Invest 99L developed into Tropical Depression #5 yesterday, and has maintained about the same intensity since then. Thunderstorm activity partially covers the center, but is still clearly being limited by wind shear being imposed on the northern side of the circulation by an upper low over the central Atlantic. This wind shear will continue to be a moderate issue for TD 5 over the next few days as the upper trough expands westward a bit to the north of the big Caribbean islands, following TD 5 as it too moves westward. The bigger issue for TD 5 though will be the strong trade wind flow that it is entering. The depression is now moving at 21mph, indicating the increasing flow that it is becoming embedded in, which will make it difficult for its circulation to survive and for thunderstorms to develop over it. This is a typical pattern for the eastern Caribbean that makes this region unfavorable most of the time for developing tropical systems, but especially during El Nino years like this one. Due to this pattern, I don’t expect TD 5 to really strengthen at all for the next 3-4 days. It may get named Ernesto, but should remain a very weak tropical storm for the time being, and could even degenerate back into an open wave at some point if it loses the westerlies on the southern side of its circulation. The lesser Antilles will get minimal tropical storm conditions from this starting tomorrow, but not a big deal there.
However, TD 5’s best days may lie well ahead of it. If TD 5’s circulation survives as a defined entity until it reaches Jamaica, the pattern starts to turn in its favor. Right now there is an upper trough over the northwest Caribbean that won’t be moving much over the next 7 days, and neither will the upper trough over the central Atlantic. As TD 5 moves westward, the NW Caribbean upper trough will start backing away to the southwest in front of TD 5, a situation that is almost always favorable, since it allows upper-level ridging to expand over the system and cause light wind shear and divergence aloft that promotes convection and lowering of pressures. The positioning of the TUTT-like trough to the northeast of TD 5 at that time would further improve ventilation of the area in general. The GFS ensemble mean supports this, as I show in the video. Such a pattern could rapidly become conducive for significant strengthening if TD 5 is organized enough to take advantage down the road in the northwest Caribbean and quite possibly the Gulf of Mexico.
As far as the future track goes, much will still depend on how strong TD 5 can get west of Jamaica if it is still alive, but some general ideas can be discussed. The GFS specifically and now the ECMWF have been leaning northwestward with TD 5’s track in the long term, and it is worth investigating whether this could make sense. I show in the video how the typhoon pattern in the western Pacific looks like it is connected with this idea, causing amplification in the pattern that ripples downstream and strengthens a trough over the eastern U.S. in 7-10 days that may break down the ridge over the Gulf of Mexico and allow TD 5 to turn northwestward towards the gulf. The strengthening of the eastern trough also looks like it could force the Texas heat ridge far enough back into the Rockies that Texas itself could be open to a hurricane hit in this kind of a situation, but such a pattern leaves everywhere from central America to Louisiana open to a hit, and specifics cannot be known this far in advance.
Overall, TD 5 is no big deal in the near term, and will be directly affecting only the Lesser Antilles as a tropical depression or minimal tropical storm tomorrow and Saturday. Jamaica and the rest of the NW Caribbean may have to deal with TD 5 in 4-6 days, possibly as a restrengthening system of higher caliber than it is now if it survives the trip through the Caribbean and is able to take advantage of what should be a much more favorable pattern for intensification in that area. Interests in the NW Caribbean, central America, and the Gulf of Mexico should keep a wary eye on this situation due to its possible long-range implications.
We shall see what happens!