23 August 2012

Isaac passing south of Puerto Rico, TD10 organizing

Isaac passed over the Leeward Islands on Wednesday afternoon as a tropical storm, and remains a relatively weak 40mph tropical storm as of the 8am EDT advisory today.  It's located about 225 miles south of Puerto Rico.  You can find radar loops of Isaac from Martinique, Puerto Rico, and other sites in the future here.

Two noteworthy things have happened since yesterday's update: the storm turned a bit south of due west for 12 hours yesterday evening, and the forward speed decreased to a more normal 11kts (13mph).  The southward bump could be significant because it should take that much longer to start getting pulled northward toward Cuba and Florida, increasing the odds of a more western track... and the slower motion is more conducive for strengthening.

The plot below shows the morning model runs, which haven't shifted very much.  It's never a good idea to dwell on a single model or a single run too seriously; but rather, to assess the spread in the various models (indicates uncertainty in the large-scale steering), and the trends of the models over several runs and several days.  Over the past few days, there has been relatively little spread, and the trends have been somewhat steady too, with a track over or near Haiti, then eastern-central Cuba, then southern Florida.  The exact details for a US impact are impossible to know this far out... just watch Isaac closely if you're in Florida, the central-eastern Gulf coast, or even the Carolinas.  As you can see, just a small shift in the track means a difference between the FL panhandle and the SC coast!

Isaac is still forecast to acquire hurricane intensity prior to reaching the Hispaniola area, then weaken to a tropical storm during the encounters with Hispaniola and Cuba.  After Cuba, it should re-intensify as it approaches southern Florida.  In southern FL, expect conditions to go downhill on Sunday, with the worst weather coming on Monday.

The figure below shows the probability of tropical storm force winds occurring within the next 5 days, based on this morning's advisory package.  The bright yellow shading that nudges into south Florida is the 30-40% bin, and each contour is a different 10% bin from there.  As the storm's position and path get closer to a given location, the probabilities go up, as you'd expect.  You can find the full/latest figure, as well as others, at the NHC website.

Elsewhere, TD10 has changed little in the past day, and is now located about 1100 miles west of the Cape Verde islands, or about 1300 miles east of the Leeward Islands.  It is forecast to slowly strengthen, and continue its northwest motion.  It's quite likely that this will be upgraded to Tropical Storm Joyce today.

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