A few people have recently asked the Local 10 Weather Staff why we sometimes refer to tropical depressions and sometimes to tropical cyclones when we give the Tropical Update. I have been using the tropical cyclone term for decades and didn’t know there was a problem. But based on the questions from viewers, a short explanation seems in order.
A tropical cyclone, as used in the Atlantic Basin, includes tropical depressions, tropical storms and hurricanes. The more formal National Hurricane Center (NHC) definition states that a tropical cyclone is “A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperature and heat export at the low temperatures of the upper troposphere. In this they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects).”
Our Tropical Updates typically give the probability of development over the next 2 days which is consistent with the NHC Tropical Weather Outlook. It is usually more technically correct to say that a given disturbance has a certain probability of becoming a tropical cyclone. One reason for this is that there is usually so much uncertainty in forecasting intensity, we certainly can’t guarantee that the disturbance, if it does develop, will not become a tropical storm within 48 hours. Another reason is that sometimes, the NHC starts advisories at tropical storm strength based on observations.
In the rare case where there is high confidence that a disturbance, if it develops, will most likely remain extremely weak, we might only refer to the probability of development into a tropical depression.
If you live in South Florida, you might as well add tropical cyclone to your vocabulary and understand that the term includes tropical depressions, tropical storms and hurricanes.