Bill's Blog: Isaac In-Depth

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HOUSTON - Tropical Storm Isaac is getting a lot more attention now that it is in the Caribbean and forecast to be a threat to Florida.  The early part of the forecast is rather clear-cut as the center of Isaac is steered just south of Puerto Rico today, then right over the Dominican Republic and Haiti on Friday. Isaac has moved into an environment more conducive for further strengthening, and could easily intensify to hurricane force before reaching Haiti.  Regardless of intensity, Isaac is a large storm and will produce very heavy rains over mountainous terrain, and will likely produce dangerous flash flooding and mudslides in Puerto Rico today, and over the Dominican Republic and Haiti on Friday.

The forecast becomes more complicated this weekend into early next week. The exact track cannot be forecast with certainty this far out in time and the area depicted by the "cone" in the forecast graphic indicates the range of most likely paths for the center of Isaac.  Our scientific skill at forecasting the track of a storm center works out to an average error of about 45 miles per day.  By days 4 and 5, a typical error in forecasting the center location is around 200 miles.  So, for this forecast as it relates to someone in south Florida, there is still plenty of uncertainty of whether it will be a hit at all, much less the degree of impacts.  Per this forecast, the likelihood of the center of Isaac passing along the west coast of south Florida Sunday or Monday is only slightly higher than the likelihood of it passing over the Miami area, or of passing 100 miles offshore in the Gulf of Mexico.

How strong Isaac might be as it approaches south Florida is even more complicated.  Passage of a hurricane through mountains usually disrupts and weakens it. If Isaac's center only crosses part of Haiti and then moves into the Florida Straits north of Cuba, it could remain or re-intensify to a hurricane.  If it's track takes it along the mountains of Cuba all day Saturday, Isaac could be considerably weaker by the time it enters the Florida Straits.  Historically, some storms regain intensity after reemerging over warm tropical water, many do not.  Intensity forecasting is difficult at best, and for this forecast, even more difficult than usual. 

I often hear people who see the forecast graphic like the one shown here say about the cone "it is larger than usual" or "smaller than usual". This is an optical illusion.  The cone is constructed by drawing circles around each forecast point out to five days, where each circle is 2/3 the average error for that time.  Tangent lines are added to complete the cone.  If the storm is forecast to move slow, the cone will look fat, if the storm is forecast to move fast, the cone looks skinny, thus the illusion that the cone is larger or smaller.  It does not depict forecaster confidence, storm size, or impacts -- just the most likely path of the storm and forecast maximum intensity.


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