Tropical depression, tropical storm and hurricane are terms you hear every hurricane season, but "potential tropical cyclone" is a relatively new phrase with which you might not be very familiar.
Introduced by the National Hurricane Center in 2017, a potential tropical cyclone permits the NHC to issue routine advisories on a system that has not yet developed into a tropical depression or tropical storm but brings a threat of 39-plus-mph winds to land within 48 hours.
This falls within the time window of posting tropical storm or hurricane watches and warnings, allowing the NHC to issue them with additional lead time and provide at-risk people with more advance notice of possible impacts to their area.
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Previously, the NHC could not issue any watches or warnings until a system had developed into at least a tropical depression.
"Advances in forecasting over the past decade or so ... now allow the confident prediction of tropical cyclone impacts while these systems are still in the developmental stage," the NHC said.
In 2020, Potential Tropical Cyclone Nine and Potential Tropical Cyclone Sixteen went on to become hurricanes Isaias and Nana, respectively.
The NHC publishes a projected path and a full suite of text, graphical and watch/warning products every six hours for all potential tropical cyclones, just as it does for hurricanes and tropical storms. These updates occur at 5 a.m., 11 a.m., 5 p.m. and 11 p.m. Eastern daylight time. When watches or warnings are in effect for any land areas, which is almost always the case with potential tropical cyclones, the NHC also provides additional updates at the three-hour intervals between those times.
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"Advisory packages on potential tropical cyclones will be issued until watches or warnings are discontinued or until the threat of tropical-storm-force winds (39 to 73 mph) for land area sufficiently diminishes, at which point advisories would be discontinued," the NHC said. "However, if it seems likely that new watches or warnings would be necessary within a short period of time (say 6-12 hours), then advisories could continue for a short time in the interest of service continuity."
The same naming conventions currently in place for tropical depressions, which are assigned a number from a chronological list, are used for potential tropical cyclones. That number always corresponds with the total number of systems that has occurred so far during the hurricane season.
For example, if the NHC had already issued advisories on eight systems in the Atlantic Basin that year, the next land-threatening system in the Atlantic would be deemed Potential Tropical Cyclone Nine. If it eventually organized into a tropical depression, it would retain the same number and become Tropical Depression Nine. Then, if it achieved 39-plus-mph winds, the system would become a tropical storm and earn the ninth name on that year’s name list, which would be the "I" storm.
The only hiccup in this naming convention would be if Potential Tropical Cyclone Ten or Tropical Depression Ten became a tropical storm before Potential Tropical Cyclone Nine or Tropical Depression Nine. In this case, Ten would earn the "I" name while Nine would earn the "J" name because Ten became a tropical storm first even though Nine existed first.