The fifth tropical depression of the 2012 Atlantic Hurricane Season has formed east of the Lesser Antilles. The system doesn't look too terribly organized at the moment, but the system has strong enough winds and a low-level circulation center, so the National Hurricane Center has classified it as a tropical depression.
A large area of high pressure is sitting over the middle of the Atlantic Ocean which is helping to steer the tropical depression westward into the the Caribbean Sea. The system is in a moist area of the atmosphere, with the Saharan Air Layer ("SAL"...the area of dry, dusty air that blows over the Atlantic Ocean from the Sahara Desert) staying off to the north and east of the system, not currently impeding development. The SAL is depicted by the yellow/orange coloration.
Given the lack of dry air, a general westward motion into the warm Caribbean Sea, and low wind shear, Tropical Depression 5 is expected to slowly strengthen into Tropical Storm Ernesto over the next few days. The below image is called a "spaghetti plot," or a graphic that plots all the model forecast tracks of the system. Ignore the outliers (like the one taking it into the Gulf Coast), and you can see that the models are in pretty good agreement of where the system will go.
The National Hurricane Center is pretty much following the guidance on this one. They're taking the system further north in the Caribbean than some of the systems would have it, which makes it more of a threat to the Greater Antilles, Jamaica, Cuba, and perhaps the Gulf as we get into next week.
The system is progged to slowly strengthen and potentially reach hurricane strength by next Monday, although tropical storm intensity forecasting is a very imprecise science and a storm's strength can fluctuate wildly with little or no warning.
The National Hurricane Center will come out with advisories on this system every 3 hours as long as Tropical Depression 5 threatens land. It's worth watching. I'll post more updates on my Facebook page as needed, and post more diaries later on during the weekend as US/Puerto Rican impacts become clearer.