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Delta Was an Extratropical Cyclone

Delta 2005 track

According to the final report from the meteorologists, reported in La Opinion, what we have been calling Tropical Storm Delta, was actually an extratropical cyclone and the damage caused by the phenomenon was the fruit of the orography of the archipelago. We're glad to have that one cleared up, aren't we? But, unless you're an expert in these things, they may as well have said it was caused by extra-terrestrials. In other words, you may be wondering, as I was, what all those big words really mean, so I've done my best to make sense of this news.

The final technical report from the National Meteorological Institute (INM) on Delta concluded that it was, on its arrival in the Canary Islands, an extratropical cyclone and not a tropical storm as had been said at the beginning. They also say that the destruction caused in some parts of Tenerife and which left some areas without electricity for a week and without water for some days, was caused more by the physical geography of the island than by the weather system itself.

Researchers from the National Meteorological Institute presented their report last Tuesday at the Museum of Science and the Cosmos in La Laguna, Tenerife during a conference called Profundizando en Delta: un estudio avanzado (Getting to the bottom of Delta: an advanced study). During this chat, INM technicians made scientific data available for the first time, covering this phenomenon that wipped the island last November 29th.

The information was collated two or three months earlier and, despite its importance, had not been made public and was presented in a discrete conference.

During the presentation, Francisco Martín León, of INM, explained that at the beginning Delta was a tropical storm, with all the characteristics of one and was generated in subtropical waters. Between November 23 and 27, "it behaved like a tropical storm", explained Martín León and added that on November 28th at 12:00 hours, the National Hurricane Centre in Miami [the centre with jurisdiction over this type of phenomenon in the Atlantic] catalogued the phenomenon as a tropical storm. (Note: I expect this was also when they gave it a name.)

After that moment, a number of atmospheric circumstances came about together, which caused Delta to arrive in the Canary Islands and, the form in which it did, which was that it became an extratropical cyclone.

For a tropical storm to travel, it normally needs water with a warm temperature of 27 degrees centigrade. One of the first curiosities of Delta was that it developed over waters with temperatures inferior to 25 degrees and even water at 21 degrees. Also, Delta had a singular movement and was conducted over a course that brought it to Canary Islands latitudes and, it was precisely this course that prevented it from dying out in mid-Atlantic, as is usual with tropical storms.

Later, Delta evolved with a warm frontal system and with this injection of dry air, it began to displace rapidly, where before that, it had been moving slowly and erratically. On November 28th, Tropical Storm Delta began its transition to extratropical and lost its symmetry (another of the characteristics of tropical storms). On arrival in the Canary Islands on the night of November 28th, it had already converted into an extratropical cyclone.

Martín León assured, in turn, in response to questions, that a similar phenomenon to Delta had been produced in the Canary Islands in 1975 and that, with these two events, it is fitting to think that this is something which could be repeated in the islands.

Orography and Wind

Juan José Bustos, also from INM, explained that the grave destruction that was produced in the Canary Islands, especially on the islands of Tenerife and La Palma, had more relation with the physical geography of the terrain that with the weather system. That is, that in less mountainous islands, there were few problems reported. He also explained that the largest gusts of wind were on the leeward side (On or toward the side to which the wind is blowing. The side sheltered from the wind.), as in the case of the Valle de Güímar.

Javier Calvo explained that the various methods that exist at the moment to predict this type of phenomenon do not have the capacity to predict exactly what time they will arrive or the force of the winds they will bring with them.

Reference and Definitions

Orography: the physical geography of mountains and mountain ranges.

Cyclone: An atmospheric system characterized by the rapid inward circulation of air masses about a low-pressure center, usually accompanied by stormy, often destructive weather. Cyclones circulate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Extratropical Cyclone: The extratropical, or middle-latitude, cyclone originates as a wave, or perturbation, in the polar front separating the cold polar easterly winds from the warmer prevailing winds farther toward the equator.

Tropical cyclones, formed over warm tropical oceans, are not associated with fronts, as are the middle-latitude wave cyclones, nor are they as large as the latter. A tropical cyclone that has matured to a severe intensity is called a hurricane when it occurs in the Atlantic Ocean. Tropical cyclones usually move toward the west with the flow of the trade winds during their formative stages.

To be fair, the information on Tropical Storm Delta (2005) at Wikipedia (in English) reflects this information. The Spanish version contains more information, but, curiously, doesn't mention the extratropical word.

Report of the preliminary evaluation of the front, which explains how the lie of the land added to or caused the turbulence, causing the level of damages (in Spanish, but with ever so pretty pictures, diagrams and graphs.)

El informe final del Meteorológico dice que ´Delta´ fue un ciclón extratropical

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