The National Hurricane Center (NHC) initiated advisories on Subtropical Depression Seventeen (currently centered a couple hundred miles east of the Turks and Caicos Islands as seen in the GOES visible image above) this morning. Every time “subtropical” advisories are issued, I get questions from viewers asking what the differences are between subtropical and tropical cyclones.
In the weaker stages of a cyclone, the designation between subtropical and tropical is somewhat academic. The simplest explanation is that a subtropical cyclone is a hybrid system that, as in today’s case, has an asymmetric wind field and distribution of convection, is involved with an upper-level low, and has maximum sustained winds well removed from the center. A tropical cyclone usually has a more symmetric wind field and distribution of convection, is not as directly involved with an upper-level low, and has the maximum sustained winds closer to the center.
The water vapor image above clearly shows the upper-level cyclonic circulation centered in the same vicinity as the low-level center of the subtropical depression seen on visible imagery. Computer models are forecasting this upper-level low to weaken with time, and the NHC is saying that the subtropical depression could gradually become more tropical with time. An Air Force Hurricane Hunter plane is scheduled to investigate the depression this afternoon to get a more detailed look at the cyclone’s wind structure.
The good news is that this cyclone, whether it remains subtropical or becomes more tropical, is forecast to move generally northward and then accelerate northeastward away from land. It should also be noted, however, that some of the computer models are indicating that yet another low pressure system will develop over the western Caribbean by the weekend.