TD4 still organizing...

Based on infrared satellite presentation and a QuikSCAT pass (active microwave scatterometer that can measure surface winds over water), TD4 has been held at Depression status as of 15Z today.  It is definitely on the threshold of being named though.  The latest intensity estimate is 30kts and 1007mb.

The most recent visible satellite imagery as of this writing shows deep convection over the low-level center, rather well consolidated.  There's healthy outflow everywhere, but moreso to the south, as was the case yesterday.  Two negatives for it are stable air to the west and north, and slight southeasterly vertical shear.  It's tracking WNW at 14kts, and is located just southwest of the Cape Verde Islands. 

It's still forecast to gradually intensify and track WNW for a few more days, then recurve northward through an anticipated weakness in the subtropical ridge.

Elsewhere, there are two points of interest, one in the central part of the basin, near 50W, and one still over the heart of Africa, at about 5W.  The one still over Africa obviously has a few days before it even hits the ocean, but it looks ominous.  And the one nearing the Lesser Antilles is definitely worth keeping an eye on.  In fact, there's a broad elongated envelope of enhanced cyclonic vorticity from about 45-60W along ~12N, and there appears to be at least two embedded mid-level circulations.

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Tropical Depression 4 forms in the far eastern Atlantic...

A very strong African Easterly Wave born in far eastern Africa back on August 14 exited the coast Sunday afternoon.  It quickly acquired a mid-level circulation and later on Monday developed a low-level circulation as well, prompting the upgrade to TD4.  As of this writing, it has deep convection in large spiral bands, but not too concentrated yet over the center... divergence aloft is healthy, with a stronger outflow channel to its south.

The 21Z advisory places the center at 12.3N 21.0W, or just south of the Cape Verde Islands.  The satellite-estimated intensity is 30kts and 1007mb.  Motion is toward the WNW at 11kts.

It is forecast to slowly intensify and track NW through a weakness in the subtropical ridge.  It seems unlikely as of now that this will be the type to travel the full distance across the basin before recurving.  The next name on deck is Debby.

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Possible Depression forming...

The tropical wave I had been mentioning since LAST Wednesday has finally entered a more favorable environment (the problem was primarily dry, stable air surrounding it) and has been sporting a cold CDO for the past 12-18 hours.  However, a recent aircraft flight into the system was unable to find a closed circulation, so for now, it remains a strong tropical wave.  There is decidedly a closed mid-level circulation though.  It is located over Barbados and tracking W at 15kts.  I was perusing a series of soundings from Barbados over the past 48 hours, and the precipitable water increased from 4.34cm to 5.56cm in that time... a likely sign that whatever Saharan Air Layer was present has since weakened/dispersed.

Forecast models take it WNW into the western Caribbean over the next 5 days.  Statistical models intensify it to at least a minimal hurricane, while the dynamical models don't even bring it up to a TS.  This will be an interesting "competition"!  If it develops, it would be TD4 or TS Debby.

The other two easterly waves at 38W and 86W are not of great concern right now; they suffer from dry air and high shear, respectively.

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