Tropical weather analysis - May 23, 2010

By: KoritheMan , 9:56 PM GMT on May 23, 2010

Invest 90L

Invest 90L continues to look unimpressive today, with a completely exposed surface circulation exposed to the south of the deep convection, as revealed per the latest animated visible satellite imagery. This particular circulation appears to be rapidly dissipating as it moves southward away from the deep convection. This swirl is clearly visible at 25N 70W. It appears that 90L is poised to undergo another center relocation, common of weak, sheared systems such as this one. It appears that a new center might be forming some 50-75 miles to the northeast, at around 26N 68W. Vertical shear has lessened considerably over the system today, with CIMSS analysis now depicting only 10-20 kt of southwesterly shear over the disturbance, due to the upper-level low, which yesterday evening was at 300 mb, moving atop the surface center of 90L. This is the first sign of a transition to a subtropical cyclone. Animation of water vapor imagery also confirms this. It also confirms a large amount of dry air is slowly approaching the system from the west. This will continue to hamper any rapid development, despite sufficiently warm SSTs and adequate vertical shear.

Interestingly, the loosely-organized bands of convection well to the east of the surface center appear to be more impressive and dominant right now, as both low-level convergence and upper-level divergence have increased along said band, while slightly decreasing near the center. I suspect this is because the new center, the one at 26N, has yet to fully form itself, and is thus not dominant. That should change soon as the new center strengthens.

90L has finally picked up speed today as steering currents have restrengthened slightly. Visible satellite imagery reveals a clear N component of motion, along the western extent of the subtropical ridge. Due to the influence of a shortwave trough to the west, stretching from Georgia to just offshore New Jersey, along with the aforementioned ridge, this system will continue moving in a general N fashion, at times possibly just E of N. All of the models appear to have poorly initialized the system, save the GFS, and even that particular model is a tad too far to the E. Hence, I remain to the left of model guidance for the first 6-12 hours, at which point I am in complete agreement with it, save the CMC, which erroneously keeps the ridge weaker for longer, and doesn't begin the system's forecast NW turn until around 48 hours. All of the other models, including the GFS, which has performed particularly well with this system so far, turn the system NW much sooner, by around 24 hours. Hence, I have discounted the CMC's ridiculous solution, and am in complete agreement with the bulk of the model guidance in calling for a N movement for the first 12-18 hours, after which point a gradual NW motion should commence as the subtropical ridge restrengthens somewhat.

By around 72 hours, the system will have made its point of closest approach to the East Coast of the United States, and the majority of the guidance agrees that the system will come within about 125 miles of the South Carolina/North Carolina border during that time. The CMC, being an outlier, is much further to the east, along the eastern North Carolina coast. This is considered highly unrealistic for now. By 78 hours, the system will begin to stall just offshore, due to interaction with a large and powerful extratropical cyclone that will be dropping southward from Atlantic Canada during this time. The GFS and NOGAPS forecast that the system will be well on its way into the circulation of said extratropical cyclone by 84 hours, though interestingly, the NOGAPS and ECMWF foresee that the system will ultimately be able to break free of the influence of the extratropical cyclone and meander southwestward across the western Atlantic for a couple of days. I will keep an eye on this potential situation very carefully. 90L should not make landfall.

Synopsis in layman's terms

A large and complex low pressure area, currently centered several hundred miles east of the Bahamas, near 26N 68W, has the potential to develop into a subtropical cyclone over the next 24 hours. Regardless of development, gale force winds will develop in association with the system, particularly the northern and eastern sides, producing dangerous sea conditions from Georgia to Virginia, over the next several days. The system could come dangerously close to either of the Carolina coasts by around 72 hours, before stalling and moving away from the area.

Caribbean disturbance

None of the models are predicting the possibility of a tropical disturbance in the Caribbean at this point. It's just as I suspected; 90L will steal most of the energy from the area, as will a possible low pressure area in the Gulf of Tehuantepec that could form in about 4-5 days. Though a tropical disturbance is not expected, seasonal heavy rainfall can be expected across the area. Certainly, nothing destructive appears likely at this time, fortunately. However, one very important thing that the models have been consistent on over the last several days is a gradual and significant relaxation of the vertical wind shear across the Caribbean. It starts with the southwest Caribbean, then gradually lifts northward out of the western Caribbean by around 5-6 days. Models have been very consistent on the timing of this event also, which could be a sign that atmospheric conditions are finally starting to become more favorable.

Synopsis in layman's terms

No tropical development is likely across any portion of the Caribbean over the next week. The only likely threat is seasonal heavy rainfall.


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.

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