TROPICAL LINGO

With the recent tropical system in the Gulf of Mexico, there were 3 different identifiers used to report on this system. We first knew it as Invest 92L. Later it was known as Potential Tropical Cyclone #3. Then just as it was making landfall it became Tropical Storm Claudette. What do these titles mean, and how many more are there?

Tropical depression, tropical storm, hurricane....those are the terms we are most familiar with concerning weather in the tropics. But what are all of these other terms you hear being mentioned from time to time?

Most tropical systems form from what are called Tropical Waves. There are systems that form from old frontal boundaries that get out over the water and start to spin up, but for the most part, a tropical wave is where it all starts. About 60% of named storms come from tropical waves, and 85% of major hurricanes form from a tropical wave. These waves are basically troughs of low pressure that, of course, form in the tropics, mostly coming off the African coast. The temperature variances between the much warmer Sahara Desert and the cooler, more forested areas of Africa are what causes these waves. In a typical season we'll have about 60 or so of these waves push off of Africa and traverse westward across the Atlantic along the Intertropical Convergence Zone (ITCZ), with less then half organizing into something more significant.

Tropical waves can be dry as they sometimes push off the African coast loaded with dust from the desert. Cloud formation gets inhibited with this dry air. But tropical waves can come with thunderstorm activity as well. These create a disturbance, and are the ones we usually have to keep an eye on. A tropical wave, or any area of disturbed weather for that matter, that the NHC believes has the potential to develop and become something that is more significant, gets a different classification. These are upgraded to what is called an Invest, short for investigation. Invests are given a number from 90 to 99. They start each season at 90. When Invest 99 is given, the next invest goes back to 90. There is also a letter following the number of the invest. This will indicate what tropical basin the system is in. For example, Invest 90L would indicate a system in the Atlantic basin, whereas Invest 90E tells you the system is in the east Pacific basin. Invest 90C is central Pacific, and so on. "A" is not used for the Atlantic because Invest 90A is used for the Arabian Sea (northwest Indian Ocean). Most of the time here at Texas Tropics, we will give an invest without the letter because we usually only deal with the Atlantic basin, so there really isn't the need to identify what basin we're talking about, but when you see 90L, you now know what the L means.

Again, invests are tropical waves, or areas of disturbed weather in the tropics, that are believed to have the potential to develop further, based on atmospheric conditions that lie ahead. And not all of these go on to develop.

A Tropical Depression, something we have some familiarity with, forms when one of these tropical waves, or invests, forms a closed circular center. If the winds around this closed center are less than 39 mph, we have a tropical depression. When those winds reach 39 mph, but stay less than 73 mph, we have a Tropical Storm. And, of course, anything 74 mph and above is a Hurricane. It isn't until a system becomes a tropical storm that it gets a name, and it then keeps that name regardless of it's status. For example, last year tropical depression #13, became Tropical Storm Laura. When it strengthened to a hurricane, it became Hurricane Laura. When it made landfall and degenerated back down to a depression, it didn't go back to being tropical depression #13, it was Tropical Depression Laura. So once you get a name, you keep it. 

When a tropical depression forms, it's first given a number, and not a name. Every named tropical storm or hurricane will have been a tropical depression first, no matter how brief it might be, or even if you never officially hear of it being referred to as a depression. But it does have to be a depression first, before it develops further. The numbers start at 1 every season. Tropical Storm Ana, the first named storm this season, officially was tropical depression #1, before it was named. Tropical Storm Bill was depression #2, Claudette #3 and so on. Last year, Hurricane Iota was tropical depression #31. Not all depressions go on to become named storms either.

With our recent Gulf system (Claudette), we heard the term Potential Tropical Cyclone. In 2017 the NHC started giving this designation to systems that are still just an open low invest, but are close enough to land and have the potential to reach depression or storm status before or at landfall. This is done so that the powers that be can put out watches and warnings to better inform the public of, well, the potential that is there for depression or storm formation. A potential tropical cyclone is given a number the next depression would have been given, even if it never becomes a depression. In this instance our Gulf system was called Potential Tropical Cyclone #3 while it was still just an open low invest. The next depression or PTC will be #4.

From time to time, usually very early in the season, or very late in the season, you'll hear the term Subtropical Storm, or Subtropical Depression. A subtropical depression will get the next number available for depressions or PTC's, and a subtropical storm will still get a name. There can not be a subtropical hurricane. To get that strong a system either has to become fully tropical, or fully cold core.

These subtropical systems usually form in the higher latitudes, or subtropics, where the air and the water temperatures are cooler, and are a combination of a warm core tropical system and a cold core low pressure system detached from the front it's associated with. Because of the hybrid setup, a subtropical storm's strongest winds don't form around the center, but are instead much farther away from the center. Often a subtropical storm will shed it's cold core characteristics and become fully tropical. This is what happened to Tropical Storm Ana earlier this season. Likewise, a fully tropical system can transition to a cold core low, becoming what is called Extratropical. This happens often with storms interacting with fronts. They tangle together and the system then loses it's tropical characteristics.

These systems that become extratropical can also be referred to as Post-Tropical Cyclones, but that title is not limited to just systems that get swept up by fronts. Tropical systems that degenerate into a remnant low or trough, usually after making landfall, can be referred to as post-tropical.

Again, we know about tropical storms and hurricanes. Tropical storms have just the one classification. Winds of 39 mph to 73 mph are a tropical storm. We can refer to them as weak or minimal, moderate, or strong, depending on where in the 39-73 mph spectrum they're at, but they're still just referred to as tropical storms. There is another classification for a hurricane though. When a hurricane reaches category 3 status, which are winds greater than 110 mph, it is referred to as a Major Hurricane. Not that category 1 or 2 hurricanes can't cause a lot of damage, but category 3 and higher are usually a lot more destructive with not just the powerful winds, but increased storm surge as well. Major hurricanes are part of the predictions that come out every year where you'll get expected number of named storms, expected number of hurricanes, and the number of those hurricanes expected to be major hurricanes.

So I think that about covers all the various names we use when talking about these tropical systems.