Tropical weather analysis - July 31, 2012
A vigorous tropical wave located about 1150 miles east-southeast of the Windward Islands is showing signs of organization. Last light visible satellite images and recent microwave data indicated a gradually improving circulation. Unfortunately, due to issues with the website, I have been unable to utilize ASCAT or OSCAT data, and WindSat missed the disturbance.
Figure 1. Latest infrared satellite image of Invest 99L. Image credit: NOAA
An upper low near 27N 44W has intensified over the past few hours, and has a rather large horizontal extent of cyclonic vorticity. Apparently, the southern end of this flow has reached the northern fringes of Invest 99L. As a result, upper-level outflow has improved in that direction.
All of the common factors appears to be present for intensification -- low shear, warm ocean temperatures, and a relatively moist environment. On the other hand, this low is rather close to the equator, which will limit the amount of spin the system can generate. In addition, the system is still embedded in the Intertropical Convergence Zone (ITCZ), a low-latitude belt of convection that is responsible for much of the precipitation across equatorial regions. Until 99L can break away from that, it will be slow to organize. There is currently very little indication of any northward component of motion, and 99L still remains below 10N. However, with the influence of the upper low spreading south, a slow increase in latitude should occur over the next few days. Although 99L has done well for itself up to this point, it is not uncommon for tropical waves or perturbations to look healthy while the ITCZ is aiding in precipitation. The test of time as it were, will come after the system breaks free from the grip of the ITCZ. At that point it will be forced to sustain itself on its own dignity. Since there is still dry air outside the comfort of the ITCZ, I find it a little hard to believe that 99L will rapidly organize.
Interestingly, the GFS dissipates the system as it moves through the central Caribbean. Although that portion of the Caribbean is generally unfavorable for tropical cyclogenesis, full-fledged tropical cyclones typically do a little better. Since the TUTT axis is north of the Greater Antilles, and I expect 99L to stay south of those islands, it is a little puzzling why this model weakens the system, especially considering it also shows low shear. Perhaps it is because the trade winds, as indicated by buoy reports, have picked up a little today. More likely, these are daily fluctuations within the strength of the Bermuda-Azores ridge, the sole driver of easterly winds in that region. Since these fluctuations are both natural and difficult to predict, I have not placed much weight on the GFS forecast of dissipation. In addition, there doesn't appear to be any significant low-level speed shear on water vapor imagery.
As for track, that's always the most important question, but it's also one of the hardest. Water vapor images show a large ridge over the central United States with an amplified trough downstream over the eastern United States and western Atlantic. While this would ordinarily be a motif toward recurvature, the global models are unanimous in slowly lifting out this feature over the next 72 hours. This evolution should allow 99L to avoid moving too far north through the next five days, especially considering its location. My current thinking is a generally west-northwest path through the eastern and central Caribbean. Where it goes after that is still speculative, and the model variability within the synoptic pattern favors an eventual landfall on the US east coast, the US Gulf Coast, or even Central America and/or Mexico.
I do expect gradual development of this disturbance, but I am not ready to give explicit mention of a tropical depression just yet.
Probability of development in 48 hours: 50%
Elsewhere, a convectively active tropical wave is over the northeast Caribbean. There are no signs of organization, and upper-level winds are expected to remain unfavorable for development as the system moves west-northwest toward the Bahamas and south Florida. Locally heavy rains will continue impacting portions of the Greater Antilles through Wednesday.
Probability of development in 48 hours: 0%