The National Hurricane Center has initiated advisories this morning on Subtropical Storm Laura centered over the North Central Atlantic about 1000 miles west of the westernmost Azores. As expected, this has prompted people to ask for the definition of a “Subtropical Cyclone.” I will answer by drawing from a blog from 2007.
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.”
An obvious example of a pure tropical cyclone is a major hurricane with a well defined eye. 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 designation “subtropical”. Laura is such a hybrid. Today’s subtropical cyclone originally developed from an upper-level low that eventually developed its circulation down to the surface. The global models did a good job in predicting this development.
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.