The National Hurricane Center is talking about the possibility of a Subtropical Cyclone or Tropical Cyclone developing out of the disturbance currently moving over Florida. Most people in South Florida understand very well what a tropical cyclone is, but I have had numerous inquiries asking what is meant by the term subtropical.
The National Hurricane Operations Plan (NHOP) defines a subtropical cyclone as “a non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. This system is typically an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles of more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.”
A major hurricane with a well defined eye is an obvious example of a tropical cyclone. The hurricane has its strongest winds in a limited area close to the cyclone center (in the eyewall that surrounds the eye). A large wintertime storm over the North Atlantic is called an extratropical cyclone. The strongest winds in the wintertime storm are usually spread out over a much broader area well removed from the center. But there is a category between these two types of cyclones – a hybrid – that has the name subtropical.
In the weaker stages of a cyclone, the designation between subtropical and tropical is somewhat academic. If a cyclone continues to strengthen, however, it is important to know whether the maximum winds are close to the center or spread out over a larger area well removed from the center. The subtropical cyclones can also develop into tropical cyclones as they strengthen.