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IWIC Atlantic Tropical Weather Discussion - 21 July 2004 - 8:30 PM EDT

Convection associated with the tropical wave traversing the central Caribbean Sea is beginning to dissipate this evening. Convection along the wave axis has typically decreased in the evenings, which is a natural part of the diurnal cycle of tropical cyclones. The latest SAB estimate remained at 1.5, but that estimate will likely drop slightly until convection begins to fire once again. The upper level low in the western Atlantic still hasn't lifted its grip from the wave. Moderate westerly shear is apparent. GHCC visible satellite imagery revealed a weak low level circulation pushing west, ahead of the core of convection earlier this afternoon. Over the past few hours, that circulation has dissipated and only the mid-level circulation to the east remains. The mid-level circulation will be the main area of interest over the next few days.

The upper trough conjoined with the western Atlantic upper level low is stretched across the northern Caribbean Sea. While the trough is shearing the wave to some extent, it is still creating a lot of divergence in the central and eastern Caribbean, which is still aiding in the development of nocturnal thunderstorms. Over the next 36-48 hours, the wave will seperate itself from the trough. Ridging aloft will allow for upper level winds in the western Caribbean to become more condusive for tropical development. However, we must also remember that sinking air is also playing a big roll in keeping the wave in check. As the disturbance moves away from the trough, it will have to do develop more at the surface in order to sustain convection on its own. We may not see a rapid explosion of thunderstorms as we've observed over recent days. However, persistent development would be a much better indication of tropical cyclone formation. Slow development is still possible over the next 72 hours. Another reconnaissance aircraft is scheduled to investigate the wave tomorrow morning.

The forecast path of the wave is starting to become a bit clearer. The tropical model consensus is for the wave to make landfall along the east coast of the Yucatan peninsula in approximately 72 hours. While it is still difficult to determine where the global models take our system, the majority of the models do show signs of an expected westward track. For example, the latest ECMWF and UKMET runs show faint signs of a weak area of low pressure passing through the Yucatan peninsula in a few days. These are all perfectly reasonable forecasts. As mentioned in the previous discussion, a mid to upper level ridge is in place over southern Florida and the Bahamas. This ridge is causing a strong east to west flow over the western Caribbean. I see no reason to believe that the wave will be able to take a more northerly track through the Yucatan Channel. Even if the wave were to develop into a tropical depression or storm within the next 48 hours, it will still be steered by the mid to upper level high.

What could end up being the million dollar question, is whether or not the ridge will push far enough east by the time the disturbance moves into the central/western Gulf to turn more northerly. Going back to the UKMET and ECMWF, both models also show this scenario. This is well beyond the predictable forecast range and any movement beyond the time it threatens the Yucatan is unclear.