Tropical weather analysis - July 21, 2010

Invest 97L

Invest 97L continues to struggle with strong vertical wind shear associated with a persistent upper-level low that has branched off from the Tropical Upper Tropospheric Trough (TUTT). The last of the visible satellite images suggest that the exposed low-level center is located at 21N 72W. 97L continues to battle 20 kt of southwesterly shear, associated with the aforementioned upper low. Animation of water vapor imagery and various CIMSS data depicts this upper low rather well from 350 mb upward to around 150 mb. This upper low is keeping all of the convection well to the east of the wave axis, resulting in a highly disorganized system.

However, this upper low has retreated westward over the last six hours, and as a result, shear has decreased over Invest 97L. The GFS continues to persistently indicate that the upper low will continue moving westward away from 97L, being replaced by an upper high, which would reduce the vertical shear over the system and allow for some upper-level ventilation to occur. Water vapor imagery supports the forecast of the GFS, indicating that the upper low has continued to gradually hook westward.

Theoretically, this low should ultimately outrun 97L, leaving a more favorable upper wind environment in place. The 18z GFS indicates that upper-level conditions should begin to significantly improve within about 12 hours. This agrees well with the evolution of the low on water vapor imagery and analysis of upper-air data. This, combined with a large area of dry air to the west, being channeled into the poorly defined low-level center by the upper low, makes it highly unlikely that 97L will significantly intensify tonight, and probably even into the early morning hours on Thursday.

By late morning and into the afternoon, however, the upper flow should vastly improve, as previously noted. Thereafter, upper-level winds appear favorable for some steady development until the system reaches south Florida in around 42-48 hours. Thereafter, the system will emerge into the eastern Gulf of Mexico, where upper-level winds appear to be more conducive than they were this time yesterday, though the aforementioned upper low, as it continues retreating westward, then northwestward and inland, may impart some southerly shear over the system.

I do not expect this shear to be strong enough to be a significant deterrent to development in the Gulf, however.

Given the system's current state of disorganization, it is quite improbable that the system will be able to be any stronger than a minimal tropical storm when it impacts southern Florida. The most likely scenario, however, is a strengthening low pressure system impacting south Florida with gusty winds, rough surf, and heavy rainfall.

Track wise, I expect the system to move generally WNW until around 72 hours, at which point a large weakness in the western Atlantic subtropical ridge is forecast to develop unanimously by nearly all of the computer models. This should bring the system inland somewhere from southeast Louisiana to Destin in about four days.

The CMC continues to suggest that 97L will make landfall along the upper Texas coast, but there is currently no valid reason to assume that this scenario is a reasonable one, since even the CMC keeps a remarkably weak ridge to the east of the system, which wouldn't allow it to move that far west.

None of the models significantly develop 97L, and most actually lose it while it is traversing Florida. The reason for this is unclear, but it could be that the upper low, which will be in the Gulf of Mexico by this time, will pump dry continental air into the circulation, along with natural disruption from passage overland, and finally, southerly shear. This possibility will have to be carefully assessed over the next couple of days, as the models may well be onto something here.

For now, the NHC is giving this system a 40% chance of developing into a tropical cyclone in the next 48 hours, and this is a reasonable forecast.

Invest 98L

The tropical disturbance previously located across the western Caribbean Sea has recently been designated Invest 98L by the National Hurricane Center.


Infrared satellite loops indicate the development of deep convection near the center, which I estimate is at 20N 93W based on animation of infrared RGB imagery.

There is also some weak vorticity noted at 850 mb, though the strongest signature is located inland over the western Yucatan Peninsula, to the east of the deepest convection. Additionally, the overall vorticity is broad and not well organized at the moment. However, upper-level winds are very favorable for development, and an anticyclone aloft is noted based on CIMSS wind shear data. The GFS keeps this anticyclone intact until 54 hours, at which point the system should be inland. Ergo, I believe it is quite possible that we will see a tropical cyclone out of this area prior to it moving inland near Tampico, Mexico. The farther south it goes, however, the less time it will have over water. This situation looks similar to 2005's Tropical Storm Bret, and also to Hurricane Alex earlier this year.

Indeed, this area might have a greater chance to develop than 97L does. The primary inhibiting factor will be land interaction.

Elsewhere

Elsewhere in the tropics, the reliable ECMWF continues to indicate the development of our first Cape Verde system, or at least a strong low pressure area, near the northern Cape Verde Islands on July 25. This model, along with the GFS, has been hinting at the possibility of tropical cyclogenesis in this area for awhile.

This appears to be a reasonable forecast, and upper-level winds favor development of anything that does happen to form here.