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Tropical Depression Six arrives

By: Dr. Jeff Masters, 5:14 PM GMT on August 22, 2010

Tropical Storm Tropical Depression Six is here, but it will not be a threat to land for at least the next five days. Tropical Depression Six is a classic "Cape Verdes"-type storm common during the peak part of hurricane season. Cape Verdes-type storms are so named because they form from tropical waves that come off the coast of Africa and pass near the Cape Verdes Islands just west of Africa. Cape Verdes hurricanes are the largest and most dangerous types of hurricane in the Atlantic, since they spend a long time over water have and have of opportunity to reach full maturity. Tropical Depression Six has a ways to go before it becomes a hurricane, as the storm is embedded in a strong easterly flow of wind courtesy of the African Monsoon that is generating a moderately high 15 - 20 knots of wind shear. There is also a tropical disturbance to the northeast of TD 6 that is sucking away some moisture and is interfering with the storm's circulation. Sea Surface Temperatures (SSTs) are a warm 28°C, and the storm is embedded in a moist environment, so wind shear is the primary inhibiting factor for development. The strong east winds imparting the shear are keeping any heavy thunderstorms from developing on the east side of the center of circulation, which is exposed to view in satellite imagery (Figure 1.)

Figure 1. Afternoon satellite image of Tropical Depression Six.

Forecast for Tropical Depression Six
A ridge of high pressure will force Tropical Depression Six to the west-northwest for the next five days, and the system should increase its forward speed from its current 10 mph to 15 mph by Monday night. A powerful trough of low pressure over the mid-Atlantic Ocean will begin to pull Tropical Depression Six more to the northwest late this week, and the storm should pass well to the northeast of the Lesser Antilles Islands. It remains to be seen, however, it this trough will be strong enough to fully recurve Tropical Depression Six out to sea. The GFS predicts that Tropical Depression Six may pass close to Bermuda about eight days from now, and it is also possible that Tropical Depression Six could eventually hit the U.S. East Coast 9 - 15 days from now. However, we have no skill in making these sort of ultra-long range forecasts, and the long-range fate of TD 6 is uncertain.

Elsewhere in the tropics
The ECMWF and NOGAPS models are predicting formation of a tropical depression off the coast of Africa 3 - 4 days from now.

Jeff Masters


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