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Posted at 11:58 AM ET, 09/30/2011

Ophelia becomes major hurricane; update on Atlantic season

Hurricane Ophelia, a mere remnant wave earlier this week, has made a remarkable recovery, reaching the major category 3 level today. About 620 miles south southeast of Bermuda, its maximum winds are 115 mph according to the National Hurricane Center. Short term fluctuations in intensity are likely over the next day before a gradual weakening. Its western periphery (on the weaker side of the storm) might (but might not) just clip Bermuda Saturday, and a tropical storm watch is in effect.

Ophelia is one of the 16 named storms (and one additional tropical depression) in this Atlantic hurricane season, the second busiest (to 2005) on record at this point.

But in this season’s batch of 16 named storms, there have been only 4 hurricanes, three of which (including Ophelia) have reached major status (category 3 or higher). And Colorado State researcher Brian McNoldy notes this is a record low number of hurricanes for such a larger number of total storms.

“Looking back through the records, any season with 16+ named storms had at least 7 hurricanes,” he blogged.

Indeed, this ratio between the number of hurricanes and total number of tropical cyclones is peculiarly small, and was not expected by any of the seasonal forecasts.

The consensus among these seasonal forecasts was instead for about 8 hurricanes. This at least partly explains why predictions for the accumulated cyclone energy (ACE), which is a crude way of estimating the season’s “activity” by incorporating a tropical system’s longevity and maximum wind speed, were likewise too high. (Despite this season’s unusually large number of named storms, the ACE is only about 7 percent above average)

Why have there been so few hurricanes? Has the environment been too harsh on the incipient disturbances? Has the crop of hurricane-candidates been ill-equipped internally to deal with the surroundings? Or was it some combination? Science will likely address this question very soon, if it hasn’t already. In the meantime, we can look at some clues that have emerged during the course of the season that may shed light on the issue.


Observed wind shear (blue line) compared to climatological average (black line) (NOAA)
One of the usual suspects involved in hindering hurricane development is strong wind shear. One might reasonably guess based on the lack of hurricanes so far that the wind shear over the Atlantic, Caribbean, and Gulf of Mexico has been abnormally high. But recent data on the wind shear patterns suggest that it has instead been close to normal for much of the season basin wide. The charts to the right plot the observed wind shear (blue jagged lines) and the “normal” levels of wind shear in each of these three regions.

In none of these charts do the blue wiggles (observations) depart significantly from normal (black line) for extended periods. For the most part, the wind shear has bobbled around reasonably close to the black lines, with only a few exceptions.

And though these simplistic pictures don’t tell us what the conditions were like near any particular storm, these observations do suggest there may be properties of the tropical atmosphere other than wind shear that have been more obviously biased against hurricane development.

For much of the season, in both the weekly averages and in individual storm analyses documented in our posts, exceptionally dry air has occupied the local tropical environment.

The plot below (top panel) indicates that for the last six weeks, it has indeed been anomalously dry (yellow/red shading) near the middle levels of the atmosphere (20,000 to 25,000 feet) all across the primary development regions. Though not shown, unusually low relative humidities were also noted at other altitudes much closer to sea-level. Given that the inhalation of dry air can suffocate a tropical system and brake the spin-up process, conditions like these are potentially hostile to the development of hurricanes.


Top: Mid-level relative humidity departure from average (greens, yellows, and reds indicate dry conditions). Bottom: Mid-level temperature departure from average. Warmer than average temperatures (colored in yellow and orange) at these altitudes increase stability and make it more difficult to sustain thunderstorm development. (NOAA)
In addition, there are indications that the vertical stability (temperature change with height) across this same sector of the globe has been arranged in a way that has hindered –more so than usual- the development and maintenance of organized thunderstorm complexes. As shown by the image above (bottom panel), unusually warm temperatures aloft (yellow/orange shading) have been observed throughout the Atlantic, Caribbean, and Gulf of Mexico this season. The size and scope of the territory shaded in yellow/orange is consistent with the observations of higher stability and the extraordinary absence of long-lived thunderstorm clusters in the region.

Of course, there is still opportunity for the hurricane tally to increase. The season is by no means over.

There is no doubt that atmospheric and oceanic conditions over the tropics will continue to be capable of supporting development for several more weeks at least. Whether or not they will, however, is another story. Either way, time is running out. The jet stream has already begun its discontinuous equatorward journey, bringing drier and windier midlatitude conditions to the development regions in bursts that are becoming ever more frequent. Indeed, this hurricane season may wind up being remembered more for its failure to produce hurricanes than for its success in producing a lot of tropical storms.

(Jason Samenow contributed to this report)

By  |  11:58 AM ET, 09/30/2011

Categories:  Latest, Tropical Weather

 
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