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ClarkModerator
Meteorologist


Reged: Wed
Posts: 1710
Loc: 45.95N 84.55W
A slow start to Cape Verde season ahead?
      #75892 - Sun Jul 22 2007 06:26 PM

A couple of early season Cape Verde tropical waves have come and gone thus far this season without much fanfare. Meanwhile, a chain of waves over Africa is currently looking fairly impressive for this time of year. Entering late July, we're moving into a time period where, climatologically-speaking, we start to turn our focus a bit more toward the central and eastern Atlantic. Putting those second and third elements together, one might surmise that one of those forthcoming disturbances might have a good shot at developing -- perhaps one of the later ones, after the others have 'preconditioned' the environment with ample low level moisture.

I'm not so sure of that. In fact, I think we might see somewhat of a delayed start to the Cape Verde season. Take a look at this SST anomaly map from today:
http://weather.unisys.com/archive/sst/sst_anom-070722.gif

Over the past month, the magnitudes of the negative SST anomalies in the main development region (MDR) of the tropical Atlantic have maintained themselves and expanded westward from the coast of Africa to now encompass the entire MDR between 10-20N. This is better highlighted by looking at the change in SST anomalies from mid-June to mid-July; if you use the Unisys archives at http://weather.unisys.com/archive/sst/ you can see them for yourself.

Currently, SSTs across the region look like this: http://www.nhc.noaa.gov/tafb/atl_anal.gif (time-sensitive).

In general, SSTs are at about 27C along 10N from the coast of Africa to 50W and cool to about 25C north of there.

And the result? Significant low-level stability, both in terms of cold air stratocumulus forming above the cooler waters as well as a strong trade wind inversion due to significant ridging across the region. You can see the stratocumulus for yourself by using the visible satellite image from http://www.ssd.noaa.gov/PS/TROP/DATA/RT/EATL/VIS/20.jpg (updated in real-time; not particularly time-sensitive).

Disturbances south of 10N have a somewhat favorable environment in which they can develop, but north of 10N is a different story -- and it appears that the ITCZ is already at or near that latitude. That means that we're likely still some time away from a significant Cape Verde threat.

How long will this last? Not sure, though climatology argues that even if the SSTs and low levels remain 1C below normal they will still rise above the typically accepted thresholds for development into August. But, as has been shown in various threads here, there is quite a bit of heat content available in the western Atlantic, Gulf, and NW Caribbean. Conditions for development improve west of 50W, suggesting that disturbances that can hold together until the Lesser Antilles and find a favorable upper level environment could pose a threat to North America and the Greater/Lesser Antilles.

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