Tropical weather analysis - August 21, 2012
A vigorous tropical wave and associated low pressure system is located in the central Atlantic located about 700 miles east of the Lesser Antilles continues to show signs of organization. Satellite images show a small area of increasing convection near the center, with banding features becoming evident to the west.
Figure 1. Latest infrared satellite image of Invest 94L. Image credit: NOAA
Since a closed surface circulation already exists, it would not take much additional organization to denote the formation of a tropical depression. I suspect we will see this at the 5AM advisory from the National Hurricane Center.
Environmental conditions appear conducive for additional strengthening, and the system is likely to become a tropical storm later today or on Wednesday. There is still some dry air to contend with, but with the GFS adamant about a large anticyclone moving in tandem with the system throughout the next several days, my hunch is that 94L will be able to ward off most of the dry air. Steady development prior to reaching the Lesser Antilles seems likely, with the biggest impediment being the fast forward speed. This particular affliction has plagued many otherwise promising tropical cyclones, namely Ernesto and Helene. However, 94L is large enough so that it will be less susceptible to hostilities than either of the two aforementioned storms were at this stage. I do not anticipate a hurricane moving through the Leeward Islands, but residents there should anticipate a strengthening tropical storm, probably in the 45 to 50 kt range. Watches and warnings will likely be required for a large portion of those islands by the National Hurricane Center later this morning, and interests across those islands should carefully monitor the progress of this system over the next couple of days.
Water vapor and UW-CIMSS steering data show a strong mid-level ridge is in place to the north. The motion of the clouds across the eastern Caribbean are uniformly east-southeast, indicating a slight weakness in the subtropical ridge, undoubtedly caused by the western Atlantic trough. Based on global model forecasts at 500 mb, as well as the current progression of the trough, I expect a turn to the west-northwest as the system moves across the eastern Caribbean. There shouldn't be a sharp poleward component of motion until the system approaches the longitude of Hispaniola. The western Atlantic trough is forecast to lift out, but leave behind enough residual cyclonic flow to maintain a general weakness in the ridge. This weakness is forecast to become reinforced by the amplification of another trough along the eastern US by Thursday. This should result in the system gradually turning more northwestward. As the system gets west of Hispaniola there are two possibilities: the first is for it to remain relatively weak, bypass the trough, and move into the Gulf of Mexico. The second, and more likely scenario, is for 94L to gradually curve into the weakness off the eastern seaboard and move northward somewhere over Cuba. This track would place the southeastern United States under the gun.
The model consensus has shifted considerably farther east today relative to yesterday, which gives me more confidence in the forecast track. One fly in the ointment will be the likelihood of interaction with the mountains of Hispaniola. If the system moves slow enough and is significantly disrupted by passage over that island, we could see a center reformation of some sort, potentially leading to a large track shift. I consider this an outlier scenario for now, though.
I prefer the southern edge of the model consensus at this time.
Interests in the Lesser and Greater Antilles should monitor the progress of this system over the next several days, as they will have to contend with it long before all of us here in the United States.
Probability of development in 48 hours: Near 100%
An elongated area of low pressure continues over the western Gulf of Mexico. This system still possesses a broad circulation, but it is not well organized. After an earlier eruption of convection Monday afternoon, dry air and northwesterly shear haven taken their toll on 95L. Looking at water vapor images, the airmass over the western Gulf of Mexico is very dry, owing to the passage of an unseasonably deep cold front ushering in an unseasonably dry airmass, and yes, even a bit of cold air advection, as denoted by surrounding temperature data over the southern US.
Figure 2. Latest infrared satellite image of Invest 95L. Image credit: NOAA
Synoptic data indicates that the cold front appears to be ebbing away. Some shallow convection may attempt to ignite in the eastern portion of the circulation, where the ambient airmass is a little more unstable. But overall, the upper air pattern over the system is not one that I consider to be conducive to tropical cyclogenesis -- although UW-CIMSS shear data isn't working for some reason, one can get a rough idea that the shear over the northwestern Gulf is probably on the order of 20 or so kt at this time. While the GFS shows this shear abating somewhat in the next 24 hours -- which would be expected subsequent to a frontal passage -- the upper flow in that model still does not look all that anticyclonic, or even diffluent. In addition, it always takes awhile for airmass modification within the boundary layer to return to normal following the passage of such intense cold fronts. Hence, dry air will still be an issue even if the shear relaxes a little. Given this, I am less enthusiastic about development potential than the National Hurricane Center is.
Synoptic upper air data suggests that this system will move slowly westward, ultimately ending up back in Mexico over the next day or two.
Probability of development in 48 hours: 10%
The tropical wave that emerged from Africa yesterday is showing signs of organization, and it appears we could be talking about another tropical cyclone over the next few days. Satellite imagery indicates that the center is along the eastern side of the gyre, away from the deep convection. This is due to moderate easterly shear. The GFS forecasts the current shear to abate, and the system may find itself situated beneath a small anticyclone in about 48 hours. I mentioned yesterday that outflow from 94L could hamper development of 96L in the long range. A more thorough analysis today suggests that 96L is moving slower than 94L. So even if 94L begins to develop an appreciable outflow channel in that direction, it is unlikely to have a significant affect on 96L.
Having said that, environmental conditions are favorable for steady development over the next couple of days, and this system could become a tropical depression over the next 48 hours. I have no idea where this system will go, but initial model prognostications indicate yet another possible threat to land down the road. Stay tuned.
Probability of development in 48 hours: 60%