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2005 Atlantic Season-in-Review: November/December/January
      #64307 - Fri Dec 30 2005 01:13 AM

We wrap up today with a look at the closing days of the 2005 Atlantic Hurricane season, in this case November and December. We closed the season as active as we started it, seeing three storms form during November -- Gamma, Delta, and Epsilon -- and another in late December, Zeta. The last three of these storms were quite alike their development, while the first was more akin to the development we usually see during the month of November in the basin.

Tropical Depression 27, Gamma's predecesor, formed in the extreme eastern Atlantic late on November 13th in the same general vicinity and under the same general circumstances as did Alpha in October. It moved generally toward the west of the next two days without developing, ultimately dissipating southwest of Hispaniola on November 16th. Its remnant circulation moved toward the west-northwest, entering the northwest Caribbean on November 18th. Hurricane hunter aircraft were dispatched to the region and found that the system had developed a closed circulation once again and was now a tropical storm, Gamma. The tropical storm appeared prime to take a path similar to the one that Wilma took three weeks prior, moving slowly toward the northwest and then accelerating toward the northeast near Cuba, but with a much weaker intensity. Indeed, the onset of fall that Wilma helped usher to the region had taken its toll on the environment, resulting in cooler SSTs, less oceanic heat content, and a dry and stable atmospheric environment in the northwest Caribbean. As a result, Gamma never became very well organized, maintaining the intensity it reached just after classification -- 40kt/1004-6mb -- for a day and a half as it drifted north of Honduras. Due to this weak intensity, the storm was not vertically deep enough to be captured by an upper-level trough that cleared the state of Florida; instead, Gamma was sheared by this trough, resulting in it weakening to a tropical depression and ultimate declassification through the day on November 20th. The remnant circulation moved toward the southeast, skirting the coast of Central America, for several days thereafter, never redeveloping any convection or organization and ultimately dissipating around the time that Delta got going. Despite the weak intensity of the storm and it never truly making landfall, Gamma is listed by the NHC as responsible for 37 deaths in Central America.

Delta, like Vince that came before it, formed on the tail-end of a midlatitude trough that fractured over the open waters of the northeastern Atlantic; similarly, Epsilon formed on the tail-end of the midlatitude trough that captured the remnants of Delta several days prior over the eastern Atlantic. The same persistent pattern identified in the October wrap-up led to these systems developing as they did; readers interested in more information about this type of development are referred to the October writeup available from my blog page (http://www.flhurricane.com/blog.php?met=Clark).

Something not mentioned in that October writeup that is worth noting for the Vince-Delta-Epsilon trio of storms is their resilency against the negative impacts (on track and intensity) of midlatitude troughs that would pass by after the systems had developed. The shallow vertical depth of these storms served to shield them from the strongest of upper-level winds found in the jet stream; what depth was there to the storms helped to minimize the shear found in the mid-levels of the atmosphere. We know that there are optimal steering levels for storms of a given intensity; what this season helped to teach us is that there may also be optimal levels at which to look at shear values when judging storm intensity. Such an exercise proved worthwhile for these higher-latitude storms as well as many Atlantic storms earlier in the season that were impacted by mid-level shear. These storms served to miss being picked up by several midlatitude troughs after development -- also keeping them away from the most unfavorable upper-level winds -- at least partially in response to the strong blocking ridge of high pressure aloft over the British Isles. The storms tended to slide underneath the ridge, allowing for weaker troughs to pass them by to the north; only the stronger, deeper troughs would be the ones that would ultimately capture their circulations.

Delta was the weaker and less significant of the two storms. It was first identified as a suspect region for development around the 20th of November, or about three days prior to development. Models -- in the cyclone phase space -- suggested some sort of warm-core conversion of the surface feature located at the tail end of the upper-level trough that spawned it. The feature finally separated from the midlatitude frontal environment enough to be classified as a tropical storm on November 23rd in the central Atlantic. Initial motion was erractic but generally toward the south and southeast. Shortly after classification, Delta reached its peak intensity of 60kt/980mb. As it moved toward the south, Delta lost some of the favorable upper-level environment that helped support its development, particularly the cooler temperatures found aloft, causing it to weaken despite gradually warmer SSTs along its track. It bottomed out in latitude near 22N just prior to it reaching its weakest intensity of 35kt/998mb on November 26th. Delta then turned to the northeast and accelerated in advance of a deep upper-level trough, heading toward the Canary and Madiera Islands -- and the coastline of Morocco. It managed to maintain its identity ahead of the upper-level trough for two additional days, even reintensifying in a more favorable thermodynamic (temperature) upper-level environment to a 60kt tropical storm, before becoming extratropical as it approached the Canary Islands midday on November 28th. Wind gusts to hurricane force and isolated heavy rains were found over the Canaries, Madieras, and inland in Morocco, a somewhat uncommon occurence. The remnant circulation dissipated in Algeria near the end of the month.

As previously noted, Epsilon formed from the tail end of the same trough that captured Delta. It was much quicker to be classified by the NHC than was Delta, based on a small but well-defined surface circulation largely detaching from the midlatitude environment in a fairly quick manner. As a result, on November 29th, Tropical Storm Epsilon formed. Early predictions called for Epsilon to drift slowly westward and then quickly be captured by a midlatitude trough and accelerated toward the northeast as an extratropical system. These predictions did get one part right -- an inital slow drift westward. As Epsilon did so, it strengthened to near hurricane intensity. Epsilon then performed a small cyclonic loop, missing the midlatitude trough as a result, and began to steadily move toward the east and northeast. As it did so, the season ended and we were left with yet another out of season system in the basin. An eye feature began to appear within the storm, resulting in it being upgraded to a hurricane on December 2nd. This eye feature persisted for quite some time despite predictions for the system to weaken; in fact, Epsilon took on the appearance and characteristics of an annular hurricane, a quality usually only seen with very intense storms. Given the cooler SSTs and marginal thermodynamic environment, it's not surprising that Epsilon was not able to reach major hurricane intensity despite its stable nature. Epsilon reached its peak intensity of 75kt/979mb on December 4th and maintained hurricane intensity in the best-track data from December 2nd-7th, a new record for a hurricane in December in the Atlantic. Finally, on December 7th, Epsilon finally began to be sheared by a midlatitude trough to its northwest, resulting in a quick dissipation of the eye feature and a rapid weakening. By midday on December 8th, Epsilon dissipated in the eastern Atlantic. The remnant circulation stuck around for a few more days, ultimately becoming entrained into another cut-off feature that came close to developing itself, but was not heard from thereafter.

For quite some time, it seemed as though the season would end with Epsilon being the last storm of the season. But, Mother Nature had other plans. In late December, a subtropical upper-level low pressure system located over the central Atlantic about 1200 miles to the east-northeast of the Lesser Antilles began to fire convection to the east and northeast of its center, largely over the same waters that supported Delta and Epsilon a month prior. Frequent readers will recall this as being one of the paradigms of tropical cyclone formation when aided by an upper-level trough -- and indeed, despite all indications (no model support, highly sheared environment, climatology), gradually a surface low pressure system formed in the region. As it became more coincident with the upper low and cut off from the westerlies to its north, the storm-relative shear decreased, allowing the storm to quickly become better organized in the early morning hours of December 30th. Early that afternoon, the season's 27th -- and final -- tropical storm, Zeta, had formed in the eastern Atlantic.

Zeta, much like Epsilon, defied predictions of its demise. Initial forecasts called for it to dissipate as the calendar turned to 2006; needless to say, Zeta defied those odds and lasted through midday on January 6th. Continually, troughs in the westerlies passed the storm by to the north; on one occasion, we even saw a favorable scenario for trough-aided intensification set up and lead to some strengthening of Zeta to a strong tropical storm (55kt/994mb on Jan. 3/4). Ultimately, however, as it moved toward warmer waters northeast of the Lesser Antilles, a sharp midlatitude trough passed directly over the system, shearing it of all of its convection and leading to the dissipation of the storm on January 6th. The remnant circulation headed northwest for another day or so before dissipating in advance of a cold front as well. Zeta extended the records for most storms in a season to 27 and most depressions in a season to 30 as well as set a new record for longevity into January, surpassing the 2nd Alice of 1954/1955 by the slimmest of margins. Quite a fitting end to the season, if I do say so myself.

All in all, the 2005 hurricane season was an extremely active and historic season, one which we are not likely to see of such magnitude for some time to come. We are in a period of enhanced tropical activity, yes, and are likely to see seasons reach 15-20 storms in many years over the next 15-20 years...but 27 storms is quite unlikely. Many records were set or tied this season, most of them records we do not care to reach again, and many lives were affected by the storms that comprised the season. Here is hoping for a safe season in 2006. Happy New Year, everyone!

In late winter and early spring, we'll take a look at some tropical cyclone theories, forecasting and understanding topics, and the like (such as the "Trough Interaction" piece posted to the blogs on January 6th). Any suggestions (or comments or questions) are welcome. Thereafter, it's on to the 2006 Atlantic hurricane season -- barring any out-of-season activity before then!

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