You can also see from this air pressure graph from a nearby buoy that surface pressure has been dropping steadily all day. And is just starting a slight rise tonight at 10pm eastern.
Visibly Arlene is not the most impressive storm at this point. There are a few banding features to the west over Mexico. It's largest blob of convection is to the east over the Yucatan. And to its north-northwest is were the least amount of convection is found. That is were that pocket of dry air is, although that is starting to move off to the northwest.
The center of circulation is located at 21.2N 93.7W and is moving at a slow 7 mph. At that rate it will be able to stay in open water for at least another 24 hours. With the wind shear relaxed now it with have favorable conditions for further development. The Hurricane Center's track takes Arlene into Mexico late Wednesday night or Thursday morning, much further south than 2010's Hurricane Alex.
Here's a quick look at the rest of the models. The GFDL takes a sharp turn south and a couple other models bring her in just a bit further north.