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IWIC Atlantic Tropical Weather Discussion - 19 July 2004 - 5:30 AM EDT

A vigorous tropical wave has emerged from the Intertropical Convergence Zone near 55ºW. Shortwave infrared imagery indicates that a low to mid level circulation is present. I might add that the current satellite signature is very impressive. Outflow is fair in all quadrants. Upper level winds are almost non-existant and a strong ridge aloft will likely keep upper level winds favorable for development over the next several days. Convection is not a problem either. Over the past month, several waves have flared as a result of divergent winds aloft. That does not appear to be the case this time around. In fact, much of the thunderstorm activity developing along the wave axis is likely surface induced, a good sign for barotropic (tropical) development.

As for the inhibiting parameters, subsidence ahead of the wave and the wave's quick forward motion could cause some problems. the wave is racing westward at 25MPH, and that will hinder the organization process. Meanwhile, the wave will have to contend with dry air as it approaches the eastern Caribbean Sea. These are the two parameters that raise doubt in terms of development potential.

Interestingly enough, none of the global models are forecasting development at this time. You can debate that this may be one of the reasons why the Tropical Prediction Center has been so hesitant to discuss this feature in detail. An invest has been issued, and that is probably an indication that TPC is also interested in the current signature of the wave. While the available model guidance does not show much in the way of development, they do show hints of the wave continuing to progress west-northwest towards Puerto Rico and Hispaniola. This is in response to the weakening of the subtropical ridge as a strong trough pushes slightly east. This will create a weakness over Hispaniola and Bahamas. This could lead to the wave taking a similar path. The low level steering flow is coming out of the east underneath the ridge, but that may change in the eastern Caribbean over the next 72 hours.

In summary, the tropical wave is fairly organized, with strong ridging aloft that will keep upper level winds condusive for development over the next several days. Sea surface temperatures also support further development. Meanwhile, dry air and a quick low level steering flow will try to keep the wave in check. The wave will need to develop a well-defined central core so that it can sustain itself and further organize. The surface circulation must become easily apparent and convection must continue to persist near the center. Basically, we need persistent convection near the low level circulation. If these things mentioned do not occur, then the potential for development will rapidly decrease. The critical timeframe for development will be the next 24 hours. If the wave doesn't continue to organize over the next 24 hours, then any potential for development will likely be limited at best. Residents in the southern Windward Islands should be prepared for heavy rainfall and strong winds over the next 36 hours. Slow development is possible within the next 24-36 hours.

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