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IWIC Atlantic Tropical Weather Discussion - 21 July 2004 - 12:00 PM EDT

A vigorous tropical wave in the central Caribbean Sea has flared for the third morning in a row. Dry air is encompassing the wave, but the subsidence appears to be having only a limited effect on thunderstorm activity. In all likelyhood, the remaining wind shear aloft is causing enough divergence to enhance convection. Additionally, a surface circulation may also be triggering bursts in activity. Latest Satellite Analysis Branch Dvorak estimates are 1.5, the highest the estimate has been for this wave.

Morning visible imagery reveals that the overall structure has become much better organized over the past 6-12 hours. This is a clear indication that upper level winds are becoming more condusive development. Shear levels in the low levels of the atmosphere are now fairly light, with only moderate winds aloft. The mid to low level circulation is located along the western side of the strongest core of convection. Visible imagery also suggests that a surface circulation may be in the beginning stages of development. There is some evidence of more turning at the surface based on low level cloud features. Regardless, it may not be long before the first tropical depression of the season is classified. A NHC reconnaissance aircraft is scheduled to investigate the area late this afternoon. If the recon can find a closed LLC, we could very well have tropical storm Alex, since winds may already be stronger than 39MPH.

Looking down the road, conditions are expected to become more condusive for development. Over recent days, the tropical wave has had to contend with dry and and moderate wind shear mainly due to a strong upper low situated over the western Atlantic. An upper level anticyclone will develop over the western Caribbean within the next 72 hours. Winds in all levels are expected to decrease, which will make ventilation a lot easier. Therefore, the potential for development increases as the wave enters the western Caribbean. Some dry air will still be in place, and that could become a problem, only if sinking air is entrained into the surface circulation. But we don't expect this parameters to completely inhibit development. The SHIPS model takes the low up to 76 knots, but SHIPS forecasts must be taken with a grain of salt, at least until we have a classified tropical storm. Tropical depressions are not included in the SHIPS database, so these forecasts are not very reliable.

It is rather obvious that the wave is not taking a northerly track into the Bahamas or southwest Atlantic. The low level steering flow is pushing the wave west. This general movement should continue for the next several days. However, the amount of latitude gained over the next 48 hours is crucial for the long term. The BAM models keep the developing low on a westerly track. At 96 hours, the BAM models place the system over Belize. The 5-day GFS 500MB forecast shows similar results. While the GFS does not pick up on the wave very well, it does show that the easterly flow will remain in place. The main steering feature is a mid to upper level ridge currently in place over the Florida Straits area. If you take a look at a water vapor loop, you can see faint signs of the ridge. The ridge is expected to remain in place over the next few days, causing the easterly flow to remain strong. Based on this data, the most reasonable forecast is a track into the Yucatan peninsula. But what we have to watch for, is if the wave can gain enough latitude, it may have a slightly higher chance of moving into the central Gulf. It must be noted that none of the global models have been able to grasp the wave very well. Model data should become much more reliable within the next 24 hours.

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