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Posted at 12:35 PM ET, 08/10/2012

An active tropical Atlantic spawns two new systems as Ernesto dissipates

After watching Ernesto for the past 10 days, we wrap up the week with a duo of active systems in the central and eastern Atlantic. One is headed for the Lesser Antilles this weekend, and the other is just off the coast of Africa.

Satellite image of tropical depression 7 in the central Atlantic. On the left part of the screen are the Lesser Antilles. (Naval Research Laboratory - Monterey)
Advisories began on tropical depression 7 at 5:00 p.m. EDT yesterday, and as of 11:00 a.m. EDT today, it is still a depression with maximum sustained winds of 35 mph and a 29.80“ (1009 mb) central pressure. It is located 775 miles due east of Martinique in the Lesser Antilles and racing westward at 23 mph.

Track forecast for tropical depression 7
It is not forecast to intensify very much because the large-scale environment is fairly hostile along its projected path. Although the ocean is plenty warm, it is ingesting low-to-mid level dry air from the north and west, and the vertical shear (the difference of winds from the surface to the top of the storm) is expected to remain high for the next several days.

The latest forecast track from the National Hurricane Center (above) brings it to the Lesser Antilles early Sunday morning, then into the northern Caribbean as a tropical storm. If it intensifies and earns a name, Gordon is next on the list.

East Atlantic tropical wave

The disturbance that has been rumbling across Africa since August 2 finally made its way to the Atlantic Ocean last night, and as anticipated, is a noteworthy feature.

I’m showing a visible satellite image from 8:15 a.m. EDT today, and have placed red ‘L’s on what I believe to be two distinct circulation centers. The one to the north is stronger and has a 29.65“ (1004 mb) central pressure, but the one to the south is a bit of a surprise yet potentially significant. Typically, a disorganized disturbance might have multiple centers, or an elongated center, but this was such a high-amplitude wave that there appears to be two separate and viable lows.

Satellite image of tropical wave from Africa with two areas of low pressure
The model guidance was run on the dominant northern center, and there is good agreement for a WNW track over the next five days, with just slight intensification if any.

If the northern low develops, it would not be a threat because it would very likely recurve to the north long before reaching land. Since the southern circulation is a fairly new and unexpected feature, models do not even “know” about it yet, and as such, it won’t be represented accurately in them. Of course, it may turn out to be too weak and transient to be a concern, but it bears watching.

Although the southern circulation is weaker and may not persist, the environment is better for it to develop. There is a large amount of dry air (the SAL, or Saharan Air Layer) surrounding the primary northern circulation. The environment down by 9N is moister and more conducive for deep convection.

The grayscale infrared image below has the strength of the SAL overlaid in yellows and reds. The more red the shading, the stronger the SAL (dustier and likely drier) and the worse it is for a developing tropical cyclone. This system, whether it intensifies or not, is not a threat to land any time soon.
Illustration of the amount of low and mid-level dry air over the eastern tropical Atlantic (UW/CIMSS)

* The author, Brian McNoldy, is a senior tropical weather researcher at the University of Miami/Rosenstiel School of Marine and Atmospheric Science. He is a new tropical weather blogger for the Capital Weather Gang.

By Brian McNoldy*  |  12:35 PM ET, 08/10/2012

Categories:  Latest, Tropical Weather

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