Tropical weather analysis - August 9, 2010

Invest 93L

Invest 93L continues to churn out in the central Atlantic Ocean, about 875 miles east-northeast of the Leeward Islands. Latest shortwave infrared imagery reveals that this system is still disorganized, with the well-defined surface circulation clearly evident at 23N 50W, moving WNW at 10 to 15 mph. This center is completely exposed from the pulsating deep convection, as has been the case over the last couple days. However, it looks even less organized today than 24 hours ago, with any deep convection displaced over 100 miles to the east and northeast of the system.

Additionally, CIMSS vorticity data suggests a vertical decoupling of the 850 and 500 mb circulations. CIMSS vertical shear analysis indicates only 5 kt of southwesterly shear over the center of the system, with 20 kt on the western side. Certainly, satellite-observed low-cloud motions seem to indicate a relaxation of the shear, confirming the analysis from CIMSS.

93L is being steered by a low- to mid-level tropospheric ridge to the north. A weakness is evident in the ridge near 60W westward to nearly 75W. The models suggest that this general motion should continue until around 24 hour, at which point a turn to the NW with some deceleration is expected as 93L rounds the aforementioned weakness in the Atlantic subtropical ridge. By 36 hours or so, a turn to the N should occur, followed by a turn to the NE by 48-54 hours. This is a rather fortunate track scenario, taking the system completely out to sea. The models differ drastically as to how strong 93L will get, and when.

The GFDL and HWRF models take the system to hurricane status on day four as the system interacts more strongly with the trough. We have seen trough interactions fuel intensification in the past, so this possibility cannot be ruled out. And given that sea surface temperatures (SSTs) in this area, 35 to 40N 45 to 50W, where the aforementioned models foresee hurricane status, are running at 26 to 28C, this is probably not baroclinic strengthening being foreseen in these models.

The remainder of the models are less bullish, and peak the system about a day earlier. I will compromise between these extremes, and still, for the third consecutive day, forecast this system to peak at 65 mph in about 84 hours. I tend to lean toward the latter camp (GFS, ECMWF, UKMET, CMC, and NOGAPS) more strongly, based on previous and current convective trends, along with shear forecasts from the GFS. Nevertheless, environmental conditions are slowly becoming more favorable for development, with a lowering of the shear today (with further lowering expected), along with a gradual dissipation of the dry air, as noted on water vapor imagery.

I still expect this system to eventually become a tropical cyclone.

Invest 94L

The basin's biggest threat is still Invest 94L, a broad area of low pressure located in the extreme eastern Gulf of Mexico off the southwestern coast of Florida. My best estimate of the broad surface center, based on the remaining visible satellite images, doppler radar animations, and CIMSS 850 mb vorticity data is 25N 81W. Recent convective trends suggest that 94L is attempting to organize, with the development of a small area of deep convection over its estimated center.

Some modest pressure falls have been noted across the area this afternoon, and buoy observations across the eastern Gulf suggest that the system maintains its closed surface circulation, and that it might have become even better defined today.

The key player in the genesis, or lack thereof, of Invest 94L will be a newly-developed upper low just to the north of the northern Bahamas. Currently, this low is moving west-northwest, while 94L is moving west-southwest. Steering data from CIMSS suggests a WSW motion should continue for the next 6-12 hours, after which point a turn to the WNW should occur, followed by a turn to the NW in about 72 hours as the system nears the northern Gulf Coast. The majority of the models still bring this system into southeast or eastern Louisiana in about 72 hours, and the 18z GFDL makes this system a hurricane and brings it west of New Orleans. I will not go that high just yet, because I want to see the evolution of the aforementioned upper low first, as if it gets close enough, it could shear 94L, and inhibit significant development.


The 18z GFS suggests that the upper low will move generally westward and gradually weaken and deamplify as it does so. Additionally, steering data from PSU's e-wall suggests that the upper low could well outrun Invest 94L, leaving behind a very favorable upper environment as upper ridging builds overhead. In addition, the shear is forecast to be very light, and the currently northerly shear is forecast to abate soon, and veer to easterly or southeasterly by 24 hours. The shear may switch back around to northeasterly and become stronger again before the system moves inland.

There are three possibilities right now with 94L:

1. It ultimately becomes sheared and dissipated due to the upper low.

2. It becomes a tropical cyclone.

3. It becomes a subtropical cyclone.

Based on what I've seen, I'm expecting tropical cyclogenesis to occur with this wave by tomorrow night or Wednesday morning. However, this will be highly dependent upon the evolution and strength of the upper low. Subtropical development could also occur if the two become vertically aligned, or near so. A reconnaissance aircraft is scheduled to investigate this system tomorrow, if necessary.

Residents along the northern Gulf Coast from Biloxi to Morgan City should closely follow the evolution of this system over the next several days. Regardless of development, locally heavy rainfall, above normal tides, and strong gusty winds will overspread portions of the aforementioned areas during the next few days.