Hanna the Headache

There’s a lot of eyes on Hurricane Gustav this weekend, and rightfully so — it’s become a Category 3, 125 MPH beast, and the forecast just does not look good for Texas and Louisiana around, unfortunately, New Orleans. Gustav appears to be in position to make this Labor Day weekend a very memorable one for all the wrong reasons. Here’s hoping folks are heeding the call to leave the coast early; the last thing anybody wants to see is a duplication of the tragedy of Katrina three years ago yesterday.

Here in Charleston, we may have a tropical issue of our own to deal with by the end of next week. I’ve been spending a showery morning in West Ashley playing with computer model runs for Tropical Storm Hanna and the end result has been a little heightened concern for our neck of the woods.

Before we dive into Speculationland, let’s first start with the official 5-day forecast track for Hanna as provided by the National Hurricane Center:

Five-day forecast track for Tropical Storm Hanna

Hanna’s currently expected to do something unusual for tropical cyclones and actually take a southward dip toward the Bahamas. I need to reiterate just how weird that is — it doesn’t happen often. (This southward dip is actually much less pronounced than what was being forecast yesterday through 5am.) One thing to note is the gigantic cone of uncertainty, especially as we get into Days 4 and 5. A lot of it has to do with a high pressure system that’s built up over the Southeast. This high pressure is what’s likely to steer Gustav into the Gulf Coast. It’s also expected to move out more into the Atlantic during mid-week, breaking down Gustav’s steering (and stalling it out over Louisiana and Texas, making for a potentially catastrophic flooding event) and also starting Hanna on a northward trajectory. It’s the degree of the high’s movement that makes all the difference.

The long-range models show a variety of northward-turning scenarios. The GFDL has Hanna doing a loop-dee-loop before resuming a northwest motion about a week from now. The HWRF model shows a southward dip and then a resumption of northwest motion. The GFS shows, so far at least, a track that shows a Savannah landfall roughly Thursday night into Friday morning as a fairly deep area of low pressure. Conversely, the CMC shows a dip into Cuba before straddling Florida’s west coast by Friday. Those are just a few of the scenarios out there — it’s really anybody’s guess.

Another area of Hanna’s uncertainty is the intensity forecast. NHC’s official forecast keeps it at tropical storm strength thanks to some persistent upper-level shear; however, the models continue to try to make Hanna at least a Category 1. For a time, in fact, the GFDL shows Hanna touching Category 3 strength before backing off into a Category 2 over the Bahamas. The HWRF is a bit more conservative with intensification, but still makes Hanna a Category 1. The confidence in all intensity forecasts is fairly low and this is definitely a low-confidence forecast.

Needless to say, anywhere from both of Florida’s coasts up the Eastern Seaboard should keep an eye on this one. This has the chance to be even more unpredictable than Fay proved to be, so it’s definitely one to watch especially as the week wears on.