Thursday, August 23, 2012

Isaac Is A Difficult Call

Tropical Storm Isaac moved into the eastern Caribbean Tuesday afternoon and was moving westward during the evening.  The storm was getting better organized late in the evening, but the winds were not much different from earlier in the day.  It is only a matter of time before the winds respond to the drop in pressure.

Tropical Storm Isaac at 0215z August 23, 2012.  This is an enhanced infrared picture of the storm.  The dark areas represent the colder cloud tops meaning that they are higher in the atmosphere.  Image Credit: NESDIS.

There has been considerable concern about the future of this storm due to the impact it could have on Florida and the Republican National Convention in Tampa next week.  Any evacuation would greatly alter the plans for the convention.  A general idea of where the storm is going can be gleaned from the models.  However, the devil is in the details.  It is far too uncertain at this time frame to pin down where the storm will strike land.

So let’s take a look at how the forecasts have fared thus far.  The 5-day forecast from the first advisory early Tuesday morning projected that the storm would continue on generally westward course until reaching a position south of the Dominican Republic where it would turn more to the west-northwest.

The 5-day forecast from the National Hurricane Center for 0900z August 21, 2012.  Click on the image for a larger view.  Image Credit: NOAA/NHC.

The 8 p.m. advisory for Wednesday evening showed the current position plus the forecast from the late afternoon advisory.  Isaac was clearly south of the forecast as it moved due west instead of moving west-northwest.  This may be due to the center being located farther south, but the current movement was still due west.

The 5-day forecast from the National Hurricane Center for 0000z August 23, 2012.  Click on the image for a larger view.  Image Credit: NOAA/NHC.

Isaac is moving around the periphery of a large high pressure system to the north.  The models have this right, but all over the models have been too far north in their initial position of the storm.  Does this mean that the path should be shifted south? Yes, albeit a small shift.  Yet, even a small shift at this time frame can make a big difference 5 days from now.

The latest forecast from the National Hurricane Center projects that Isaac will start to slowly curve to the west-northwest tonight.  If by daybreak it is still moving west, then it is more likely that the eventual path will be more in the southern half of the cone of uncertainty (labeled potential track area).

The 5-day forecast from the National Hurricane Center for 0300z August 23, 2012.  Click on the image for a larger view.  Image Credit: NOAA/NHC.
Note that I have not said anything about the intensity or changes in intensity.  Much depends on the eventual track.  If the storm tracks across Hispaniola and the mountains of Cuba, then the storm will struggle to remain a storm.  However, if it stays south of the mountainous regions, then this storm could become a monster.  Conditions will be right for the next several days for Isaac to intensify.  This may be the most difficult aspect of the forecast because of the possible interaction with land.  Forecasters at the National Hurricane Center have noted as much.

Fortunately, Isaac is still days away from the U.S. mainland.  It will probably not reach south Florida before Sunday night.  Evacuations from the Keys may begin by Friday if the storm stays close to its projections.  The state of Florida will likely begin ramping up ongoing emergency operations during the day on Thursday.

It is still not entirely out of the picture that the storm could turn move northwest and move through the Bahamas and up the East coast, however this is a low chance.  It is more likely to eventually move into the Gulf making landfall somewhere in the eastern half of the Gulf Coast area.

The main threat to South Carolina could be heavy rains in the middle to latter part of next week.  This would lead to flooding of rivers and could cause some flash flooding of small creeks.  The recent rains mean that the ground may not be able to absorb as much water leading to more flooding.

All of this will depend on the path over the next few days.  Stay tuned!