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.
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.
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.
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.
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!
The historical record shows the September 1 – 10 period to be the most active in terms of annual cyclogenesis in the Atlantic basin, followed closely by the 10-day periods of August 21-31 and September 11-20. This 30-day period, from August 21 through September 20 is the peak of the Atlantic hurricane season, at least statistically in terms of the number of tropical cyclones that originate within this time frame. Records maintained by NOAA show that of a total of 1370 tropical storms and hurricanes generating in the Atlantic from 1851 through 2006 fully 482 or 35.2% actually originated during the 30-day period from August 21 through September 20.
So, what is happening with the 2011 Atlantic hurricane season? With just about 60% of the 2011 season completed on this 16th of September there are some interesting facts to note. There have already been fifteen tropical depressions in the Atlantic in 2011, and of these fourteen became tropical cyclones; this is quite a high percentage of cyclogenesis. On the other hand only three of the fourteen named storms so far have reached hurricane strength, this is less than 22% a much lower rate than had been anticipated based on historical records and early predictions out of NOAA and Colorado. Also of interest is that 8 of the 15 tropical depressions and 7 of the 14 named storms were generated during the month of August. The ten-day period of 1-10 September only saw three tropical cyclones generated.
In addition to the above, today on 16 September 2011 as we look around the larger Atlantic basin there are no signs of tropical waves or other areas of disturbed weather that look as if they would warrant closer attention or possible investigation for potential further development. In fact there is so much dry air in most of the basin, that some of the seeds for rainstorm generation are missing, which in turn weakens the prospects for tropical cyclone formation over the next several days. There are two large areas of disorganized collectives of small and medium sized rain storms, one to the east and northeast of the Lesser Antilles and the other, an elongated one, in ‘hurricane alley’ to the south and southwest of the Cape Verde islands, but conditions do not appear favorable for any significant further development of these areas.
Looking east toward the eastern Atlantic and equatorial Africa we do not see anything near the Cape Verde islands or over equatorial Africa along the ‘tropical-wave assembly line’ that may generate tropical waves or seeds for tropical cyclone formation at least for now and over the next couple of days. In fact it is interesting to note than the ‘belt of tropical activity’ is once again “broken” [interrupted] toward the western fringe of ‘hurricane alley’.
There is one region however that has continued to show persistent storminess and some potential for tropical cyclone development. I am referring to the region that includes Central America, the northern portion of South America [Colombia and Venezuela], southern Mexico, and adjacent waters in the Caribbean and eastern east Pacific. In this regard it is interesting to note that historically cyclogenesis during the month of October and early November appears to favor activity in the Caribbean sub-basin, so if current stormy rainy weather conditions were to continue along with the rather warm surface waters in the Caribbean there may be some potential for tropical cyclone formation in that region.
Color-enhanced infrared GOES satellite view of Central America and adjacent regions, on 16 September 2011, showing the persistent area of rain, thunderstorms and otherwise disturbed area that has prevailed over the region for most of the summer of 2011
How do we qualify the 2011 Atlantic hurricane season so far? Do we categorize it as a “flop” because it’s been mostly tropical storms and weak ones at that and only 3 hurricanes so far? Or do we think of the millions of people that went without power, and suffered floods throughout several states along the mid-Atlantic and northeastern USA under the impact of Irene? Certainly in terms of number of named tropical cyclones the 2011 Atlantic hurricane season qualifies as a higher than average season, but in terms of actual cyclonic energy it’s been a mild one, although we have had more than 50 deaths so far in the USA and throughout the Caribbean. Also, while the Atlantic season appears to be in a ‘lull’ for now, there is plenty of time left as the Sun above progresses southward as the Earth’s axis tilts toward the north and the tropics continue to be primed for potential tropical cyclone activity. I guess we will have to wait and see until after the end of the season to review what took place, and then decide what kind of a season it has been.
In my view, what is clear is that all of the statistical games we play in trying to compare the current season to what has been designated as an “average season”, if there is such a thing, based on the historical record, really mean nothing until there is a land-falling hurricane or tropical storm that hits one or more urban communities in the Caribbean, Central America, Mexico or the USA, and then the structural damage, death, injuries and always shocking human suffering, really bring the whole season into focus, and we all realize it only takes one impact to make it “a season” for the historical records!
Relative to the impact of hurricanes, and tropical storms, we must continue to search for a better method of communicating to residents of vulnerable coastal communities what the “potential impact” of an approaching tropical cyclone really means, moving away from focusing on a number on a scale that means different things to different people. We had interesting discussions on this topic at the Colorado Hazards Conference two years ago, and there have been some notable papers and presentations on this including one by current NHC Director Bill Read, but there is no “new hurricane impact index” yet. Consequently as each hurricane season progresses, potential impacts continue to be colored by the experience or lack thereof of each resident in a vulnerable coastal community, or by the opinions and at times mystifying pronouncements of electronic of printed media reporters or meteorologists; as a result many continue to be “surprised” by what they actually experience during the impact of a hurricane regardless of its category, and by what they see in terms of damage and suffering in the aftermath of such an event.
The challenge to find a more effective way of communicating potential damage from hurricane impact is out there, in front of all of us that work in hurricane research, forecasting, predicting, vulnerability and mitigation, or that study hurricanes from the perspective of their human or social impacts or that of the structural and physical damage they can cause. Relative to this I’d like to issue a challenge of my own, to all those bloggers or members of discussion groups in various social media outlets, please make this issue of defining and designing a better method to convey hurricane impact one of your priorities!
Two Days Later
It now Sunday 18 September 2011 and it would appear mother Nature took issue with my statement that “we do not see…..anything that may generate tropical waves or seeds for tropical cyclone formation at least for now [16 September] or over the next couple of days”, for as we look toward hurricane alley and the eastern Atlantic there is some activity out there that may warrant closer attention.
In the satellite image above notice three areas of disturbed weather, identified by heavy yellow dashed outlines and the numerals 1, 2 and 3, which have flared-up in the past few hours and may warrant closer attention and possibly investigation by our colleagues at the National Hurricane Center. Keep looking east and watch for updated postings here in www.mitigat.com !
U.S. DEPARTMENT OF COMMERCE – NOAA: Tropical Cyclones of the North Atlantic Ocean 1851-2006; Historical Climatology Series 6-2. Sixth Revision – July 2009: BLAKE, Eric S., DAVID, Joan E., LANDSEA, Christopher W., MCADIE, Colin J., and NEUMANN, Charles, of the National Hurricane Center; HAMMER, Gregory R., of the National Climatic Data Center. http://www.ncdc.noaa.gov