What began as Tropical Depression Nine early this morning has developed into Tropical Storm Isaac this afternoon. To be honest, TD 9 was not that impressive this afternoon and I was skeptical as to whether it would be upgraded to a tropical storm today. The center of circulation was “exposed” on satellite images. In other words, you could see the circulation in the clouds near the surface without any significant convection around the center. Most of the convection was off to the south and west, indicating there was wind shear present in the atmosphere that was preventing the storm from growing.
Nevertheless, a hurricane reconnaissance aircraft that investigated TD 9 this afternoon found enough evidence of tropical storm intensity. Since then, convection near the center of the storm has increased – another sign the storm is intensifying. It is forecast to slowly strengthen over the next few days. The official forecast calls for Isaac to reach hurricane strength in the Caribbean Sea after crossing the Lesser Antilles, but forecasting exactly when it will transition from tropical storm to hurricane strength is a difficult proposition. Let’s put it this way… Isaac is expected to become a hurricane in the next few days and all island nations in the northern Caribbean Sea should monitor the movement and development of the storm.
Right now, Isaac is not forecast to strengthen beyond a category one hurricane during its’ time in the Caribbean. However, that is due to the expected interaction with Hispaniola and Cuba toward the end of the week. It’s possible that a slight shift in the track (either direction or speed) could leave Isaac over water longer and increase the chance it strengthens beyond category one. At the same time, a shift northward in the track could lead to increased land interaction and weaken Isaac further.
You will note this forecast track stops after 5 days, and for good reason. Forecast track errors grow immensely as you go out further in time. You can see this with the white shaded areas on the map above. So, right now, the NHC is not going to speculate on the potential impact along any US coast. Of course, that won’t stop your local TV meteorologist from speculating. If you live in Florida, get ready to see a picture like this on your local news this evening:
Note the European model forecast is 3.5 days behind the GFS model in bringing Isaac toward the US coast. The Euro model also brings Isaac between Cuba and the Yucatan peninsula, which seems pretty wacky given all the other model forecasts. That said, it’s super scary and makes for good TV, so don’t be surprised if someone shows it tonight.
I believe average model track error 7-10 days out is on the order of 500 miles. Thus, it’s really not worth considering Isaac’s potential impact on the US until the end of this week. However, with the Republican National Convention taking place in Tampa, FL next week, we’ll probably get more speculation than we ever would for a hurricane so far out to sea. Let’s just wait until the end of the week before considering potential US impacts.