Small town USA guy. Politics nerd. Soccer fan. Interested in eyewalls, deformation zones, and hook echos.
By: TropicalAnalystwx13, 3:20 AM GMT on September 18, 2011
For the first time since August 18, there are no active tropical cyclones in the Atlantic basin at this time. However, that doesn't mean we aren't watching any areas. In fact, we are watching quite a few tonight. The first is a tropical wave located nearly midway between the Leeward Islands and Africa. This area is producing scattered shower and thunderstorm activity, and has broad cyclonic rotation embedded within the monsoonal trough. The National Hurricane Center is currently giving this area a 10% chance of developing into a tropical entity over the next 48 hours. Given its satellite appearance and overall structure, I will agree with these percentages for now. According to the latest 850 mb. vorticity map from CIMSS Tropical Cyclones, there is a broad area of vorticity stretching from 30W to 40W, with the highest concentration located near 38W. This elongation is likely due to the fact that there are two low pressure areas in the vicinity, per the 18z Tropical Atlantic surface analysis. In my opinion, these areas of low pressure will begin to consolidate as we head into tomorrow, and begin to strengthen as well. Wind shear over the tropical wave is a low 5-15 knots, and SAL is very little at the very best. Sea Surface Temperatures are very warm, near 28 °C, which is approximately 2 °C what is needed to sustained a tropical cyclone. All things considered, I believe this tropical wave will become the next tropical cyclone of the already above average 2011 Atlantic hurricane season. However, this wave may run into troubles down to road, as wind shear begins to increase thanks to an Upper Level Low currently situated near Puerto Rico.
Figure 1. Central Atlantic Rainbow Imagery shows deep thunderstorm activity associated with a broad area of cyclonic rotation embedded within the monsoonal trough.
The second area I am watching, which is farther east, is Invest 97L. This area has virtually no thunderstorm activity, and has very lackluster organization. The National Hurricane Center is currently giving this area a 20% chance of developing into a tropical cyclone over the next 48 hours. I think these percentages may be a little too generous, and I would put them more near 10%. Wind shear values, Sea Surface Temperatures, and the amount of SAL are nearly the same as the wave in front of it, but it may have to compete to develop over the coming days. The reason I say this is because there is a strong tropical wave located over Western Africa, which seems to be pulling 97L's energy in that direction. The chances of development would increase if they both combine, which is definitely possible over the coming days. If 97L becomes a tropical cyclone, and quickly, then it would likely curve out to sea early, without affecting anybody. However, if it waits until the Central Atlantic to develop, if it develops at all, then it may be more of a threat to the Leeward Islands. There is broad rotation with the invest, so it will definitely need to be watched for development as we head into this coming week. All things considered, I think there is a fair chance it becomes our next tropical cyclone of the 2011 Atlantic hurricane season.
Figure 2. Nighttime Visible Satellite imagery of Invest 97L, associated with lackluster thunderstorm activity at this present time.
Lastly, I wanted to touch briefly on the possibility of Caribbean trouble by the end of this month and into October. After jumping on and off the development train, some of the reliable computers once again show the potential for Caribbean development by the last week of September and into the first week of October. What the models disagree on is what octant the Madden-Julian Oscillation (MJO) will be in. Some have it being in an area that brings sinking air to the Atlantic basin, which would tend to suppress thunderstorm development, while others take it into the area that brings upward motion into the area, promoting thunderstorm development. However, one thing that we will likely see is a more moist Caribbean as we head into the last week of September, and things can quickly spin up in that region because of the Sea Surface Temperatures and Tropical Cyclone Heat Potential. All things considered, we will have to watch the Caribbean in 7-10 days or so from now.
Figure 3. The GFS 15-day Madden-Julian Oscillation (MJO) forecast from the Climate Prediction Center.
Thanks for stopping by,
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.