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What is a Subtropical Cyclone?

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The National Hurricane Center has initiated advisories on Subtropical Depression Seven centered just under 600 miles south of Bermuda and is forecasting it to become a Subtropical Storm as it moves generally northward over the next few days.   Most of us in South Florida understand very well what a tropical cyclone is, but I have had numerous inquiries over the years asking what is meant by the term subtropical.

A subtropical cyclone is a non-frontal low-pressure system that has characteristics of both tropical and extra tropical cyclones (i.e., winter storms).  Subtropical Depression Seven fits the classical definition of a subtropical system because it is involved with an upper-level low and has a radius of maximum winds around 100 miles from the center.  The satellite image shows no central dense overcast over the center of circulation, and the moderate to deep convection is not at all symmetric.  The wind field is also not symmetric.

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 extra tropical 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.

If Subtropical Depression Seven strengthens into a subtropical (or tropical) storm, it will get the name Fay.  This system is no threat to South Florida.