Tropical weather analysis - June 17, 2010

Began writing at around 7:40 PM eastern

Invest 92L simply refuses to give up. Deep convection has once again reignited, though it is still disorganized due to strong upper-level shear. My best estimate of the center location, based on the last of the visible satellite animations, a recent AMSUB microwave pass, and 850 mb vorticity data, courtesy of University of Wisconsin CIMSS is 17N 58W. Satellite images strongly indicate a struggling, sheared system, with the center displaced well to the west of the large unorganized convective mass. CIMSS diagnoses this westerly shear at 30 kt. A slight weakness is noted in the subtropical ridge to the north of the system around 700 mb. This weakness can also be recognized at 500 mb, as well. This weakness can also clearly be seen on the limited availability of water vapor animation, though this weakness is very slight.

Steering current forecasts from PSU are divided. I personally believe the GFS is too far north, and the CMC too far south. Hence, I have opted to split the difference and forecast a gentle WNW movement throughout the forecast period.

At the end of the forecast period, after the system enters the Gulf of Mexico, there are indications within the models that the subtropical ridge could strengthen a bit, forcing a more W component of motion thereafter. It is much too early to tell where exactly this system will go when it reaches the Gulf of Mexico, but residents all along the Gulf Coast from Mexico to Florida should carefully monitor the progress of this system.

My current track takes the system across the northern Leeward Islands and Puerto Rico in three days, and then across extreme northeastern Hispaniola after that. Because of the likelihood of land interaction, any development will be highly limited in the short-term. Long-term, it appears that vertical shear will be considerably lessened as the system continues WNW north of the Greater Antilles. Indeed, upper-level winds will be very favorable in the Gulf of Mexico at the end of the forecast period, with the GFS indicating a 200 mb anticyclone over the eastern portion of the region.

However, despite this forecast, I am not expecting 92L to rapidly regenerate once it clears the islands and the upper-level flow improves.

Why? Because of its structure. Although I must say, a weak tropical storm ultimately impacting a portion of the Gulf Coast is certainly not out of the realm of possibility. However, it is much too early to assert this with any reasonable degree of confidence. Just keep an eye on it for now, and don't panic due to the oil spill.

In the short-term, the central and northern Leeward Islands, Puerto Rico, and Hispaniola, particularly the northern and eastern reaches, will experience heavy rainfall and wind gusts in excess of 30 to 35 kt as 92L passes through.

Elsewhere

The GFS has been hinting, off and on for the last day or so, the possibility of a tropical cyclone developing southeast of the Windward Islands in about a week. From 0z to 12z, it had the system developing (three consecutive runs). However, in the most recent run, the 18z run, it drops it. The ECMWF also no longer shows it as it did around two days ago. However, we can still get an overall idea of what the atmospheric pattern is likely to be during this time, and it is one of an active ITCZ in the area, so this certainly bears monitoring. Upper-level winds will be near zero, with the GFS building a weak anticyclone over the eastern Caribbean by day seven.