99L Organizing, Will Likely Bring TS Conditions to Lesser Antilles
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The main feature of interest in the Atlantic continues to be Invest 99L east of the Antilles islands The system has gradually become better organized over the last couple of days, with moderate-deep convection persisting over a developing area of surface low pressure. This convection is not yet well-organized, however, and 99L is still largely attached to the ITCZ, and it will need to detach from this in order to close off its own surface circulation. This could happen within the next day or two, and would likely result in the NHC designating this a tropical depression. The system has not exploded with strengthening, and thus has not been able to gain enough latitude to pass north of the big Caribbean islands, and will instead be trekking through the Caribbean itself. 99L, depending on exactly where the surface center develops, looks like it will pass into the Caribbean through the southern half of the Lesser Antilles, and we should get a good look at 99L through the new Barbados doppler radar system.
There will be numerous struggles for 99L in the central-eastern Caribbean. The biggest one will be strong trade winds redeveloping in the Caribbean behind the tropical wave that is running in front of 99L, that will make it difficult for 99L's circulation to hold itself together. There is also still large-scale sinking in the Caribbean as the MJO is still in the Pacific, and is not supporting the Atlantic with upward motion yet. There will also be some wind shear imparted by a TUTT-like upper trough currently situated north of 99L that will be expanding westward as 99L moves into the Caribbean. 99L's eventual separation from the ITCZ will also open it up to entrainment of the dry air to its north and to its west in the Caribbean. Due to all of these things, it seems likely that any strengthening of 99L will halt after it passes the Antilles Islands, and the system will likely struggle to survive after that point, possibly even dissipating.
However, 99L's story may not be that short. If 99L becomes sufficiently organized to survive a trip through the central Caribbean as a defined entity, conditions may improve in 7-10 days in the western Caribbean and Gulf of Mexico. This region will be west of the TUTT axis, in a good position for being ventilated by upper-level ridging that the GFS ensembles are now showing should develop west of the TUTT late next week. In addition, the MJO will have moved farther into the eastern Pacific by that time, and may decrease the large-scale sinking over the area, making the environment more hospitable for a tropical system. Such a situation could result in 99L restrengthening west of 80W, as long as it can avoid hitting Nicaragua or Honduras straight away. No global model currently strengthens 99L into anything significant, but most models do keep it as a defined enough entity throughout its journey that once into the western Caribbean, it may be able to cause problems, even though the models do not show it yet. It is too soon to know whether 99L will curve northwestward into the Gulf of Mexico in the long range or move into Central America. That will depend on how well 99L handles the Caribbean, and whether it survives to make it that far west.
Overall, we still have a lot of time to monitor this system. The Lesser Antilles, especially the southern half, will deal with 99L first, likely receiving tropical storm-like conditions late Friday well into the weekend, but not a big deal for them. The rest of the Caribbean farther west along with central America and the southeastern United States should keep a wary eye on the situation due to its long-term potential.
We shall see what happens!