SOUTH MISSISSIPPI (WLOX) - While talking about the tropics, you may have heard recent meteorologists using words like ‘invest,’ or ‘tropical wave.’ What do those words mean? Why does it seem like those terms are suddenly used so often?
Well, to start, all tropical activity can be easily split into two categories: unnamed disturbances and named tropical systems.
‘Tropical wave,’ ‘invest,’ and some other terms are typically used to describe disturbances that have not been named yet, though each phrase has its own specific meaning. These can cause heavy rain showers but usually do not have the ability to strengthen rapidly.
The National Hurricane Center is responsible for naming tropical systems, also called tropical cyclones.
It takes at least three things for an unnamed tropical disturbance to develop into a named tropical system: a closed circulation, well-defined center, and organized thunderstorms.
- A tropical disturbance must have a closed wind circulation in order to be named. If there are winds blowing in a complete counter-clockwise circle around the storm’s circulation, then it’s considered closed.
- In order to get named, it must also have a well-defined center. It’s the point at which the entire storm spins around and can typically be viewed on satellite.
- Finally, along with a closed circulation around a well-defined center, it must also have organized clusters of deep thunderstorm activity surrounding its center.
At that point, the National Hurricane Center will either declare it a tropical depression and give it a number (if its winds are less than 39 mph) or a tropical storm and give it a name (if its winds are between 39 and 74 mph).
"The National Hurricane Center has been calling unnamed disturbances 'invests' for years," explained WLOX First Alert Meteorologist Wesley Williams. "It's just that now with TV meteorologists able to use their graphics systems to share that information as well as weather enthusiasts online and on social media sharing products from the NHC, it just feels like those terms are more common all of a sudden."
“Keep in mind that unnamed disturbances often do not get official advisories and forecasts from the National Hurricane Center,” Williams said. “But, once a tropical system is named the National Hurricane Center is guaranteed to issue an official forecast track that lets you know where the storm will be and how strong the storm will be. Up until that point, us TV meteorologists will likely use other unofficial computer model projections to help tell the forecast story.”
“Unfortunately, computer models often struggle with their projections for unnamed disturbances since there is not much information for them to make a outlook off of,” continued Williams. “So, these model outlooks will often flip-flop with uncertainty until the disturbance for which they are forecasting becomes better organized.”