Ed. note: Edited 12/29/05 to add more information on . --Clark
The month of October was the busiest month of the 2005 season in the Atlantic basin, featuring one carryover and seven unique systems within the basin. This month's storms were a combination of features induced by tropical waves, interacting with upper-level lows, or even forming from upper-level lows. Several of this month's storms were memorable for various reasons, though unfortunately not all of them pleasant ones.
The month started with Tropical Depression 19 meandering in the central Atlantic far from land. It never did develop into a named tropical cyclone and dissipated on October 2nd. A bit more on this storm may be found in September's recap.
However, a more substantial feature would form early in the month in the NW Caribbean. By midday on October 1st, a tropical wave had acquired enough organization to be classied as the season's 20th tropical depression. Early forecasts called for it to follow a similar track to the systems that came before it -- Bret, Gert, and Jose -- albeit with a greater potential for significant development due to slower movement in the Bay of Campeche and slightly more favorable upper-level conditions. Overall motion early on was toward the west-northwest. As the system made landfall along the Yucatan Peninsula south of Cozumel early on October 2nd, it acquired tropical storm status -- named Stan -- an intensity it would maintain most of the way across the landmass, albeit largely due to stronger winds found along the eastern coast of the Yucatan long after the storm had made landfall.
As Stan emerged off of the coast of the Yucatan, it briefly weakened back to a tropical depression as we moved from the 2nd to the 3rd. It did not take long for Stan to reacquire tropical storm strength, however, as it did so early on the third. Around this time, however, Stan unexpectedly turned more toward the west and west-southwest. It continued on this motion during the day on the 3rd, gradually turning more toward the southwest with time. As noted, this went in the face of almost all of the available model guidance and required continual shifts in watches in warnings to the east along the coast of Mexico. During this time, Stan was continually becoming better organized, reaching minimal hurricane intensity early on October 4th as it neared land. Fortunately in terms of future development, Stan made landfall a day or two earlier than initially predicted midday on October 4th at its peak intensity of 70kt/979mb. Unfortunately, the landfall location just west of Chevala Pass and slow motion once inland brought about massive flooding to parts of the region. Though Stan dissipated by early morning on October 5th and parts of its remnants did not redevelop in the Eastern Pacific, it will be remembered as this season's 2nd deadliest storm with around 100 deaths attributed to the storm (per the ).
As Stan dissipated inland, yet another tropical-wave-interacting-with-an-upper-level-low feature was organizing just off of the coast of Florida. Reconnaisance aircraft were dispatched to the feature early in the morning on October 5th providing data that, in conjunction with data from the NWS radar in Melbourne, FL, led to the classification of this feature as Tropical Storm Tammy later that morning. Tammy was a borderline tropical cyclone, exhibiting many characteristics of cyclones -- and moreso as it went along. In particular, Tammy exhibited poor organization, a flat wind field (e.g. winds nearly the same from the center out to larger distances away from the center), and no temperature gradient between the center of the storm and the outside environment (e.g. it was not a true warm-core system like a tropical cyclone). This is merely semantics, however. Tammy paralleled the coastline of Northeast Florida during the day on the 5th, making landfall during the evening hours near Jacksonville at its peak intensity of 45kt/1001mb. It turned toward the west once inland, dissipating over SW Georgia on October 6th. Only minor damage was noted from Tammy.
Between Tammy and the next system, we had a brief respite before the season's next feature, Subtropical Depression 22, formed in the Central Atlantic southeast of Bermuda on October 8th. Forming from an upper-level area of low pressure, STD 22 was forecast to intensify slightly and potentially complete tropical transition as it headed in a general west-northwest fashion toward Bermuda. However, as quickly as it was classified, the subtropical depression quickly became disorganized, dissipating as the 8th turned into the 9th. It's remnants continued west to west-northwest across the western Atlantic, ultimately interacting with a frontal boundary near the Carolinas but not contributing to any further development.
Even before this, however, curious onlookers had an eye turned to the extreme northeast Atlantic, where a cut-off area of low pressure was becoming more organized and appeared to be displaying an occasional eye feature. The theory of "if it quacks like a duck and walks like a duck, it must be a duck" came to the forefront initially, with satellite appearance and warm-core development gradually following suit. By midday on October 9th, the followed suit as well and dubbed the feature Tropical Storm Vince. Forming at 19.2W, it was one of the furthest east-forming storms on record in the basin. Even more bizarre were the predictions for the storm -- it was projected to head to the east-northeast toward the Iberian Peninsula, an unprecedented feat for a storm in the Atlantic basin. Shortly after classification, the satellite appearance continued to improve with an eye feature reappearing, resulting in the system becoming a minimal hurricane that evening. At 17.8W, this was the furthest east for a hurricane in recorded history in this basin. Vince continued to the east-northeast, gradually weakening as it did so over cooler waters, ultimately making landfall along the extreme southwest coast of Spain early on October 11th as a tropical depression. This marked the first known landfall in Spain for a storm in this basin, the furthest east-located tropical storm in the basin (9.5W), and brought the basin to within one storm of tying the record for the most of all time. Vince quickly dissipated after landfall and its remnants were swept across Spain and southern France by a frontal boundary across Europe.
Vince's formation in the far eastern Atlantic would be followed in November by two additional storms forming in much the same manner, Delta and Epsilon, that we'll take a look at in the last installment of the season-in-review series. However, it is worth a look at the factors that contributed to their development. During much of the mid to late fall, a strong, deep layer ridge of high pressure was found over western Europe and the northeastern Atlantic. As midlatitude troughs would approach these ridges, they underwent 'trough fracture,' a process that basically is what the term says. As a result, the main part of the trough would lift to the north around the ridge axis while a cut-off upper-level low would be left trapped on the southwest side of the ridge. This cut-off low would gradually become entirely detached from any frontal boundaries but would lead to the development of a surface region of low pressure underneath itself. While this is normally a process that leads to decay, the surface area of low pressure would gradually acquire a low-level warm core as the upper-level low gradually weakened and became broader in scope. This warm core developed despite subcritical SSTs (generally 21-24C) as a result of the upper-level low providing colder temperatures aloft, resulting in a rather efficient heat distribution cycle for the surface low pressure system. Gradually, this low pressure system would acquire more tropical characteristics, ultimately developing into a tropical cyclone embedded within a larger cyclonic gyre. Without such strong blocking at higher latitudes, we never would have seen any of these three storms form. This is yet another unique aspect of the 2005 season.
As we moved toward mid-month, a late season tropical wave was gradually becoming better organized south of Jamaica. Late on October 15th, this feature was classified as the season's 24th tropical depression while located west of Jamaica. Initially, the depression was slow to organize, only becoming a tropical storm during the early morning hours of October 17th. At that time, it took the name , the last name in the list of names for the 2005 season and tying the record set in 1933 for the most storms in the basin during one season. While slowly organizing, drifted toward the south and southwest before stalling out for some time halfway between Jamaica and Central America. As it did so, about a day after becoming a tropical storm, became classified as a hurricane midday on October 18th. Satellite appearance was of a fairly large storm, though reconnaisance aircraft data indicated that looks were indeed deceiving and that was actually a fairly small storm. Nevertheless, as the evening of the 18th approached, began to look very impressive on satellite imagery with the following suit by upgrading to a strong category 2 hurricane (95kt/945mb) late that night with the caveat that, with the lack of recent aircraft data, it might be even stronger.
Little did they know how strong actually was. Aided by several strong outflow channels and very high oceanic heat content, was quickly becoming a very dangerous storm. The next reconnaisance aircraft found a very small storm with a 2 nautical mile (in diameter) eye and a height of the 850mb surface of 500m -- compare this to most minimal major hurricanes near 1000m and normal midlatitude heights near 1500m. The extrapolated pressure from this measurement was 902mb -- one of the most intense storms on record in the basin and tied with for this season's 2nd most intense storm -- with a corresponding advisory intensity of 130kt, category 4 intensity. As the aircraft continued to sample the storm into the morning hours on the 19th, the winds continued to increase and the pressure continued to fall. Once all was said and done, the minimum pressure of 882mb made the most intense storm in recorded history in the Atlantic basin, besting Gilbert's 888mb. The peak winds during the storm's lifetime were noted as 150kt; indications are that this may be upgraded to 160kt (185mph) in the final analysis. As it strengthened, began to move toward the west-northwest, albeit slower than expected, toward the Yucatan Peninsula. It maintained category 5 intensity for much of the day on the 19th before weakening back to a strong category 4 storm as we moved into the 20th.
Over the course of the next two days, maintained a west-northwest to northwest course and category 4 intensity, ultimately nearly stalling out over Cozumel midday on the 21st as it became entrenched within a region of weak steering currents between a ridge of high pressure to the east and a trough well toward the northwest. Intensity during this period was largely regulated by eyewall replacement cycles, hovering generally around 125-130kt/925-930mb, before slowly weakening over land. After battering the island, grazed the coast of the northeast Yucatan Peninsula as it finally began to turn toward the northeast in advance of the trough on the 22nd. The trough served to limit reintensification of the storm -- now of category 2 intensity -- until the 23rd as neared Florida. Against all predictions, reacquired category 3 intensity as it approached southwest Florida and the Naples region, making landfall there as a 110kt system during the overnight hours of October 24th. rapidly moved across the state during the morning hours of the 24th, only weakening slightly across land. Impacts were felt all across south Florida from such a strong, northeastward-moving storm owing to its large size -- much larger than when it was a category 5 storm -- and being on the side of the storm going with its motion.
After impacting south Florida, rapidly accelerated -- moving 50-70mph at times -- toward the northeast across the Gulf Stream, reacquiring category 3 intensity in the process. As moved north of the Gulf Stream during the morning of October 25th in advance of the trough, it finally began to weaken and become absorbed by the trough, being declared midday on the 25th still with winds of 75kt. later battered the Canadian Maritime region, albeit not to the extent that some model forecasts had predicted for the region when the storm was a monster in the NW Caribbean Sea. The slower forward motion and not-quite-ideal interaction timing with the trough led to this evolution, probably thankfully for the NE United States and Canadian Maritime region. Behind and the trough, however, much of the east coast of the US saw the coldest weather of the season (to date), breaking the warmth of summer and fall. The tropical phase of was quite memorable for many reasons and is a name we are not likely to see in use in this basin ever again. The attributes 22 deaths and $14.4 billion in US damages alone to , making the storm the 3rd costliest (unadjusted for inflation) in the US behind and Andrew.
It should be noted that , at its most intense, was a very small tropical cyclone. Besides the 2-n.mi. wide eye, hurricane-force winds extended outward no more than 45 miles from the center and tropical storm-force winds extended outward no more than 140 miles from the center. Microwave imagery confirmed the small size of the storm, despite the appearance of a much larger storm on infrared satellite imagery. Unfortunately, other than through continual observation, we do not have a way of predicting the size of a storm before it develops. Smaller/larger depressions tend to end up as smaller/larger hurricanes, yes, but even that is not fail-proof. What we do know is that smaller storms tend to have greater fluctuations in intensity, both up and down, and that most of the most intense storms are small to average sized systems. Many intense storms can be large systems, particularly the annular storms like Isabel. What we are starting to find out is that eyewall replacement cycles seem to be an effective mechanism for expanding the size of a tropical system and providing a limit upon its intensity (though we've know this latter point for some time); what is still to be determined is why the new eyewalls do not contract all the way to where the old ones were previously located. Nevertheless, presented quite an extreme example of rapid intensification -- 100mb in 24hr, at or above the recorded record for any basin -- and to some degree challenged what we know about storm structure. While storms like are no joy for anyone in their path, they also provide a lot of data and motivation toward advancing the science -- something we'll certainly see after this season.
While battered Cozumel and the Yucatan, a late-season tropical wave developed into a tropical depression south of Puerto Rico on October 22nd. This feature quickly became a tropical storm -- Alpha, the first letter in the Greek alphabet and a new record for number of storms in a season in the Atlantic basin. Alpha moved toward the west-northwest, making landfall over southern Hispaniola at its peak intensity of 45kt/998mb early on October 23rd. Alpha turned toward the north-northwest and quickly moved across the island, weakening to a tropical depression in the process. It never could regain tropical storm intensity in a moderate shearing environment ahead of as the latter storm began to accelerate toward Florida, ultimately becoming absorbed by during the afternoon of October 24th southwest of Bermuda. Twenty deaths were attributed to Alpha in Hispaniola by the .
October was not done, however, as yet another system would form deep in the southwest Caribbean late on October 26th. This depression, the 26th of the season, moved erratically toward the north (paralleling the Central American coastline) over the next few days, becoming better organized as it did so. During the morning of October 27th, yet another tropical storm -- Beta -- formed. Defying model predictions, Beta traveled a bit further north than was forecast, reaching 14N on October 29th. Also defying predictions to some degree, Beta did not organize as fast as forecast, only reaching hurricane intensity overnight early on the 29th; this was likely due to relatively low heat content in the region despite very warm sea surface temperatures. At this point, Beta slowed, turned toward the west and southwest, and made its final approach toward land. As it did so, Beta briefly reached category 3 intensity before weakening to a category 2 storm and making landfall along the Nicaraguan coastline late on October 30th. Beta dissipated after moving inland on October 31st, capping off a very active month of October. Fortunately, no deaths have been attributed to Beta in Central America. Beta did not develop once moving into the Eastern Pacific despite maintaining itself as a trackable entity for a few days in that basin.
Once October was complete, we had seen a record 23 storms for the season-to-date. November and December would bring about three more, which we'll take a look at next time in the last installment of the 2005 Atlantic Season-in-Review. Look for it before the end of the year (and week).
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