October Activity: 2011 Atlantic Hurricane Season

Historically the period from late September through mid October has marked the peak of the annual Atlantic hurricane season, but so far this year October has been somewhat of a flop in terms of tropical cyclone generation.

Color-enhanced infrared GOES satellite image taken on 9 October 2011 showing the large low-pressure system that brought copious precipitation to Florida and point beyond, in what has so far been the main tropical event in the month of October in what has been historically considered the peak of the annual Atlantic hurricane season

It is now Monday 17 October 2011, and all the Atlantic has generated so far this month by way of cyclogenesis has been a  large rather wet disturbance, which for a while was borderline between a  low pressure system and a tropical storm, but never made it. What was interesting about that particular weather event in early October, besides what it could have been, but never was, is that it brought much needed rainfall to the Kissimmee – Okeechobee – Everglades system helping relieve a persistent drought in South Florida. This event caused significant precipitation north of Lake Okeechobee, actually over the river system that feeds the lake, which helped raise the level of the lake close to two feet. So, despite the inconvenience of localized flooding in urban areas in South Florida as well as Central Florida this was by and large a beneficial event for Florida. However in numerous mid-Atlantic states and northeastern USA communities the same event caused wide-spread damage with rain, flooding and gusty winds.

For the past week or so there has been a large cell of low pressure slowly moving over Central America and the Yucatan Peninsula of Mexico, which has caused persistent precipitation leading to several instances of flooding across the region. Recently, say over the past two days, this system has gotten much better organized and stronger. In the morning of 17 October, the system is showing characteristics that may lead to tropical cyclone formation over the next 12-24 hours as it moves generally, albeit slowly,  north by northwest over the northeastern Yucatan Peninsula, Northwestern Cuba and the Western Florida Keys and Florida Straits.

Color-enhanced infrared GOES satellite image of 17 October 2011 showing the low pressure system between the northwestern Caribbean and the Gulf of Mexico, which may become a tropical cyclone over the following 12-24 hours although atmospheric conditions beyond that time-frame are not favorable

Much of Florida has already been feeling the impact of this weather system in the form of persistent, and at times quite heavy, precipitation over the past 24 hours as it moves over the Yucatan channel  and western Florida Straits toward the Gulf of Mexico. In the early morning  hours of today, Monday 17 October 2011 the National Hurricane Center continues to monitor this system and it is giving it a high probability of developing into a tropical cyclone over the next 12-24 hours. However a large mass of dry air and high pressure over most of the Gulf of Mexico and points beyond toward the west and north would present an unfavorable environment for further development of this system.

GOES satellite image showing the weather system over the Yucatan channel and eastern Florida Straits on 17 October 2011, as well as other regions of disturbed weather over Central America

Elsewhere in the tropical Atlantic basin there are a couple of large areas of disturbed weather over ‘hurricane alley’ and the far eastern Atlantic, and a smaller one over the extreme southeastern Caribbean, but nothing which so far presents much potential for tropical cyclone development. There is a need for continuing monitoring of the northern tropical Atlantic however as tropical waves continue to appear over equatorial Africa and farther beyond over the Indian Ocean, which could carry the seed for cyclogenesis over the Atlantic as they continue to move generally toward the west.

Composite satellite image showing a swath of the tropics from the eastern Pacific to the Indian Ocean on 17 October 2011; several areas of disturbed weather, which may have potential for cyclogeneis are visible of this image showing water vapor in the atmosphere

In summary. it would appear the so-called “peak” of the annual Atlantic hurricane season may have already taken place back in September when there were several storms, but we’d do well in continuing to monitor the various regions that may contribute to cyclogenesis over the larger Atlantic basin in days and weeks to come. By the same token, events so far in October appear to be following what has by and large been the norm for tropical activity in the 2011 Atlantic hurricane, which is one characterized mainly by tropical storms and less than a hand-full of hurricanes. Relative to this it becomes relevant to again ask if we may be witnessing a gradual upward shift in the threshold for hurricane generation, under the influence of changes in atmospheric physics driven by global warming. While we ponder how research may provide answers to such question, it is important to keep in mind that the 2011 Atlantic hurricane season is still in progress, consequently let us pay attention, be prepared, and keep practicing mitigation!

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