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IWIC Atlantic Tropical Weather Discussion - 19 July 2004 - 6:45 PM EDT

We are continuing to monitor a vigorous tropical wave that is approaching the lower Winward Islands. The overall appearance of the wave has not changed much within the past 24 hours. However, there are a few interesting things that should be noted. An earlier SSMI pass clearly indicated that some dry air is wrapping into what appears to be mainly a mid-level circulation, since scatterometer winds are showing no signs of a surface circulation. The dry air entrainment will keep thunderstorm activity in check for the time being. Thunderstorms will continue to fire but they won't blossom enough for further development within the next 24 hours. In addition to dry air entrainment, infrared satellite imagery now indicates that the western fringe of the wave is being inhibited by light to moderate southwest shear. There is still some upper level ridging in place, so the wave is not expected to be sheared off completely anytime soon. Furthermore, the wave is still moving westerly at 25 miles per hour, and the bulk of the convection is just west of the center, certainly not a great sign for organization.

The wave will pass through the islands within the next 12 hours, with the potential for heavy rainfall and wind gusts as strong as tropical storm strength. Caribbean water vapor imagery reveals that dry air will continue to be a problem as the wave enters the eastern Caribbean. It would not be surprising if some of the convection were to diminish within the next 48 hours. However, upper level winds will still likely be marginally favorable for slow development.

The expected path of the wave is up in the air at this point. The global model consensus is for more of a west-northwest track through Puerto Rico and Hispaniola. The globals weaken the western periphery of the subtropical ridge, allowing the low level steering flow to shift from due east to east-southeast. The globals also indicate that upper level ridging will still be in place over the wave once it approaches the Bahamas. If this solution were to verify, all bets are off. There are signs that the subtropical ridge may build west later in the period, and that would cause the potential low to stall for the time being. Even with the ridge in place, the globals still indicate no development. As already said, all bets are off if this solution were to pan out. On the other hand, the tropical models keep the wave on a westerly path, placing the center south of Cuba and Jamaica in 120 hours. This would decrease the potential for development, as the central and western Caribbean are bone dry at the moment. There are no signs of conditions changing in the near term.

The most reasonable solution at this time appears to be that suggested by the global models. While the wave is still racing westward, the low level steering flow has changed slightly over the past 12 hours, in favor of the global model consensus. This keeps the door open for slow development. The eastern Caribbean is known as the graveyard for tropical storms, and conditions aren't exactly supportive for development in that area. But the wave will have to be monitored since it still appears to be heading into the western Atlantic, where a lot can happen. Very slow development is possible over the next several days.

Elsewhere, activity behind the wave is also beginning to increase. These features will have to be monitored as they gain latitude while approaching the lower Lesser Antilles.

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