97L East of Windwards Being Watched

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IWIC Atlantic Tropical Weather Discussion - 20 July 2004 - 3:00 PM EDT

The strong tropical wave that we have been closely monitoring has passed through the Windward Islands and is currently located in the eastern Caribbean Sea. A weak mid-level circulation is still present, but the low level cloud movement in visible satellite imagery and the most recent QuikSCAT pass both show nothing more than general easterly flow. Furthermore, wind observations in the lower Windward Islands does not indicate even the slightest wind movement from the west, so all of the available data confirms that there is no low level circulation associated with the tropical wave. As far as organization is concerned, we have not seen any improvement in the satellite appearance over the past 24 hours. If anything, the system has become less organized, as there is no longer discernable anticyclonic flow aloft. A rather sharp outflow boundary has developed north and west of the axis, which is disabling ventilation. Convection on the other hand has been persistent. However, it is being blown to the northwest by strong south-westerly shear induced by an upper level low the north. To some extent, the same atmospheric mechanisms shearing the wave are also helping to enhance the convection by creating divergence. To add to the strong shear problems, moderate subsidence is present around the wave. This entire environment is not ideal for a tropical cyclone to form. Given that and the tropical wave's current organization, no development is expected in the near-term. But the system still needs to be watched closely for a few reasons. There are two possible tracks it will take over the next few days. Either one of them could bring the tropical wave into more favorable conditions for development.

The first possible track is through Hispaniola and into the southwestern Atlantic just east of the Bahamas. This movement will probably occur if the wave becomes deep enough to be felt by a strong trough currently exiting the United States eastern seaboard. The trough will erode the western prehiphery of the subtropical ridge, therfore altering the mid-level steering flow in the eastern and central Caribbean Sea to more south-southeasterly. If the wave is deep enough, this would allow for it to escape the Caribbean Sea and get picked up by the trough. This motion is indicated by most of the global models, such as the CMC, NOGAPS, and UKMET. The major problem with this track is that the wave is currently not deep or strong enough to be influenced by something besides the easterly flow.

This is why a second possible track has to be considered. In the second scenario, the tropical wave is pushed further west into the western Caribbean Sea and maybe eventually into the southeastern Gulf of Mexico. This track is likely to happen if the wave remains shallow enough to escape the mid-level south-southeasterly flow in response to the trough. All of the tropical models, including the BAMM, BAMD, and GFDL forecast this movement. Additionally, the ECMWF and now possibly the GFS show a more westward track. The GFS was earlier in agreement with the other global models on the first scenario, but the latest 12Z run actually has the main energy associated with the tropical wave continuing westward after a slight interaction with the trough. It will be interesting to see if the GFS continues to show this in the upcoming runs, but in any case, there currently appears to be more model support for the second possible track than the former.

Putting models aside and focusing back to the current and progged steering currents, we are leaning towards the westward track. While the trough may pull some of the energy associated with the wave, there is no doubt the wave itself will continue westward. As mentioned further above, conditions will not allow any intensification over the next day or so at least, so therefore, it will probably not become any deeper than it is now anytime soon. Even if the wave is more organized, being pulled by the trough means passing through Hispaniola, which will weaken or destroy any low level circulation that is present. Therefore, we could see the wave initially be pulled by the trough only to become more shallow by terrain interaction and once again resume a westward course. In either case, a motion governed by the low-level easterly steering flow is likely, and is what we expect to occur.

With a continued westward track in mind, this means the wave will be located the northwestern Caribbean Sea or near Cuba within the next few days. Around this time, a broad upper level anticyclone is progged to advance southward into the area. The exact positioning will be crucial as to how much shear the wave is exposed to, but the overall idea is that the environment will be at least partially favorable for intensification. Keep in mind there may not be much left of the wave in terms of convection due to the absence of divergence that it is currently experiencing. Any mid-level circulation may be gone as well from the close encounter with the trough. However, with fairly conducive conditions expected in place, slow intensification is a possibility once in the northwestern Caribbean Sea. Again, development is highly unlikely in the near-term, but it is the long-term where things may become a little more interesting. Our current thinking of a westward course raises the possibility of a track into the Gulf of Mexico. It is too soon to say with certainty where the system will move and how strong it will be at that point, but in any case, it is a system that still bears watching. No development is expected at this time, but some slow development cannot be ruled out later on as it moves westward.

Elsewhere, activity behind the wave is also beginning to increase. Nothing shows signs of developing at this time.

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Rob Mann
IndependentWx.Com

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