Bill's Blog: Isaac's Path

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HOUSTON - Tropical Storm Isaac will be impacting Haiti and the Dominican Republic Friday, then eastern Cuba and Jamaica on Saturday as it moves steadily towards the northwest. Because the center of the circulation will be moving across mountainous terrain, it is unlikely that Isaac will strengthen any through Saturday. Nevertheless, very heavy rain leading to flash flooding and mud slides is unfortunately likely to occur over these vulnerable islands. On Sunday, Isaac is expected to emerge off the north coast of Cuba and approach the Florida Keys and south Florida. How much, if any, Isaac intensifies before reaching the Keys is difficult to determine at this time. Much depends on how organized the center circulation is when it moves back over the water. Preparations are already underway in the Keys as it is wiser to prepare for the worst and hope for the best.

At this time, the forecast for Isaac from late Sunday through mid week is very interesting. As shown here, the models are fairly consistent in forecasting a general southeast to northwest steering current right through the five day forecast. Again, it is important to remember that errors of 100 to 200 miles are common for the day five forecast. Subtle features in the atmosphere not evident in this morning's data can arise which can impact the direction or speed the storm will move. Also, the size and strength of Isaac can have an impact on the surrounding environment, leading to changes in the steering current. A valuable asset in assessing and forecasting the steering current for Isaac is the high level hurricane plane flown by NOAA. This aircraft flew around the storm last night gathering high resolution data of the environment. The data is then used in the computer track forecasts, often improving them by 10% or more. These flights will continue to be flown until Isaac has made landfall.

Forecasting the eventual size and intensity for Isaac after crossing Cuba is considerably more difficult than forecasting the general environment. A lot depends on how organized it remains after spending a day in the mountains of Cuba and Haiti. Given the relative lack of organization this morning, I suspect it will be weaker coming off Cuba. Isaac is already fairly large and I suspect it will be a large circulation as moves through the eastern Gulf of Mexico. Environmental factors that can impact intensity such as wind shear and dry air are difficult to forecast accurately 3-5 days in advance. Computer models this morning suggest the environment in the eastern Gulf of Mexico would favor Isaac intensifying, as would the very warm waters that are always present in late August across the Gulf, hence the National Hurricane Center forecast for Isaac to become a hurricane in the Gulf.

Thursday, as the track forecasts started showing a trend toward the toward the north central Gulf coast, folks began asking "What is the chance of Isaac coming to Houston"? While our best models still indicate Isaac staying east of our area, there is enough uncertainty in the forecast to not completely rule out the possibility this morning. Stay tuned!


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