of a tropical cyclone.
Let's compare this to the GFS forecast for a slightly earlier time (09Z Thursday; the GFS model develops this storm more quickly):
Here the precipitation might be slightly less clearly comma-shaped, though there is a large dry slot to the south of the low. The warm-core idea is again shown with higher thickness values in the center of the storm. So different models have varying degrees of warm-core-ness.
The thickness plots above are based on temperatures in the 1000mb to 500mb layer. Let's explore temperature further by looking at the 850mb temperatures from the ECMWF forecast, which would be right in the middle of that layer:
One tool to help decide if a cyclone is tropical or extratropical (or something else) is looking at something called tropical cyclone phase diagrams (from work by Robert Hart at Florida State). Here's the forecast phase diagram for the cyclone in the GFS forecast:
The type of cyclone basically depends upon the thickness (the y-axis on the diagram) and the thermal wind (which is related to temperature gradients; the x-axis on the diagram). By measuring these two things and plotting them as the cyclone evolves over time, we can see what "region" of the plot the storm falls into. Here, the storm's trajectory (the dots along a line going from A to Z) are all in the lower right part of the chart---that "red" sector is labeled "symmetric warm-core", aka tropical cyclone. An extratropical cyclone would be "asymmetric cold-core". So, interestingly enough, according to this one method of looking at cyclones, they would forecast the storm forecast to develop off the southeast coast as a tropical cyclone.
As we've seen above, the latest forecasts show this particular cyclone to have aspects of both a tropical (warm-core) and extratropical (cold-core) cyclone. So what do we do? We call it a "subtropical" cyclone, sort of a catch-all term that describes storms that have both warm- and cold-core attributes. More thorough definitions are at this page and this page. If the maximum winds are less than 34 knots, it will be labeled a "subtropical depression". Greater than that, it will become a "subtropical storm" and actually given a name from the 2015 hurricane name list by the NHC (which would make this Subtropical Storm Ana, should it get strong enough).
Regardless of which model you believe, both bring heavy rain and strong winds to the Florida, Georgia and Carolina coasts Thursday and Friday. The ECMWF forecast has it just reaching subtropical storm strength at this time...