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TD 4 Discussion, Aug. 22nd
      #70423 - Tue Aug 22 2006 03:43 PM

As of this writing, TD 4 remains just that -- a tropical depression -- situated to the west-southwest of the Cape Verde Islands. Two factors are at play hindering its development: its broad size and ingestion of stable air at low-levels. What is not at play is Saharan dust; while there is some evidence of another dust surge located to the northeast of the depression, the TD itself is situated in a relatively favorable area dust-wise. The relatively large size of the storm is causing it to take longer to get organized and, perhaps more importantly, is causing some stable air located to the north of the depression to be drawn into its circulation. A long-term visible imagery loop of the region highlights this best, such as that at Despite this, TD4 continues to have a very well-defined signature on satellite imagery and in the wind fields.

The models have shifted, somewhat predictably, back to the west since yesterday, recurving TD 4 more along 50-55W than 40-45W. They also have been trending weaker with the storm, likely due to slower organization and somewhat unfavorable atmospheric conditions ahead in the vicinity of 40W with a deep trough located over the central Atlantic. The ridge near and to the northeast of TD 4 continues to build somewhat toward the west, but at a slower rate than seen yesterday. The GFS still wants to develop a mid/upper-level low to the west of TD 4 over the next day or so, slinging the depression off to the northwest through time. The UKMET is more muted with this, with the depression slowly gaining latitude over the next two days before being turned more toward the west underneath a building ridge near the Azores. It saves significant recurvature for further west, ahead of a developing trough in the west/central Atlantic. I think the UKMet has a better handle on current evolution than the GFS, especially given the westward push of everything shown in current water vapor imagery in the central Atlantic ( Ultimately, recurvature is likely -- it's just a matter of how fast the depression gains latitude and where it makes that turn. I still tend to think the models are a bit too fast with recurvature and latitude gain as I'm not sure how accurate their handle on the upper level features out there is and will be, but I think recurvature by 55-60W is likely.

Intensity remains a tricky question, dependent both upon how much development TD4 does in the short-term and the evolution of the upper level pattern ahead of it to some degree. Given its broad state and ingestion of stable low-level air right now, only slow development is likely for the next couple of days until it can get further west, though I do anticipate seeing TD4 become T.S. Debby sometime in the next two days. After that, depending upon how much shear actually is found in the environment, some additional strengthening may take place or it could remain as a weak-moderate tropical storm. Models are starting to favor the former moreso than the latter, but I'll wait for evidence of a strongly sheared environment to manifest itself before buying that. The NHC intensity forecast may be a tad too aggressive, as noted in the 11a discussion, but not far from the realm of possibility.

Elsewhere in the tropics, the wave out ahead of TD 4 is showing some signs of organization and some weak turning in the low level wind fields, but is currently located in a somewhat inhospitable environment due to the presence of a relatively significant Saharan dust outbreak. It may prove frisky a bit further to the west, though, as the effects of the dust layer likely begin to wane and the wave slows down its forward progress. Some of the model solutions aren't out to lunch on this one, I think, but it's still a long ways from threatening as anything significant (re: classified). Keep an eye on it for down the road. Nothing else is threatening at this time.

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