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July is closing out as a very quiet month in the Atlantic, which is to be expected in an El Nino year, but we are now heading into August where activity starts to pick up as the peak of the season nears. Evidence of this has been emerging over the last week or so as stronger tropical waves have moved off of Africa and started moving across the Atlantic. One wave is moving across the Antilles right now, bringing showers and thunderstorms, but will be quickly torn apart by the Caribbean trade winds and will not be a significant threat for development. Behind this wave, however, is an area of low pressure embedded within the ITCZ that has more potential for development over the next few days. This low has been designated Invest 99L. The CMC for a couple of runs now has shown development, and the GFS and UKMET at one point or another have both supported development of a tropical storm prior to 99L reaching the islands. The ECMWF has been relatively unimpressed. 99L is currently broad, and needs to gain latitude away from the ITCZ if it is to develop a tighter circulation. If it is going to try to develop, it will likely be about 48 hours before it tries in earnest.
There is still a lot of sinking air in the tropical Atlantic right now as the MJO upward motion pulse is over in the Pacific, away from our area of the world, so dry air and suppression of convection will be struggles that 99L will have to deal with. The system will be crossing the Antilles Islands in about 5 days, though how much latitude it will have gained by then is uncertain, and largely depends on how much it strengthens up until that point. A stronger system will tend to gain more latitude. This will become important because if 99L spends any significant amount of time in the Caribbean, it will likely die off, suffering the same fate as the wave ahead of it due to the strong trade winds there. If, however, 99L can quickly strengthen and lift north of the Caribbean island chain, it may have a better chance at becoming or remaining a tropical cyclone. The HWRF and GFDL models are supporting the idea that an on approach to the Antilles 99L will already be weakening due to the increasing trade winds in which it will be embedded, which is significant given that these two models often strengthen storms even in the face of unfavorable conditions in the deep tropics once they get going.
If 99L does move north of the Caribbean, it will likely recurve somewhere from the U.S. eastern seaboard to Bermuda due to a mean trough that is forecasted to be over the eastern U.S. during the next 10 days or so, a pattern that prevails often in summers like this one. It is too early to speculate on any potential effects that 99L could have on land masses west of the Antilles Islands.
Overall, 99L is a threat for potential development during the next 4-5 days, and may affect the Antilles Islands with tropical storm-like conditions even if it is not a tropical storm by this weekend. Explosive development is unlikely given some of the unfavorable conditions the system will be dealing with, but this opportunity does herald the start of the peak of the hurricane season, where more of these disturbances and tropical waves in the central Atlantic will try to develop, especially once they get north of 20N.
We shall see what happens!