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Vigorous tropical wave .. Invest #96L .. starting to affect Northerrn Leewards. TC development less certain, but some impacts regardless
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ClarkModerator
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Reged: Wed
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Development in the East Atlantic?
      #70353 - Mon Aug 21 2006 02:15 PM

Slowly but surely, the tropical Atlantic is starting to open for business. Taking a look at a 10-day water vapor imagery loop (http://cimss.ssec.wisc.edu/tropic/real-time/wavetrak/movies/m8wvupper/m8wvupperjava5.html), the propensity for cut-off upper lows to develop in the central Atlantic has been steadily declining over the past 6-8 days, with ridging slowly building west across the region. Coincidentally, this is largely occurring in conjunction with the development of the tropical disturbance located southeast of the Cape Verde Islands right now. Saharan dust has been a theme for the past two seasons, but the SAL is currently a neligible factor for this feature's development (and only slightly moreso for the one out ahead of it).

Trades are fairly significant across much of the basin, thanks to a relatively strong low-level ridge centered near 32N/50W (http://www.nhc.noaa.gov/tafb/QUNA00.jpg), but we are start to see some signs of enhanced convergence along the ITCZ in the tropical Atlantic. The strong trades and relatively high pressures across the tropical Atlantic have also been mitigating factors for development this season; a slowing of the trades and lowering of pressures, perhaps aided by the wave in advance of the current disturbance and an approaching weak MJO pulse, will help to activate things. Also note the significant 850hPa low pressure center associated with this wave already. Wind shear is relatively weak in the vicinity and SSTs, while not optimal, are sufficient for continued development.

Models suggest that anything that develops out there should head to the west-northwest to northwest, recurving well out at sea. I'm not sure how much stock to put into that one, as they've been overdoing such tracks so far this year and there is no indication of a weakness for anything to move into at this time. The satellite & model derived steering flow products from UWisconsin (http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/winds/winds-dlm.html) suggest a more westerly to west-northwesterly motion based solely off of upper-level steering patterns. The GFS, which many of the steering-pattern models such as the BAM series are derived from, shows the development of an upper-level low out ahead of this disturbance, likely interacting with it and forcing it on a more northwesterly pattern. This shows up well in the 500hPa vorticity fields from today's 06Z run (http://moe.met.fsu.edu/cgi-bin/gfstc2.cgi?time=2006082106&field=500mb+Vorticity&hour=Animation) and has been a pretty persistent feature in the model looking back through the past few runs. Other models don't depict the disturbance and its environment well from the outset or show a similar evolution to the GFS.

Question is, is there any indication of this feature forming? It's tough to give the eyeball method much sway when looking at how upper patterns are evolving because it's tough to break down the winds (current and forecast) without the assistance of a numerical model, but my eyeball method suggests a firm "maybe." There is a weak upper low stationary near 22N/42W, but this is a separate feature. More to the point, near the Cape Verde Islands as of this writing, the upper-level pattern is taking on the characteristic "S" shape indicative of a developing upper-level feature. The trough to the disturbance's north near the Madiera Islands may have been the catalyst for this as it completed trough fracture earlier today, but I can't attribute everything to that without additional evidence. Nevertheless, the significance of this upper evolution remains to be seen; the disturbance is in its diffluent region and sufficiently far away to likely be aided a tad by it, but also likely to be steered somewhat by it depending on their mutual interaction and development.

If the current development trend holds, and I don't see much reason why it would not other than perhaps SSTs leading to the weakening of the convective structure, I expect we'll see our fourth tropical depression -- and ultimately storm -- out of this disturbance within the next couple of days. Track will ultimately depend on whether or not that cut-off upper low ahead of it actually does develop and forces the disturbance on a more northwesterly track. Otherwise, it's likely to ride more westward (or west-northwestward) under the building ridge. Storms that form well to the east end up ultimately being fish storms, but it bears watching nonetheless to see how far west it does make it (if it makes it terribly far west at all).

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